Plasmon enhanced photoelectron spectroscopy and the ... · Plasmon enhanced photoelectron spectroscopy and the generation of isolated ... binding potential of the atom’s valence

Plasmon enhanced photoelectron spectroscopy and the generation of isolatedattosecond XUV pulses for use with condensed matter targets

by

Phillip Michael Nagel

A dissertation submitted in partial satisfaction of the

requirements for the degree of

Doctor of Philosophy

in

Chemistry

in the

Graduate Division

of the

University of California, Berkeley

Committee in charge:

Professor Stephen R. Leone, ChairProfessor Daniel M. NeumarkProfessor Roger W. Falcone

Fall 2011

Plasmon enhanced photoelectron spectroscopy and the generation of isolatedattosecond XUV pulses for use with condensed matter targets

Copyright 2011by


1

Abstract

Plasmon enhanced photoelectron spectroscopy and the generation of isolated attosecondXUV pulses for use with condensed matter targets

by


Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Stephen R. Leone, Chair

Surface plasmon resonances (SPRs), collective oscillations of quasi-free electrons in met-als, can produce strong electric field enhancements at the surface of nanoparticles. Theseoscillations typically occur at optical frequencies (thus having a period on the order of oneto a few femtoseconds) and only remain coherent for a few to tens of femtoseconds. Becauseof their increasing importance in various applications, it is important to understand SPRsat a fundamental level. The ultrafast nature of the processes involved with SPRs maketime-resolved spectroscopy an important tool for probing their dynamics.

Recently developed light sources capable of producing isolated attosecond (10−18 s) pulsesof light can provide snapshots of electron dynamics on a sub-femtosecond timescale. Fewerthan a dozen laboratories in the world currently have the ability to produce such lightpulses. In this dissertation I discuss the development and construction of an experimentalapparatus capable of producing and utilizing isolated attosecond pulses to study condensedmatter, including surface plasmon dynamics. The ultimate goal of the experiments presentedhere is to laser-excite plasmonic resonances in metallic nanostructures and to detect the fieldenhancement at the surface of the nanostructures by measuring photoelectron spectra.

In the first experiment presented, electron photoemission from lithographically preparedgold nanopillars using nominally few-cycle, 800 nm laser pulses is described. Electron kineticenergies are observed that are higher by up to tens of eV compared to photoemission froma flat gold surface at the same laser intensities. A classical electron acceleration modelconsisting of multiphoton ionization followed by field acceleration qualitatively reproducesthe electron kinetic energy data and suggests average enhanced electric fields due to thenanopillars that are between 25 and 39 times greater than the experimentally used laserfields.

In the second experiment presented, attosecond streaking from a W(110) single crystaland from an amorphous Cr thin film is demonstrated. In addition, a novel concept for SPRenhanced attosecond streaking is proposed and evaluated with the aid of a numerical model.

i

To Aisling, for always being by my side.

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Contents

List of Figures iv

1 Introduction 11.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Attosecond Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 History and mechanisms of high-harmonic generation . . . . . . . . . 21.2.2 Attosecond pulse trains from HHG . . . . . . . . . . . . . . . . . . . 41.2.3 Isolated attosecond pulses from HHG . . . . . . . . . . . . . . . . . . 51.2.4 Experiments using isolated attosecond pulses . . . . . . . . . . . . . . 9

1.3 Surface plasmon resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.3.1 Definition of the surface plasmon resonance . . . . . . . . . . . . . . 121.3.2 Propagating and localized surface plasmons . . . . . . . . . . . . . . 121.3.3 Plasmon lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.4 Overview of photoelectron spectroscopy from surface plasmon systems 18

1.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2 Experimental Apparatus 202.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2 Laser System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2.1 Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.2 Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.3 Prism Compressor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.3 Carrier-Envelope Phase Stabilization . . . . . . . . . . . . . . . . . . . . . . 262.3.1 f-2f Mach-Zehnder Interferometer . . . . . . . . . . . . . . . . . . . . 262.3.2 CEP Locking Electronics . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.4 Spectral Broadening in Hollow-Core Fiber . . . . . . . . . . . . . . . . . . . 282.5 Chirped Mirror Compressor . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.6 Vacuum System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.7 High Harmonic Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.8 Metal filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.9 XUV Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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2.10 Time-of-Flight Electron Spectrometry . . . . . . . . . . . . . . . . . . . . . . 392.10.1 TOF Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.10.2 Signal analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.10.3 Sample TOF data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.11 XUV Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3 Surface plasmon electron acceleration 493.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.2.1 Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.2.2 Nanopillar Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.3.1 Photoelectron Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . 523.3.2 Total Electron Emission Scaling with Laser Intensity . . . . . . . . . 543.3.3 Classical acceleration model . . . . . . . . . . . . . . . . . . . . . . . 55

3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

4 Condensed Matter Attosecond Streaking 594.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.2 Previous experiments in the literature . . . . . . . . . . . . . . . . . . . . . . 594.3 Streaking results from a W(110) single crystal . . . . . . . . . . . . . . . . . 63

4.3.1 Photoelectron background emission . . . . . . . . . . . . . . . . . . . 634.3.2 Demonstration of attosecond streaking . . . . . . . . . . . . . . . . . 66

4.4 Streaking results from amorphous Cr thin film . . . . . . . . . . . . . . . . . 694.5 Surface plasmon enhanced attosecond spectroscopy . . . . . . . . . . . . . . 714.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Bibliography 75

iv

List of Figures

1.1 Illustration of the three step model for high-harmonic generation put forth byCorkum [16]. Step 1: The intense laser field is strong enough to distort thebinding potential of the atom’s valence electron and allow tunnel ionization.Step 2: The free electron is accelerated in the laser field back toward theparent ion. Step 3: Recombination of the free electron with the parent ionreleases a photon with energy up to the cutoff at Ip + 3.17Up. . . . . . . . . 3

1.2 A 5 fs FWHM, 800 nm Gaussian laser pulse with CEP = -π/2. The CEPis defined as the phase difference between the peak of the Gaussian pulseenvelope and the maximum of the carrier wave. . . . . . . . . . . . . . . . . 5

1.3 Demonstration of the carrier-envelope offset frequency (CEO) taken from Ref.[25]. The CEO results from the gap between zero frequency (ν = 0) andthe lowest peak (νCEO)when the laser frequency comb is extended to zerofrequency. frep is the inverse of the repetition rate of the laser. The absolutefrequency of any spectral line, ν(m) can be determined by ν(m) = νCEO+mfrep. 6

1.4 Comparison between a 5 fs FWHM, 800 nm Gaussian laser pulse and a 25 fsFWHM pulse. The difference in electric field strength of adjacent half cyclesof the laser pulse becomes much less for longer pulse durations and causes thevalue of the CEP to lose effect. . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.5 Schematic of the effect of CEP on the ability to isolate a single attosecondpulse. a) A cosine pulse (CEP = 0) creates a situation in which one halfcycle of the driving field is of higher intensity than all other half cycles. b) Asine pulse (CEP = π/2) where there are two half-cycles of equivalent intensity.When the spectral filter is applied an isolated attosecond pulse can be obtainedfrom the cosine pulse but not from the sine pulse. . . . . . . . . . . . . . . . 7

1.6 Principle of attosecond streaking taken from Ref. [36]. Electrons are releasedby the near-intantaneous attosecond pulse and receive an ultrafast sub-cyclemodulation of their momentum from the few-cycle streak field. . . . . . . . . 9

1.7 A typical streak trace taken in sulfur hexafluoride gas. The streaking ofelectrons ionized from the outer valence orbitals appears around 76 eV whilethe faint signal of inner valence streaking can be seen around 50 eV. . . . . . 10

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1.8 Diagram of the plasmon dispersion relation for bulk plasmons and surfaceplasmons, modified from Ref. [54]. ωp is the bulk plasmon frequency, ωs isthe surface plasmon frequency, and the light line is k = ω/c where ω is theangular frequency of light and c is the speed of light in vacuum. Couplingof the light line to the plasmon modes via momentum transfer by surfaceroughness, kr, is shown for both plasmon modes. . . . . . . . . . . . . . . . . 13

1.9 A schematic of the Kretschmann configuration for exciting propagating surfaceplasmon waves in a flat metal surface. Total internal reflection of the laserbeam at the ε0/ε1 interface launches an evanescent wave which can excite aSPR at the ε1/ε2 interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.10 Schematic depicting localized SPRs oscillating in gold nanospheres. In a local-ized plasmon the electron cloud oscillates back and forth across the particle,concentrating the incident electromagnetic energy into a small physical vol-ume. In the schematic the incident field is traveling in the direction of thearrow and driving the plasmon oscillations in the particles. The shaded arearepresents the electron density as the particles are macroscopically polarized. 15

1.11 An example of persistent spectral hole burning from Ref [52]. As the oblategold-nanoparticles on a sapphire substrate are irradiated, an increasingly largehole forms in the measured spectrum. From this hole the homogenous plas-mon linewidth can be determined. In this case the homogenous linewidth is94 meV, which corresponds to a dephasing time of 14 fs. . . . . . . . . . . . 16

1.12 An example of second-harmonic generation from gold nanoparticles in Ref-erence [48]. The bold line is the autocorrelation measured using a standardBBO crystal and representing the laser pulse duration. The thin line is theautocorrelation measured with second-harmonic light generated from a goldnanostructured surface. The broadening is from the plasmon dephasing life-time and corresponds to a lifetime of 6± 1 fs. . . . . . . . . . . . . . . . . . 17

1.13 Surface plasmon-based electron acceleration demonstrated by Irvine, et al.[60]. In this experiment a 27 fs laser oscillator pulse is used to both excite aSPR in a gold film in the Kretschmann geometry and ionize photoelectronsinto the enhanced field. The electrons are then classically accelerated to highkinetic energies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1 An overview of the experimental apparatus. . . . . . . . . . . . . . . . . . . 212.2 Schematic of the Femtolasers Femtosource Scientific Pro oscillator. PL - pump

laser, L - lens, Ti:S - titanium sapphire crystal, CM - chirped mirror, WP- wedge pair, OC - output coupler, CP - compensating plate, BS1 - 50:50beamsplitter, BS2 - 30:70 beamsplitter, FI - Faraday isolator, TOD mirrors- chirped mirrors for third order disperson (TOD) compensation, GB - 10cmlong SF10 glass block, RR - retro-reflector, PO - pick-off mirror, PD - photo-diode. Modified from Ref. [70] . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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2.3 Schematic of the Femtolasers Femtopower Compact Pro amplifier. L1,2 -telescope for pump beam, L3 - lens for focusing of pump beam, PBFM - pumpbeam focusing mirror, P1-4 - periscopes, IRFM1,2 - infrared focusing mirrors,RR1,2 - retro-reflectors, PBS1,2 - polarizing beam-splitters, PC - Pockel’s cell,BC - Berek polarization compensator, PO1,2 - pick-off mirror, VC - vacuumchamber, BW - Brewster window, Ti:S - titanium sapphire crystal, C - Peltiercooling, PD - photodiode. Courtesy of Ref. [70] . . . . . . . . . . . . . . . . 22

2.4 Schematic of the Femtolasers Femtopower Compact Pro prism compressor.Courtesy of Ref. [70] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.5 Spectral fringes measured in the slow loop. The first 8 minutes show spectracollected while varying the offset voltage that is sent to the phase lockingelectrons. The changing voltage varies the CEP and thus the positions of thespectral fringes. From 8 minutes onward, the offset voltage is only controlledby the slow loop error signal and long-term locking stability of the CEP isdemonstrated. Inset: Fast loop beat signal measured on a spectrum analyzer. 24

2.6 A schematic of the Mach-Zehnder interferometer used in the CEP fast loop.MO - microscope objective, PCF - photonic crystal fiber, L - lens, DC -dichroic mirror, P - polarizer, BBO - beta barium borate crystal, BPF - bandpass filter, PD - photo diode. . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.7 A schematic of the beam stabilization system for input into the hollow-corefiber (not drawn to scale). PZ1, PZ2 - Piezo actuated mirrors, L - focusinglens, QPD1, QPD2 - Quadrant photodiodes, BS - Beamsplitter, HCF - Hollow-core fiber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.8 Laser pulse spectrum measured after spectral broadening in a hollow-core fiberfilled with 1.9 Bar of Ne gas but before temporal recompression. . . . . . . . 30

2.9 A measured autocorrelation trace of the few-cycle laser beam after spectralbroadening in the Ne filled hollow-core fiber and temporal recompression inthe chirped mirror compressor. The black circles are experimentally measuredpoints while the red line is the calculated fit to the measured data. Thecorresponding FWHM laser pulse duration is 6.5 fs. The deviation from thefit in the wings of the pulse results from higher order phase terms that manifestas pre- or post-pulses. These are not well characterized by autocorrelation butcould be characterized using a pulse-reconstruction technique such as SPIDERor FROG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.10 Schematic of the vacuum chamber . . . . . . . . . . . . . . . . . . . . . . . . 322.11 Typical HHG cutoff spectrum generated by an 800 nm laser pulse in Ne gas.

This spectrum was generated by laser pulses with an unlocked carrier-envelopephase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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2.12 Generation of a continuum in the HHG cutoff region by using a CEP locked800 nm laser pulse. The two spectra are taken at zero and π relative CEP.The continuum at π relative phase is indicative of an isolated attosecond pulsein that energy region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.13 Calculated transmission of a 200 nm thick Zr foil filter. For use with the XUVspectrometer, two such filters are used in-line. . . . . . . . . . . . . . . . . . 36

2.14 200 nm thick Zr filter mounted on an aluminum coated pellicle. This filterserves to separate the copropagating XUV and IR light into an inner beamand an outer beam so that a time delay can be introduced between the two. 37

2.15 Multilayer Mo/Si XUV mirror reflectivity shown compared to the HHG con-tinuum. The mirror reflectivity is designed to spectrally select only the con-tinuum region of HHG, leaving an isolated attosecond pulse after reflection. . 37

2.16 Schematic of the cored-mirror used in this apparatus. The central portion ofthe mirror has a multilayer XUV coating and can be moved in the beam axisindependently from the outer mirror by a piezo-translation stage. The outermirror (not shown here for clarity) is gold coated and can be moved in thex- and y-directions by picomotors to allow for precise spatial overlap betweenthe inner and outer beams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.17 Circuit diagram of the MCP apparatus used in TOF detection. Providedcourtesy of Jordan TOF Products, Inc. . . . . . . . . . . . . . . . . . . . . . 41

2.18 Kinetic energy resolution of the TOF electron spectrometer as a function ofelectron kinetic energy. For valence electrons emitted directly by the HHGproduced XUV pulse, around 90 eV, the energy resolution is 0.9 eV. The plotonly accounts for the instrument resolution and does not include the band-width of the XUV pulse or any other contributions to the final experimentalresolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.19 A sample TOF photoelectron spectrum collected from ionization of a goldnanopillar sample by the few-cycle IR laser and integrated over 60000 laserpulses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.20 Raw data for a TOF photoelectron spectrum collected from ionization of aW(110) crystal by HHG produced ≈93 eV photons and integrated over 500000laser pulses. The peak at 104 ns is caused by scattered photons and can beused for calibrating the spectrometer time zero. To the right of the photonpeak a sharp peak from XUV emitted photoelectrons can be seen, followedby a large low energy electron background. . . . . . . . . . . . . . . . . . . . 43

2.21 The TOF data from Figure 2.20 after conversion to a kinetic energy scaleand correction for the Jacobian. The peak centered at ≈86 eV is from XUV-induced valence band photoemission while the large low energy signal is theresult of inelastically scattered electrons within the metal. . . . . . . . . . . 45

2.22 Schematic of the XUV spectrometer. . . . . . . . . . . . . . . . . . . . . . . 46

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2.23 Measured zero order transmission of the HHG radiation generated in Ne gas,averaged over 10 scans with 1 s integration each scan. 1/e2 diameters of the x-and y-cross sections are measured as 6.86 mm and 5.4 mm, respectively. Thewhite dashed lines show the positions at which the line-outs were measured. 47

2.24 First order dispersion spectrum of HHG XUV radiation generated in Ne gas,averaged over 10 scans with 1 s integration each scan. . . . . . . . . . . . . . 48

3.1 Schematic of the experimental apparatus. 30 fs FWHM, 800 µJ laser pulsesare spectrally broadened in a gas-filled hollow-core fiber (HCF) and temporallycompressed to ≈7 fs FWHM with a series of multilayer chirped mirrors (CM).The laser is focused onto the sample surface and photoelectrons are detectedusing a linear time-of-flight spectrometer (TOF). . . . . . . . . . . . . . . . . 50

3.2 (a) Scanning electron microscope (SEM) image of the gold nanopillar array.(b) Dark-field scattering measurement of a single nanopillar from an identi-cally prepared sample with a larger pitch to allow for measurement of a singleparticle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3 (a) Photoelectron kinetic energy spectra taken from a flat gold surface as afunction of excitation intensity. (b) Photoelectron spectra taken from the goldnanopillars at the same intensities as (a). Strong acceleration of photoelec-trons to high kinetic energies is indicative of photoelectron emission in thepresence of plasmon-enhanced electric fields. Because of the inability of pho-tons to directly excite a SPR in flat gold, a minimal increase in kinetic energyis present in (a). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.4 Log-log plot showing the total number of detected photoelectrons as a func-tion of excitation intensity, I, for (a) the flat gold surface and (b) the goldnanopillar sample. While the flat gold surface demonstrates the expectedthird order multiphoton dependence, only a second-order dependence is ob-served in emission from the nanopillars. . . . . . . . . . . . . . . . . . . . . . 54

3.5 (a) Spectra modeled from classical electron trajectory calculations (blacklines) compared to the experimental data (symbols). Each trace is offsetby one order of magnitude from the previous trace for clarity. In the model,multiphoton emission is followed by classical acceleration in an enhanced field.An average field enhancement of 32 brings the model in close agreement withthe experimental data. The intensities shown in the legend are the enhancedintensity values, (I ∝ E2), used for the calculation. The experimental data isthe same as shown in Figure 3.3a. (b) The experimental data (symbols) com-pared to a range of modeled spectra calculated for average field enhancementfactors from 25-39 (shaded areas). Each trace and shaded area is offset bytwo orders of magnitude from the previous trace for clarity. . . . . . . . . . . 55

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4.1 (a) Photoelectron spectra collected at different time delays between the XUVattosecond pulses and the IR laser pulses. The positions of the delays areshown as dashed lines in (b). Peaks from the tungsten valence band (83 eV)and 4f state (56 eV) can be seen in a spectrum taken far from zero timedelay (blue line), and the Fermi level is denoted by Ef . This same spectrumis also shown after subtraction of the large multiphoton background emissionand numerical smoothing (red line). These peaks broaden out from streakingby the IR laser field in the spectrum measured at zero time-delay (blackline). (b) The full streaking spectrogram after subtraction of the multiphotonbackground emission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.2 Streaking data from a W(110) crystal from Ref. [40]. (a) Streak traces follow-ing cubic-spline interpolation of photoemission from the valence band (upperpanel) and the 4f state (lower panel) from a W(110) single crystal. A verysmall time delay between the two streak traces is highlighted by the dashedwhite lines. (b) Center-of-mass plots for the valence and 4f streak tracesin (a). The resulting delay is 110 ± 70 as, where the error results from thecalculation of the center-of-mass. . . . . . . . . . . . . . . . . . . . . . . . . 62

4.3 The universal curve for electron inelastic mean free path (IMFP) taken fromRef. [81]. For electrons with kinetic energy around 90 eV, the IMFP is only≈ 0.4 nm. Because this length is shorter than the penetration depth of XUVradiation into the sample, a background of inelastically scattered electronsresults. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.4 A schematic illustrating the source of the large inelastically scattered pho-toelectron background resulting from XUV photoemission. The 93 eV lightpenetrates ≈ 3 nm normal to the surface (z-axis), releasing electrons all alongthe path of the light, while the IMFP is only ≈ 0.4 nm for 90 eV electronkinetic energy. Electrons released deeper than this have very little probabilityof escaping the surface without inelastically scattering. . . . . . . . . . . . . 64

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4.5 (a) Photoelectron kinetic energy spectrum from a W(110) single crystal sur-face ionized by HHG generated XUV pulses centered at 93 eV. The peak cen-tered at≈ 86 eV is from electrons that escape from the surface without scatter-ing (indicated approximately by grey shaded area), while the large low energybackground results from electrons that are inelastically scattered within themetal. (b) Comparison of XUV only photoelectron emission from W(110)(black line) to photoemission from the XUV plus the few-cycle, 800 nm laserpulses (red line). Instability in the HHG flux has led to a slight decrease inoverall signal between the two measurements. The two pulses are positionedat zero time overlap and the intensity of the 800 nm laser pulses is typicalfor a streaking experiment. The broad peak centered at 34 eV is the result ofmultiphoton emission by the 800 nm laser pulses and is saturating the detec-tor below 34 eV (indicated by the arrow). In addition, increased amplitudeabove ≈ 95 eV shows streaking of electrons from the valence band peak tohigher kinetic energies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

4.6 First demonstration of attosecond streaking from the apparatus developed inthis dissertation. The spectrogram is constructed from a series of photoelec-tron spectra collected from a W(110) single crystal at varying time delaysbetween the attosecond XUV and the IR laser pulses. Negative time delaysrepresent the time at which the attosecond XUV pulses arrive before the IRlaser pulses. Each time step (200 as) is integrated over 30000 laser pulses. . . 67

4.7 Spectral centroid analysis of the streaking trace presented in Figure 4.6. Thecentroid is calculated from data between 70 eV and 110 eV and clearly demon-strates the sub-optical-cycle resolution of the streaking spectrogram. . . . . . 68

4.8 Comparison of attosecond streaking from (a) a W(110) single crystal and(b) a 10 nm thick amorphous chromium thin film. The 800 nm streak fieldintensity was not the same in both measurements, which accounts for thedifferent amounts of streaking in the kinetic energy domain. . . . . . . . . . 70

4.9 Summation of the photoelectron yield between 93 eV and 100 eV for (a) aW(110) single crystal surface and (b) a 10 nm thick amorphous chromiumthin film. Sub-optical-cycle resolution is clearly visible in both spectrograms. 71

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4.10 Simulation of a proposed measurement of field-induced attosecond time-resolveddipole potentials. a) Schematic of the dipolar charge distribution induced ina metal nanoparticle by the exciting laser field. Electrons are freed by the at-tosecond pulse with an initial velocity v0 and sample the induced electric field,experiencing a force Fsurface, before they are detected by a time-of-flight spec-trometer with a small collection angle of 15. b) Simulated time-dependentphotoelectron kinetic energy spectrum as a function of time-delay betweenthe attosecond and the 530-nm laser pulse, where E0 = v2

0/2 is the kineticenergy in the absence of the plasmon excitation. A 6 fs laser pulse (whiteline) at 9.8×109W/cm2 intensity excites the plasmon which is then probed bya time-delayed 500 as pulse. A temporal broadening of the dipole potentialresponse function — mapped out by the intensity maxima of the photoelec-tron spectral distributions — compared to the driving pulse shows the finitelifetime of the plasmon resonance (sustained dipole oscillations at late timesafter the driving laser pulse is over). In addition to resolving the decay timeof the plasmon resonance, individual plasmon oscillations are observed withsub-cycle resolution, permitting the possibility to unravel nonlinear dynamics. 73

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Acknowledgments

After six years of grad school I can’t count the number of people who have touched my life inways that I will never forget. Thank you first to my amazing and beautiful very-soon-to-bewife Aisling. She has been there beside me in good times and bad, helping me celebrate orkeeping me going. Her faith in me is what led me to complete this work. I owe her all of mylove. I also especially want to thank my family for all the support that they have given me,not only in my graduate work, but throughout my entire life. They gave me the opportunityto be where I am today, and I hope that I’ve made them proud.

Thank you to my advisor, Steve Leone, for all of the opportunities he has provided meover the years. From him I learned the importance of being a rigorous scientist and to neverstop asking questions of myself. I also never once had to worry about getting my paycheckon time or about not having enough funding to fix my broken laser, and there is a lot to besaid for those things. Dan Neumark, an unofficial advisor to me in many ways, has providedme with a lot of guidance and a few good days of skiing as well. Robert Kaindl has beenkind enough to let me collaborate with his group and work in his lab for a number of yearsnow. I owe him thanks for giving me space to work and for providing expertise in condensedmatter materials, something that none of the rest of us were very familiar with. I would alsoreally like to thank my undergraduate research advisor, Alex Kandel at the University ofNotre Dame. Without the opportunities that he gave me, I would never have known aboutthe joys of working in a laboratory and I certainly would not have ended up here.

I am greatly indebted to my two biggest influences in all things related to the laboratory,Thomas Pfeifer and Joe Robinson. I’m pretty sure that no postdocs in history have everbeen more patient with stupid questions from grad students or more willing to take time outof their schedule to help with whatever I needed. It has been an honor (honour for Joe) anda pleasure to work with both of them.

I have had many great friends that have always supported me and have made my lifequite enjoyable. Ben Doughty and I started at Berkeley together and were officemates forover four years. I can’t think of a better person to have spent so much time with. Ourdiscovery of Lucky House and Pad Ke Mao (well done, of course) was probably the discoveryof the century at Berkeley. Mark Abel and I learned about attoseconds together, but we alsomanaged to drink (and make) some great beer and listen to some great music. I also have tothank my roommate, Noah Bell, for putting up with me for all of these years and for beinga great roommate. Thanks also to my newest officemates, Justine Bell and Annelise Beck.You guys may listen to some insane music, but we do laugh an awful lot and that makes itworth coming to work every day.

I would like to thank Adam Bradford and Kathleen Fowler for their amazing help innavigating the absurd bureaucracies that are LBL and UC Berkeley. Without them I’dprobably still be waiting on laboratory equipment to arrive from the vendors. Lastly, Iwould like to thank Professor Roger Falcone for taking time out of his very busy schedule toserve on my thesis committee and help me finish what I came here to do.

1

Chapter 1

Introduction

1.1 Overview

Surface plasmon resonances (SPRs), collective oscillations of quasi-free electrons in metals,have received a significant amount of attention over the last half century [1, 2, 3, 4]. Theseoscillations typically occur at optical frequencies (thus having a period on the order of oneto a few femtoseconds) and only remain coherent for a few to tens of femtoseconds [5]. SPRshave enormous potential for applications in medicine, communications, and electronics [6, 7].Most of these applications take advantage of the strongly enhanced electric field created bythe plasmon at the metal surface. Techniques such as surface-enhanced Raman spectroscopy(SERS) exploit this near field enhancement and even enable spectroscopic detection of singlemolecules [8]. Because of the increasing importance of this phenomenon, it is important tounderstand at a fundamental level. The ultrafast nature of the processes involved with SPRsmake time-resolved spectroscopy an important tool for probing SPR dynamics.

Time-resolved spectroscopy, the use of short bursts of light to illuminate processes as afunction of time, has been used to shed new light on scientific questions since the invention offlash lamps and eventually pulsed lasers [9]. From milliseconds to picoseconds, new regimes ofstudy have been opened up with every new generation of technology. The 1980s and 1990ssaw the widespread growth of femtosecond (10−15 s) laser systems. With this technologycame unprecedented access to many previously mysterious ultrafast processes, including anisomerization that triggers the first steps of human vision in less than 200 fs [10] and thedynamics of molecules and transition states during chemical reactions [11], for which AhmedZewail won the Nobel prize in 1999. The turn of this century saw the development of thevery first light sources to produce isolated attosecond (10−18 s) pulses [12]. The attsecondtimescale is the natural timescale of electron motion inside of atoms and molecules; forinstance, the time for an electron to complete one orbit of a Bohr hydrogen atom is ∼150 as[13]. Electron dynamics are the driving force behind much of chemistry and these new lightsources offer an opportunity to understand the basic processes of the world around us at the

CHAPTER 1. INTRODUCTION 2

most detailed level yet possible. From gas phase to condensed matter, important processesnot accessible through the frequency domain are now being uncovered with unprecedentedtemporal resolution.

By ionizing electrons with isolated attosecond XUV pulses in the presence of enhancedSPR fields, potential nonlinearities in the oscillation of the plasmon electron cloud may beuncovered with sub-cycle resolution. With sub-cycle resolution the mechanisms that lead tothe decoherence of the plasmon oscillation can be investigated more clearly. Of particularinterest is what happens when various molecules are adsorbed onto the surface of the particle.Through a process known as chemical interface damping, electrons from the metal can tunnelinto and out of surface states created by the adsorbed molecules [14]. When electrons thathave been trapped in surface states re-enter the metal particle, they do not necessarily do soat the same phase as the plasmon electrons. This addition of this dephasing channel to thedephasing channels of the bare particle decreases the overall plasmon lifetime. If the plasmonfield is monitored with sub-cycle resolution using attosecond pulses, the loss of electrons tosurface states might be observed directly. Additionally, the amount of enhancement that canbe achieved in a technique such as SERS has been shown to be proportional to the plasmoncoherence lifetime [15]. It is thus of great interest to understand what controls this lifetimeso that substrates for SERS can be designed to achieve maximum enhancement. These arejust a few of the reasons that surface plasmon resonances present a highly interesting targetfor study with isolated attosecond pulses. The ultimate goal of probing plasmon resonancesusing isolated attosecond pulses could provide new insights into the basic physical propertiesof SPRs such as the dephasing rate of the oscillating electrons or the effects of surfaceconditions on the plasmon electron population.

The overview goal of the experiments presented here is to laser-excite plasmonic reso-nances in metallic nanostructures and to detect the field enhancement at the surface of thenanostructures by electron ejection. Three kinds of experiments are anticipated: visible laserelectron ejection by multiphoton processes, attosecond streaking at the surface of metals us-ing an extreme ultraviolet (XUV) attosecond pulse coupled with a visible laser field, andpump-probe experiments with visible laser excitation of the plasmon and attosecond electronejection. Results for the first two goals are achieved in this dissertation. The work consistsprimarily of two parts: the construction of an apparatus capable of producing and utilizingisolated attosecond light pulses in the XUV regime and the subsequent application of thisapparatus to the study of nanostructured plasmonic systems and metallic surfaces.

1.2 Attosecond Spectroscopy

1.2.1 History and mechanisms of high-harmonic generation

In order to understand attosecond pulse generation, one must first understand the processof high-harmonic generation (HHG). HHG in the gas phase, its most commonly used form


field-/tunnel-ionization

1

acceleration in laser field

2

E

t1

2

3x

x

x

Ec~ Ip + 3.17 Up

hν

ATI or recombinationand photo-emission

3

Figure 1.1: Illustration of the three step model for high-harmonic generation put forth byCorkum [16]. Step 1: The intense laser field is strong enough to distort the binding potentialof the atom’s valence electron and allow tunnel ionization. Step 2: The free electron isaccelerated in the laser field back toward the parent ion. Step 3: Recombination of the freeelectron with the parent ion releases a photon with energy up to the cutoff at Ip + 3.17Up.

today, was first pioneered in 1988 [17] and is a relatively simple process in principle. Byfocusing an intense femtosecond laser pulse into a gas medium, a highly nonlinear processcan emit higher energy photons at multiples of the input photon energy. The well-knownthree-step model describing the HHG process semi-classically was introduced by Corkum in1993 [16]. It is shown schematically in Figure 1.1. In the first step a strong laser electricfield distorts the binding potential of the valence electrons in the atoms in the gas. Thepotential is so distorted that electrons can tunnel through the ionization barrier and becomefree electrons in the continuum. In the second step the free electron is accelerated first awayfrom the parent ion, but then back towards it as the sinusoidal laser field changes direction.As the electron is being accelerated through the continuum it can gain a large amount ofkinetic energy. The third step is the recombination of the free electron with the parent ion.This final step does not always occur, and when it does a burst of higher energy radiationcan be released by the recombined atom. The photon energy is determined by the bindingenergy of the electron that was tunnel ionized and the amount of energy it gained in thelaser field:

~ωmax = IP + 3.17Up (1.1)


where Ip is the ionization potential of the harmonic generation medium and Up is the pon-deromotive potential, defined as:

Up =e2E2

o

4meω20

(1.2)

where e is the elementary electric charge, E0 is the laser electric field strength in V/m, me

is the electron mass, and ω0 is the angular frequency of the laser electric field. The HHGprocess has also been described in a fully-quantum-mechanical model [18].

While the three-step model describes the single atom response of HHG, emission fromsingle atoms can build up coherently to form a macroscopic response under the correct phasematching conditions [19]. Proper phase matching is the condition such that the radiationemitted by individual atoms or molecules adds coherently to the radiation emitted by otheratoms or molecules in the ensemble. If appropriate phase matching conditions are not met,the emission from individual atoms will destructively interfere and macroscopic HHG radi-ation will not be produced. Phase matching in HHG depends on a number of experimentalparameters, including the gas density, the laser intensity and the free-electron density [20, 21].

1.2.2 Attosecond pulse trains from HHG

The discovery of HHG almost immediately led to the speculation that the sharp spectralspikes may be pulses of attosecond duration in the time domain [22]. In principle, this iseasy to think about. The HHG spectrum is a series of equally spaced lines that when Fouriertransformed, if they are phase-locked with respect to each other, will result in a series ofpulses with attosecond time structure in the time-domain [23]. This is also easy to thinkabout in terms of the three-step model, in which the recombination event occurs once everyhalf cycle of the driving laser pulse and produces a burst of high energy radiation everyhalf cycle. Because the recombination of the free electron and the parent ion occurs on anattosecond timescale, the burst of radiation has an attosecond temporal structure.

The biggest question after the successful generation of HHG but before the realizationof attosecond pulses was whether the harmonic orders were in fact phase-locked to eachother. This was conclusively shown by Paul, et al. in 2001 when they were able to measurethe relative phase of harmonic orders and determine that the time structure of their HHGemission consisted of attosecond bursts of light spaced by one half of an optical cycle [24].In order to do this they used a quantum interference technique, which came to be known asRABITT, or reconstruction of attosecond beating by interference of two-photon transitions.The idea is again quite simple. Normally photoionization of an atomic gas such as argonby the HHG pulse would produce a photoelectron spectrum with peaks at the harmonicfrequencies minus the ionization potential. By overlapping the HHG pulse with the drivingfew-cycle laser field, additional quantum pathways (the subtraction or addition of a laserphoton) become possible. Because the harmonic peaks are evenly spaced at odd harmonicorders, the addition of a laser photon to one harmonic order creates a sideband that overlaps


−5 0 5−1

−0.5

0

0.5

1

Time (fs)

No

rma

lize

d E

lec

tric

Fie

ld

CEP

Figure 1.2: A 5 fs FWHM, 800 nm Gaussian laser pulse with CEP = -π/2. The CEP isdefined as the phase difference between the peak of the Gaussian pulse envelope and themaximum of the carrier wave.

with the sideband from subtraction of a laser photon from the next higher energy harmonicorder. By measuring the intensity of the sidebands as function of the delay between theHHG pulse and the few-cycle laser pulse, the relative phase of neighboring harmonic orderscan be deduced and the temporal structure of the pulse determined. The result in this casewas a train of attosecond bursts of radiation with an average duration of 250 as and the firstever measurement of a sub-fs light pulse.

1.2.3 Isolated attosecond pulses from HHG

The carrier-envelope phase

The biggest factor in spanning the bridge from attosecond pulse trains to isolated attosecondpulses is control of the carrier-envelope phase (CEP). The CEP is defined as the phasedifference between the peak of the laser carrier wave (the oscillating electric field) and thepeak of the laser pulse envelope [25], assumed to be approximately Gaussian in the case ofthe laser used in this dissertation. A diagram of the CEP principle is showing in Figure1.2. The CEP results from a frequency domain property called the carrier-envelope offset(CEO) frequency, which can be described as follows. When a train of laser pulses in thetime domain, such as that produced by a mode locked laser, is Fourier transformed to thefrequency domain, it appears as sharp, discrete frequency peaks separated by the repetitionrate of the laser cavity (the inverse of the roundtrip cavity time). This series of sharp peaksis called a frequency comb [25]. If this frequency comb is extrapolated to zero frequency,


Figure 1.3: Demonstration of the carrier-envelope offset frequency (CEO) taken from Ref.[25]. The CEO results from the gap between zero frequency (ν = 0) and the lowest peak(νCEO)when the laser frequency comb is extended to zero frequency. frep is the inverse ofthe repetition rate of the laser. The absolute frequency of any spectral line, ν(m) can bedetermined by ν(m) = νCEO +mfrep.

there exists an offset between zero frequency and the lowest frequency peak of the comb.This offset is the CEO frequency; Figure 1.3 demonstrates the concept. In the time domain,this CEO frequency causes the CEP to slip on a pulse-to-pulse basis. Isolated attosecondpulse generation depends on careful stabilization of the CEP.

In most femtosecond laser systems the CEP will vary from pulse to pulse and measure-ments will be averaged over all values of the CEP. In most older femtosecond lasers therewould have been very little effect on the measured results even if the CEP had been stabi-lized because the CEP does not typically start to become an important parameter until laserpulses reach the few optical cycle regime. In this regime, where the duration of the laser pulseis on the same order as the period of the laser carrier wave, the difference in electric fieldintensity between adjacent half-cycles of the carrier wave starts to become significant, lead-ing to a dependence on the exact shape of the waveform. In longer laser pulses of equivalentpeak intensity, the peak of the Gaussian pulse envelope flattens out the intensity differencebetween adjacent half-cycles of the carrier wave is small enough to cause little effect. Thisis illustrated in Figure 1.4.

Isolated attosecond pulses by intensity gating

The principle of generating an isolated attosecond pulse from HHG is the same as generatingan attosecond pulse train but with two additional constraints, active stabilization of the CEPand a driving laser pulse duration on the order of 5 fs. Isolated attosecond pulse generationwith CEP stabilized pulses was first achieved in the laboratory of Ferenc Krausz in 2003 [13].The method for isolating one pulse from the train of pulses generated by HHG relies on the


−30 −20 −10 0 10 20 30Time (fs)

−6 −4 −2 0 2 4 6−1

−0.5

0

0.5

1

Time (fs)

No

rma

lize

d E

lec

tric

Fie

ld

5 fs 25 fs

Figure 1.4: Comparison between a 5 fs FWHM, 800 nm Gaussian laser pulse and a 25 fsFWHM pulse. The difference in electric field strength of adjacent half cycles of the laserpulse becomes much less for longer pulse durations and causes the value of the CEP to loseeffect.

−6 −4 −2 0 2 4 60

0.2

0.4

0.6

0.8

1

Time (fs)

No

rma

lize

d In

ten

sit

y

−6 −4 −2 0 2 4 6Time (fs)

a) b)

Spectral filter

Figure 1.5: Schematic of the effect of CEP on the ability to isolate a single attosecond pulse.a) A cosine pulse (CEP = 0) creates a situation in which one half cycle of the driving fieldis of higher intensity than all other half cycles. b) A sine pulse (CEP = π/2) where thereare two half-cycles of equivalent intensity. When the spectral filter is applied an isolatedattosecond pulse can be obtained from the cosine pulse but not from the sine pulse.


significant difference in intensity between adjacent half cycles of the laser pulse. Because thephoton energy generated in the HHG process is partially determined by the laser intensity(see Equations 1.1 and 1.2), the difference in half-cycle intensities means that the HHG cutoffenergy varies depending on which half-cycle produces the electrons. By stabilizing the CEPa waveform can be used for HHG that has one half-cycle that is higher in intensity than allother half-cycles (a cosine pulse). By using a CEP locked cosine pulse, one attosecond pulseof higher photon energy than the other pulses in the train will be generated every laser pulse.A spectral filter can then be used to reject all lower photon energy pulses and leave a single,isolated attosecond pulse. This is illustrated schematically in Figure 1.5a. The importanceof the CEP can be visualized by shifting it π/2 radians to create a sine pulse. In this casethere are two equivalent maxima of the carrier wave per laser pulse. This will result intwo attosecond pulses of equivalent photon energy being generated within each laser pulse.Spectral filtering is then not possible and an isolated attosecond pulse cannot be achieved.This situation can be seen in Figure 1.5b.

The second requirement, a driving pulse duration on the order of 5 fs, is related to theamount of spectral bandwidth in the high harmonics necessary to support an attosecondpulse duration. According to Equation 1.2, the bandwidth is determined by the differencein intensity in adjacent half cycles of the driving laser pulse. The larger the difference inintensities is, the larger the difference in photon energies generated by each half cycle is. Thisallows for the spectral filter to be placed at a lower energy to increase the bandwidth of thehighest energy attosecond pulse to be isolated. According to the time-bandwidth relationshipfor a Gaussian pulse, (1/2π)∆ω∆t = 0.44 for a transform limited pulse [26]. This meansthat to support a pulse of less than 1 fs FWHM duration in the resulting high harmonicspectra, a bandwidth of at least 1.82 eV is necessary. The laser parameters necessary togenerate this bandwidth depend on the HHG generation gas and the desired cutoff energy.

A special case of intensity gating called two-color gating has also been investigated boththeoretically and experimentally by several groups [27, 28, 29, 30, 31]. Two-color gating relieson the same principles as intensity gating but uses a HHG driving pulse that is a mixture of afundamental frequency plus a harmonic or sub-harmonic of that frequency. This heterodynetechnique breaks the symmetry of the HHG driving pulse and allows for fewer recombinationtrajectories. For example, adding a 400 nm laser field to an 800 nm laser field reduces HHGrecombination to once every optical cycle, instead of twice for an 800 nm laser field alone.This reduces the frequency of pulses in the attosecond pulse train by a factor of two andthus relaxes the requirements on the pulse duration of the driving pulse by a factor of two.

Isolated attosecond pulses by polarization gating

A second technique for the generation of isolated attosecond pulses, called polarization gat-ing, was first demonstrated by Sansone and coworkers in 2006 [32]. This technique takesadvantage of the fact that a macroscopic HHG signal is only observed for linearly polarizedlight due to the fact that the free electron is driven away from the parent ion by a circularly


Figure 1.6: Principle of attosecond streaking taken from Ref. [36]. Electrons are releasedby the near-intantaneous attosecond pulse and receive an ultrafast sub-cycle modulation oftheir momentum from the few-cycle streak field.

polarized field and recombination is unlikely to occur. In polarization gating the driving laserpulse is synthesized such that it is circularly polarized except for a brief moment (typicallyaround half of an optical cycle) during which it is linearly polarized. This allows for HHGduring only one half cycle and therefore only a single isolated attosecond pulse. Because thispulse does not have to be spectrally filtered from other attosecond pulses in a pulse train,it is capable of supporting a much larger bandwidth and thus a shorter attosecond pulseduration.

Isolated attosecond pulses by double optical gating

One of the latest techniques in isolated attosecond pulse generation is double optical gating(DOG) [33, 34, 35]. DOG combines the techniques of two-color gating and polarization gatingto result in broadband supercontinuum generation supporting very short isolated attosecondpulses over a wide range of frequencies, all while relaxing the requirements on driving pulseduration.

1.2.4 Experiments using isolated attosecond pulses

Attosecond streaking

By far the largest number of experiments using isolated attosecond pulses to date takeadvantage of the technique known as attosecond streaking [36, 37, 38, 39, 40]. The principleof attosecond streaking is based on the long existing technique of the streak camera. In astreak camera a pulse of light impinges on a photocathode to produce a flow of electronswith intensity proportional to the light intensity. The electrons are then passed between a


Ph

oto

ele

ctr

on

Kin

eti

c E

ne

rgy

(e

V)

Time Delay (fs)

Figure 1.7: A typical streak trace taken in sulfur hexafluoride gas. The streaking of electronsionized from the outer valence orbitals appears around 76 eV while the faint signal of innervalence streaking can be seen around 50 eV.

pair of electrodes with a sweeping potential so that the photoelectron signal as a function oftime is mapped into a spatial dimension. In attosecond streaking experiments the electronsare produced by ionization with the isolated attosecond pulse and the sweep electrodesare replaced with the electric field of few-cycle laser pulse. A schematic of the attosecondstreaking principle is shown in Figure 1.6 [36]. As electrons are ionized from the targetmaterial they are accelerated by the few-cycle streak field, with the magnitude and directionof the acceleration dependent on the exact phase at which the electron is born. The electronvelocity as a function of time can be described by:

v(t) = − e

me

A(t) + [v0 +e

me

A(ti)] (1.3)

where e is the elementary charge, me is the electron mass, A(t) is the vector potential of thelaser field where EL = −∂A/∂t, and ti is the time of electron ionization [41]. Because of thedependence on A(ti), electrons ionized at different optical phases will achieve different finalvelocities; this is the principle which allows for time measurement on a sub-cycle timescale.By scanning the time-delay between the attosecond pump pulse and the few-cycle streakfield, different values of ti can be sampled and can then be combined to form an attosecondstreak trace that maps out the vector potential of laser field with sub-cycle resolution. Atypical streak trace taken in sulfur hexafluoride gas is shown in Figure 1.7.


An early example of the use of streaking to measure electron dynamics was performed byDrescher and coworkers in 2002 [39]. In this experiment, they used an attosecond pulse tomeasure the lifetime of M-shell core-holes in krypton atoms. In this case the XUV attosecondpulse is used to create a core hole that is known from linewidth measurements to decay byAuger decay in a few femtoseconds. In Auger decay, ionization of an inner-shell electroncreates an energetically unfavorable hole. To relax, a higher energy electron can fill the core-hole, and in the process transfer enough energy to another outer shell electron to ionize it andleave a doubly charged ion behind. This secondary electron is released at a characteristickinetic energy and can be streaked separately from the primary electron. If the electronwere released promptly (i.e. a very short core-hole lifetime), a well resolved streak trace isexpected. However, if the the core-hole lifetime is significantly long compared to the half-cycle of the streaking laser field, the sub-cycle oscillations will be blurred out as electronsare released at all optical phases of the streak field over the lifetime of the core-hole state.For long times, this results in sideband formation at ±1 photon energy. By analyzing thetemporal behavior of the sidebands, the authors were able to determine a core-hole lifetimein Kr of 7.9+1.0

−0.9 fs.Attosecond streaking has also been performed from a condensed matter surface [40] and

will be discussed in Chapter 4.

Attosecond tunneling spectroscopy

Another method of attosecond spectroscopy was introduced by Uiberacker and coworkersin 2007 [42]. In this method the authors take advantage of the fact that ionization withattosecond pulses not only liberates electrons but also leaves behind positively charged ions.Several experiments are presented in the paper, but here one is highlighted. By trackingthe yield of Ne2+ ions as a function of the delay between an XUV attosecond pump pulseand an 800 nm few-cycle probe pulse, direct observation of the timescale of light-inducedelectron tunneling was made possible. A mixture of Ne1+ and Ne2+ was initially producedby the attosecond pulse, then a strong enhancement of the Ne2+ signal was observed whenthe attosecond pulse was brought into temporal overlap with the 800 nm laser field. Sub-femtosecond steps in the ion yield, spaced by the half-cycle 800 nm laser period, were observedaround the peaks of the laser electric field and matched well with the theory of electrontunneling put forth by Keldysh in 1965 [43].

Attosecond transient absorption

The most recent measurement technique in attosecond spectroscopy is attosecond transientabsorption [44, 45]. Transient absorption takes advantage not only of the extremely shortpulse duration of attosecond XUV pulses, but also of the extremely large bandwidth thatcomes with such a short pulse duration. In this technique the few-cycle IR laser pulse isused as a pump pulse while the attosecond pulse is used as the probe pulse. By measuring


the dispersed spectrum of the attosecond pulse after interaction with the target gas, a largenumber of bound-bound or bound-free transitions can be probed simultaneously. When tran-sitions occur, some peaks will decrease (bleaches) and some new peaks will appear (transientabsorptions) in the transmitted attosecond spectrum. In this study, the few-cycle laser fieldis used to tunnel ionize krypton atoms and launch coherent electronic wavepackets. The at-tosecond XUV pulse can promote core level electrons into the hole left by tunnel ionizationand thus probe populations and coherences on a sub-femtosecond timescale.

1.3 Surface plasmon resonance

1.3.1 Definition of the surface plasmon resonance

One of the most exciting phenomena in condensed matter physics today is the coherentelectronic excitation in metals known as the surface plasmon resonance (SPR). The SPRis a collective oscillation of conduction band electrons that typically oscillate at opticalfrequencies in noble metals [46]. For a short amount of time, these electrons oscillate inphase and create a strongly enhanced electric field at the surface of the metal/vacuum ormetal/dielectric interface. This field decays exponentially from the surface of the conductor,penetrating only tens to hundreds of nanometers into space [1], and it typically decays in afew to tens of femtoseconds [5, 47, 48, 15, 14, 49, 50, 51, 52]. After this time the coherenceis destroyed by various mechanisms (discussed below) and the oscillation will eventuallydissipate to lattice vibrations on the order of picoseconds[53].

The SPR is an extension of the concept of bulk plasmons. Bulk plasmons result when thefree electrons of a metal are considered as an electron liquid that can undergo longitudinaldensity fluctuations. The oscillations have an energy described by:

~ωp = ~

√4πne2

me

(1.4)

where ωp is the bulk plasmon frequency, n is the electron density, e is the elementary chargeand m0 is the electron mass [2, 1]. The solution to Maxwell’s equations also shows thatdensity fluctuations can be confined to the surface of the metal as a propagating or localizedsurface plasmon wave. This surface plasmon resonance has an energy of:

~ωs =~ωp√

2(1.5)

where ωp is described by Equation 1.4.

1.3.2 Propagating and localized surface plasmons

Typically surface plasmons will propagate along the surface of a metal just like a light wave[1]. One major drawback to applications involving propagating plasmons on flat surfaces


Bulk plasmon dispersion

Light line

Surface plasmon dispersion

Figure 1.8: Diagram of the plasmon dispersion relation for bulk plasmons and surface plas-mons, modified from Ref. [54]. ωp is the bulk plasmon frequency, ωs is the surface plasmonfrequency, and the light line is k = ω/c where ω is the angular frequency of light and c is thespeed of light in vacuum. Coupling of the light line to the plasmon modes via momentumtransfer by surface roughness, kr, is shown for both plasmon modes.


Є0

Є2

Є1

Figure 1.9: A schematic of the Kretschmann configuration for exciting propagating surfaceplasmon waves in a flat metal surface. Total internal reflection of the laser beam at the ε0/ε1interface launches an evanescent wave which can excite a SPR at the ε1/ε2 interface.

is that they cannot be directly excited by photons because the dispersion relation of SPRsfalls below the dispersion relation of light propagating in vacuum (the light line, k = ω/cwhere ω is the angular frequency of the light and c is the speed of light in vacuum) [1, 54].This is illustrated in Figure 1.8. Here ωp is the bulk plasmon frequency and ωs is the surfaceplasmon frequency. This means that a plasmon wave travels with greater momentum than alight wave of equivalent energy. In order to excite propagating plasmon waves on flat surfacesusing photons, special geometries such as the Kretschmann configuration must be used [55].

In the Kretschmann geometry, illustrated in Figure 1.9, a thin metal film is depositedon the surface of a prism. The exciting laser beam is directed through one of the uncoatedsides of the prism and angled such that it undergoes total internal reflection from the goldcoated side. At the point of internal reflection at the εo/ε1 interface an evanescent light waveis launched in the interface with a wave vector k =

√ε0(ω/c) sin θ, where c is the speed of

light in vacuum and θ is the angle of incidence from normal to the surface. If√ε0 sin θ > 1,

the wave vector of the evanescent wave will lie to the right of the light line and can excitea SPR at the ε1/ε2 interface because the wave vector is now commensurate with the SPRdispersion curve.

A special case of SPRs, called localized surface plasmons, exists when a plasmon os-cillation is confined to a small volume such as a nanoparticle. Localized plasmons in goldnanospheres are illustrated in Figure 1.10. In these particles the free electron cloud oscillatesback and forth across the particle, confining the electromagnetic energy to a small physicalvolume and potentially leading to a much greater enhancement of the incident electric fieldstrength than in a propagating plasmon[2, 1]. Localized plasmons have the additional benefitthat they can be excited directly by photons because the particle edges and surface rough-ness allow for the necessary exchange of momentum to couple the light line to the plasmon


+

+

+

++++

+ ++

+

+

++++

+ ++_

_ __

__

__

_ _

Figure 1.10: Schematic depicting localized SPRs oscillating in gold nanospheres. In a local-ized plasmon the electron cloud oscillates back and forth across the particle, concentratingthe incident electromagnetic energy into a small physical volume. In the schematic the inci-dent field is traveling in the direction of the arrow and driving the plasmon oscillations in theparticles. The shaded area represents the electron density as the particles are macroscopicallypolarized.

dispersion curve. This is illustrated in Figure 1.8 by process 2 → 1, where the plasmondispersion curve is coupled to the light line by a momentum transfer, kr. In order to directlyexcited plasmons with laser light, the experiments presented in this dissertation are focusedon localized SPRs.

1.3.3 Plasmon lifetimes

One of the main motivating factors for studying surface plasmon resonances with attosecondpulses is the extremely short lifetime of the coherent electron oscillation. Typical plasmondephasing times for gold and silver surfaces and nanoparticles are on the order of a few to tensof femtoseconds [5, 51]. This is the time in which the electrons lose phase coherence but arestill oscillating in an excited state. After the loss of phase coherence it takes approximately1 ps for the plasmon energy to thermalize to the ion lattice [53]. The dephasing timeis an important factor because the large electric field enhancement only occurs while theelectrons are oscillating in-phase with each other. If they have random phases the individualcontributions will cancel out and there will be no macroscopic field effect. This means thatany application that relies on the plasmon field enhancement is limited by the plasmondephasing time. Because of this there is interest in accurately measuring plasmon dephasingtimes so as to understand the factors that go into it and potentially improve the customfabrication of plasmonic systems tailored to specific applications.


Figure 1.11: An example of persistent spectral hole burning from Ref [52]. As the oblate gold-nanoparticles on a sapphire substrate are irradiated, an increasingly large hole forms in themeasured spectrum. From this hole the homogenous plasmon linewidth can be determined.In this case the homogenous linewidth is 94 meV, which corresponds to a dephasing time of14 fs.

A number of experiments have been performed in both the frequency domain [51, 15, 52,14, 56] and the time domain [5, 47] to measure plasmon dephasing times. In the frequencydomain, the plasmon dephasing time is measured by taking the inverse of the linewidthof the plasmon resonance. The commonly used tool to do this is persistent spectral holeburning. In this method a narrow-bandwidth light source is used to excite the plasmonresonance for a subset of an ensemble of particles. Because of inhomogeneous broadening,the bandwidth for the ensemble will be very large but individual particles will have morenarrow bandwidths corresponding to their homogenous linewidths. The narrow-bandwidthlight source is energetic enough to physically change the structure of the excited particles andthus change their plasmon resonance frequency. This leaves a hole in the spectrum when theensemble is measured again. After accounting for power-broadening, the linewidth of thishole can be considered the homogeneous linewidth of the removed particles. An example ofthis method is shown in Figure 1.11. In this experiment, a homogenous linewidth of 94 meV,corresponding to a dephasing time of 14 fs, is determined for oblate gold nanoparticles.

In the time domain studies, the standard measurement of dephasing time is done throughsecond- or third-harmonic generation at a nanostructured surface. By measuring an auto-correlation of the upconverted light from the nanostructures, a trace that is broadened bythe plasmon dephasing time is obtained. By comparing this broadened autocorrelation traceto the non-plasmon-broadened autocorrelation obtained by using a standard frequency con-version crystal (such as a BBO for the case of second-harmonic generation), the plasmon


Figure 1.12: An example of second-harmonic generation from gold nanoparticles in Reference[48]. The bold line is the autocorrelation measured using a standard BBO crystal andrepresenting the laser pulse duration. The thin line is the autocorrelation measured withsecond-harmonic light generated from a gold nanostructured surface. The broadening is fromthe plasmon dephasing lifetime and corresponds to a lifetime of 6± 1 fs.

decay time can be extracted. A measurement made using this method is shown in Figure1.12 [48]. In this experiment a dephasing time of 6± 1 fs is determined for lithographicallyprepared gold nanostructures.

The primary mechanisms through which the plasmon electrons lose phase coherence aresurface scattering, chemical interface damping (in which plasmon electrons become trappedin surface states of adsorbed molecules), electron-electron scattering and inter-band damping[56, 15, 14, 49, 57]. While these measurements have proven extremely useful, direct probingof the surface plasmon field by attosecond light pulses will be able to provide far moredetailed information on plasmon dephasing processes. The primary advantage of this newtechnique is that it is sensitive to the phase and intensity of the plasmon electric fieldon an unprecedented timescale, whereas the previously used techniques have only providedintensity information on a much longer timescale. The addition of phase information and theimproved time resolution will allow for the direct observation of plasmon electron dynamicswith sub-cycle resolution instead of the observation of dynamics that have been averagedover the lifetime of the plasmon resonance.


Figure 1.13: Surface plasmon-based electron acceleration demonstrated by Irvine, et al. [60].In this experiment a 27 fs laser oscillator pulse is used to both excite a SPR in a gold film inthe Kretschmann geometry and ionize photoelectrons into the enhanced field. The electronsare then classically accelerated to high kinetic energies.

1.3.4 Overview of photoelectron spectroscopy from surface plas-mon systems

Recently, plasmon-enhanced photoelectron acceleration has been studied in propagating plas-mons on thin metal films [58, 59, 60, 61, 62, 63, 64, 65] and extremely sharp metal tips [66, 67].In these experiments, an ultrafast laser pulse is used to launch a propagating SPR wave. Inthe same laser pulse, photoelectrons released via multiphoton ionization are precisely spa-tially and temporally injected into the strong plasmon electric field and accelerated awayfrom the surface. Because this type of experiment is very reminiscent of attosecond streak-ing, it has been explored in this dissertation as a pre-cursor to SPR enhanced attosecondstreaking experiments.

Figure 1.13 shows an experimental demonstration of photoelectrons being accelerated tohigh kinetic energies by a plasmon enhanced electric field. In this study 27 fs laser oscil-lator pulses are used to excite a propagating surface plasmon wave using the Kretschmanngeometry. Photoelectrons are ionized via multiphoton ionization and are determined to beponderomotively accelerated to extremely high kinetic energies after ionization. Extensivetheoretical studies have shown this and similar experiments to be a classical accelerationeffect [68, 59].

Variations of the photoelectron spectra as a function of CEP due to electron accelerationhave also been predicted [69]. Because the phase of the enhanced plasmon field is related tothe CEP of the driving laser field, electrons that are accelerated in the enhanced plasmon


field should be sensitive to the CEP for sufficiently short pulse durations. At short pulsedurations, electrons ejected at different half-cycles of the plasmon electric field will expe-rience significantly different enhanced field strengths and thus have different final kineticenergies. When the photoelectron spectrum is integrated over the duration of the drivinglaser pulse, this should result in distinct cutoffs in the photoelectron spectrum correspondingto individual half-cycles of electron acceleration. As the CEP of the driving laser pulse isvaried, the distribution of photoelectron kinetic energies should vary as the distribution ofhalf-cycle intensities changes. However, recent attempts to measure such CEP variation haveso far proven to be unsuccessful [65]. The absence of the expected variation is explained bya small amount of surface roughness on the metal film that causes localized plasmon modesthat oscillate out of phase with each other. Acceleration by the out-of-phase modes servesto wash out any CEP variation expected in the photoelectron spectra. This may have con-sequences for future experiments on SPR enhanced attosecond streaking and considerationof the exact system to study must be made carefully.

1.4 Summary

The ideas presented in this chapter have described the history and theories of both at-tosecond pulse generation and surface plasmon resonances. Chapter 2 provides a detailedoverview of the construction and operation of the apparatus that is used to conduct theexperiments presented in this dissertation. In Chapter 3, I present the results of a studyusing a visible laser to eject electrons by multiphoton processes in the presence of plasmon-enhanced electric fields. Finally, in Chapter 4, I demonstrate attosecond streaking from botha W(110) single crystal surface and an amorphous Cr thin film and discuss the additionalchallenges in performing condensed matter attosecond experiments over more common gasphase experiments.

20

Chapter 2

Experimental Apparatus

2.1 Overview

As described in Chapter 1, the overall theme of this experiment is to laser-excite plasmonicresonances in metallic nanomaterials and to detect the field enhancement at the surface of thenanoparticles by electron ejection. Studies using excitation by both 800 nm infrared-visiblelaser pulses and extreme ultraviolet (XUV) isolated attosecond pulses, or the combination ofthe two, are desired. The first step towards accomplishing these goals was the constructionof an experimental system to produce such light pulses and to detect electron ejection fromsurfaces. The apparatus generally consists of three main parts that will be described below:the generation of few-cycle 800 nm laser pulses, high-harmonic generation for the productionof isolated attosecond pulses near 90 eV photon energy, and a time-of-flight (TOF) electronspectrometer and XUV grating spectrometer for detecting ejected electron kinetic energiesand the HHG spectral distribution, respectively. In order to provide as useful of a guideas possible, the apparatus is described chronologically from the laser to the TOF detectionapparatus. Figure 2.1 shows an overview of the entire system.

2.2 Laser System

2.2.1 Oscillator

The laser oscillator used is a Titanium:Sapphire Femtolasers Femtosource Scientific Prooscillator [71]. The oscillator is pumped by a diode-pumped, frequency doubled, single lon-gitudinal mode yttrium lithium fluoride (YLF) laser (Coherent Verdi V-5) that can provideup to 5 W of CW 532 nm light. The Ti:Sapphire oscillator crystal is typically pumped with4.6 W and can produce 5 nJ, ∼10 fs FWHM Gaussian pulses centered at 800 nm (∼100 nmFWHM bandwidth) at a repetition rate of 78 MHz. Multiple reflections from chirped multi-layer dielectric mirrors (CM in Figure 2.2) are used to correct for dispersion in the oscillator

CHAPTER 2. EXPERIMENTAL APPARATUS 21

Fe

mto

lase

r Co

mp

act P

ro

Oscillator Amplifier

Evo

lutio

n-1

5E

vo

lutio

n 1

5

Ve

rdi V

-5

CCD

f=1m

0.7%

Chirped Mirror Compressor

XUV Spectrometer

f=2m

f=0.5m

High Harmonic Generation

Time-of-flight

spectrometer

Slow loop

Hollow-core fiber

Figure 2.1: An overview of the experimental apparatus.

Stretcher

Oscillator

PL

PD

GB

TOD mirrors

OC

CP

Ti:S

WP

CM

CM

L

RR

PO

BS2

FI

To amplifier

BS1

To f-2f interferometer

Figure 2.2: Schematic of the Femtolasers Femtosource Scientific Pro oscillator. PL - pumplaser, L - lens, Ti:S - titanium sapphire crystal, CM - chirped mirror, WP - wedge pair,OC - output coupler, CP - compensating plate, BS1 - 50:50 beamsplitter, BS2 - 30:70beamsplitter, FI - Faraday isolator, TOD mirrors - chirped mirrors for third order disperson(TOD) compensation, GB - 10cm long SF10 glass block, RR - retro-reflector, PO - pick-offmirror, PD - photodiode. Modified from Ref. [70]


PC

BC

PD

PBS1

PBS2

Ti:S

L1L2L3

L4P2

P1

P3

P4

RR1

RR2

IRFM1IRFM2

PBFMC

VC BW

BW

From stretcher

From pump laser

To compressor

PO2

PO1

Figure 2.3: Schematic of the Femtolasers Femtopower Compact Pro amplifier. L1,2 - tele-scope for pump beam, L3 - lens for focusing of pump beam, PBFM - pump beam focusingmirror, P1-4 - periscopes, IRFM1,2 - infrared focusing mirrors, RR1,2 - retro-reflectors,PBS1,2 - polarizing beam-splitters, PC - Pockel’s cell, BC - Berek polarization compensator,PO1,2 - pick-off mirror, VC - vacuum chamber, BW - Brewster window, Ti:S - titaniumsapphire crystal, C - Peltier cooling, PD - photodiode. Courtesy of Ref. [70]

and to produce as short a pulse duration as possible. Before seeding the pulse to the ampli-fier it is temporally stretched to ∼3 ps to keep the peak power below the damage thresholdof the Ti:Sapphire crystal and other optics in the amplifier. Multiple reflections from a pairof chirped mirrors (TOD Mirrors in Figure 2.2) are used to pre-compensate for third orderdispersion induced by optical components in the amplifier. A schematic of the oscillator andstretcher is shown in Figure 2.2.

2.2.2 Amplifier

The laser amplifier is a Femtolasers Femtopower Compact Pro 9-pass Ti:Sapphire amplifier.The crystal is pumped by a diode-pumped, frequency doubled YLF laser (Coherent Evolution15) that can provide up to 15 W of 532 nm light at 1 kHz repetition rate. The amplifiercrystal is typically pumped in constant current mode at around 20 A while being cooled to∼230 K and housed in vacuum at a pressure of < 50 mbar. The first four passes through theTi:Sapphire crystal amplify all of the pulses in the oscillator pulse train. After the fourthpass a Pockel’s cell is used to select the most energetic pulses from the oscillator pulse trainand reduce the repetition rate to 1 kHz. Five additional passes through the Ti:Sapphirecrystal further amplify the 1 kHz pulse train. By only placing the Pockel’s cell after thefirst four passes, the buildup of amplified spontaneous emission is greatly reduced. Gainnarrowing, which results when the central frequencies of the laser pulse are amplified more


Translation

From amplifier

PO

Output

Figure 2.4: Schematic of the Femtolasers Femtopower Compact Pro prism compressor. Cour-tesy of Ref. [70]

than the spectral wings, decreases the pulse bandwidth to approximately 40-50 nm FWHM,substantially less than the oscillator bandwidth. The amplifier output is then directed intoa prism compressor for temporal re-compression. The amplified pulse bandwidth supports atransform-limited pulse duration of ∼25 fs, but imperfect compensation of fourth and higherorder dispersion results in a final pulse duration of ∼30 fs. Typical amplified laser powermeasured before the compressor is ∼1 W.

2.2.3 Prism Compressor

In order to temporally re-compress the amplified laser pulse, a prism based compressor is usedinstead of a more traditional grating based compressor. Prism compressors can often havehigher throughput efficiency than grating compressors, and although it was initially thoughtthat carrier-envelope phase stabilization would be problematic with grating compressors,this has been shown not to be the case [72]. Typical throughput for this compressor isapproximately 80%, giving a final amplified output of 850 µJ per pulse and 30 fs FWHMGaussian pulses at a 1 kHz repetition rate. A schematic is shown in Figure 2.3.


Time (mins )

Wa

ve

length

(nm

)

10 20 30 40 50 60

475

500

525

550

575

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Figure 2.5: Spectral fringes measured in the slow loop. The first 8 minutes show spectracollected while varying the offset voltage that is sent to the phase locking electrons. Thechanging voltage varies the CEP and thus the positions of the spectral fringes. From 8 min-utes onward, the offset voltage is only controlled by the slow loop error signal and long-termlocking stability of the CEP is demonstrated. Inset: Fast loop beat signal measured on aspectrum analyzer.


PCF

BBO

PD

Glass Wedges

BPF

DC

MO

L

L

L L

P

P

P

Figure 2.6: A schematic of the Mach-Zehnder interferometer used in the CEP fast loop. MO- microscope objective, PCF - photonic crystal fiber, L - lens, DC - dichroic mirror, P -polarizer, BBO - beta barium borate crystal, BPF - band pass filter, PD - photo diode.


2.3 Carrier-Envelope Phase Stabilization

2.3.1 f-2f Mach-Zehnder Interferometer

A special feature of this laser system is the ability to actively lock the carrier-envelope phase(CEP). The CEP results from the carrier-envelope offset (CEO) frequency of the oscillatorfrequency comb and has been defined and discussed in detail in Section 1.2.3. The CEP islocked using a method known as f-2f interferometry. In this technique it is required that thelight spectrum span at least an optical octave, meaning that the highest frequencies containedin the pulse are at least twice the lowest frequencies in the pulse. If this condition is satisfied,the pulse can be split into two copies of itself and then one of the pulses can be sent througha frequency doubling crystal. Because the highest frequencies in the fundamental pulse areat least twice the lowest frequencies, the low frequency side of the doubled laser pulse willbe spectrally overlapped with the high frequency side of the fundamental pulse. By spatiallyand temporally overlapping the pulses in this spectral region, a self-referencing beat signalcan be measured. This beat signal is proportional to the CEO frequency and can be lockedon using an electronic control loop [73].

In this system there are two stages of CEP locking. The first stage, called the fastloop, locks the CEP of selected pulses in the oscillator pulse train. A schematic of the fastloop is shown in Figure 2.6. Because the Femtosource oscillator does not span an opticaloctave on its own, the beam must be focused through a spectrally broadening material inorder to perform f-2f interferometry. In the fast loop this source of white light generationis a photonic crystal fiber (PCF). The PCF is a microstructured fiber designed to have astrong nonlinear response to spectrally broaden low energy laser pulses. The fiber consistsof a solid cladding with a microstructured center. A 50% beamsplitter is placed into theoutput of the oscillator (before the stretcher) to direct enough light to the f-2f for sufficientspectral broadening. Typical power measured after the PCF is 15-30 mW. This light isthen split in a Mach-Zehnder interferometer configuration. A dichroic mirror is used to senddifferent frequency light to the two arms so that pulse energy is not wasted by doubling lightthat won’t be in the spectral overlap region. The light in the green arm (bottom arm asshown in Figure 2.6) is focused into a 3 mm thick Type I beta barium borate (BBO) crystalfor frequency doubling. The two arms are recombined and then passed through a narrowbandpass filter (typically 510 nm with a 10 nm bandwidth) to isolate the region of spectraloverlap. A pair of glass wedges mounted on a translation stage can be used to tune thetemporal overlap between the two arms for maximum beat signal. The signal is measuredon a photodiode and fed into the CEP locking electronics. The inset of Figure 2.5 shows thetypical beat signal measured on a spectrum analyzer. The beat signal is located at ∼30 MHzand a signal-to-noise ratio of 30 dB is typically required for effective CEP locking.

The fast loop is one of the most critical components of the experimental apparatus butalso one of the most common problem areas. The main problem is drifting of the spectraloverlap region between the two arms of the interferometer. Because a narrow bandpass filter


is used before the photodiode, drifting of the overlap region in wavelength on the order ofless than 10 nm will cause the beat signal to drop substantially. Because this problem canhappen on the timescale of experimental operations (several hours), it is not practical tochange the bandpass filter in the middle of operation. Instead, two avenues are currentlybeing approached to improve the system. The first is to try to eliminate spectral driftsby actively stabilizing the pointing of the oscillator beam into the PCF. By stabilizing thepointing of the input, the spectrum of the output should also be stabilized and hopefullyreduce spectral drift. A longer term issue is drift within the oscillator itself, which can alsolead to changes in the PCF output. Ultimately the oscillator will also need to be stabilizedin one or several ways. The second avenue of improvement for the fast loop is to replacethe narrow bandpass filter with a grating and a slit so that the spectral overlap can bevisually determined and selected by an adjustable slit. This could potentially be automatedto correct for spectral drifts in real time.

The second stage of CEP locking, called the slow loop, is measured after the amplifierand compressor and is used to correct slow drifts in the CEP introduced during the ampli-fication process. The f-2f configuration is the same as for the fast loop with one notableexception. Because the amplified pulse is of much higher energy than the oscillator pulse,the octave spanning white light can be generated by focusing into a 2 mm thick sapphireplate. The signal from the photodiode is then fed into a Labview controlled proportional-integral-derivative (PID) control loop that produces an error signal. This error signal is sentinto the CEP locking electronics as an offset voltage to pre-compensate for CEP drifts in theamplifier and compressor by adjusting the fast loop appropriately. Figure 2.5 demonstratesboth control of the CEP (by varying the offset voltage, which results in a change of CEPand thus different positions of the spectral fringes) and long term locking stability of thespectral fringes from the slow loop f-2f. Typical root-mean-squared fluctuations are on theorder of 200 mrad when averaged over 10 ms. When the spectral drift of the fast loop isstable, sustained phase locking can be achieved for greater than two hours.

2.3.2 CEP Locking Electronics

The CEP locking electronics in this apparatus have been homebuilt through a collaborationwith the group of Jun Ye at JILA at the University of Colorado Boulder. The systemconsists of three main parts: a phase detector, a fast PID control loop and a fast summer.The photodiode signal from the fast loop is sent into the phase detector where the beatsignal is selected and compared to the oscillator repetition rate signal in order to determinethe phase difference. This phase is then fed into the PID loop where a phase error signalis generated. In order to keep the phase constant this error signal is used to adjust theoscillator CEO frequency. However, before being sent back to the oscillator, the error signalis combined in a fast summer with the offset correction provided by the PID controller ofthe slow loop. This combined error signal is then sent to an acousto-optic modulator (AOM)that is placed in the pump beam of the oscillator. Small changes to the oscillator pump


power will change the peak power of the oscillator laser pulse and vary the refractive indexof the Ti:Sapphire crystal due to the Kerr effect. This causes a change to the group andphase velocities of the laser pulse and thus changes the CEO frequency. By using the AOMto slightly deflect the pump beam and change the pumping power, the CEO frequency canbe stabilized according to the error signal and provide a locked CEP.

2.4 Spectral Broadening in Hollow-Core Fiber

As described previously, CEP does not typically become an important parameter until thelaser pulse duration is in the few-cycle regime. The amplified laser pulses are already ∼25 fs,but for isolated attosecond pulse production much shorter laser pulses, on the order of 5 fs,are needed. Laser pulse durations are limited by the energy-time uncertainty principle, ac-cording to Heisenberg. According to this principle, the spectral bandwidth and the temporalduration of the laser pulse must satisfy the relationship ∆E∆t ≥ ~

2, where ∆E is the spec-

tral bandwidth, ∆t is the pulse duration and ~ is the reduced Planck’s constant. Becauseof this, the spectral bandwidth of the pulse must be increased to obtain a shorter pulseduration. This is achieved by focusing the laser into a gas-filled 1 m long, hollow-core glassfiber with a 250 µm inner diameter [74]. The beam is focused using a 1 m focal length lensto achieve the optimum 64% ratio between focused beam diameter and the diameter of thecore [75]. The fiber is housed in an outer tube that can be evacuated and backfilled withneon gas, simultaneously filling the hollow core with Ne. Typical gas pressures used rangefrom 1.4 Bar-2.0 Bar. Spectral broadening occurs through the nonlinear process of self-phasemodulation. This process relies on the nonlinear refractive index of the propagation medium,n(I). Because of the intensity dependence, a Gaussian laser pulse will induce a time-varyingrefractive index in the Ne gas that will in turn induce a shift of the instantaneous spectralphase. This phase shift will introduce new spectral components and give the pulse increasedbandwidth, ultimately supporting a shorter temporal duration according to the uncertaintyprinciple. Figure 2.8 shows the spectrum measured at the output of the hollow-core fiberand before temporal recompression.

The output of the hollow-core fiber is very sensitive to the pointing stability of the inputto the fiber. One of the most important components of this apparatus was the installationof an active beam pointing stabilization system between the amplifier and the input of thehollow-core fiber. Before installation of this system, the fiber input often had to be adjustedas often as every 30 min. After installation of the stabilization system, the fiber input almostnever has to be touched, and the beam stabilization system typically only needs to be resetevery few days unless significant alignment to the laser has occurred. The stabilizationis provided by a commercial system from MRC Systems (Germany) and is diagrammed inFigure 2.7. It consists of two actively controlled mirrors mounted with piezoelectric actuatorsand two quadrant photodiodes to detect pointing fluctuations. By using two mirrors andtwo photodiodes, full control of the beam pointing position and angle is available. A fast


PZ1

PZ2

QPD1QPD2

HCF

BS

L

Figure 2.7: A schematic of the beam stabilization system for input into the hollow-core fiber(not drawn to scale). PZ1, PZ2 - Piezo actuated mirrors, L - focusing lens, QPD1, QPD2 -Quadrant photodiodes, BS - Beamsplitter, HCF - Hollow-core fiber.


Figure 2.8: Laser pulse spectrum measured after spectral broadening in a hollow-core fiberfilled with 1.9 Bar of Ne gas but before temporal recompression.

electronic loop provides feedback.

2.5 Chirped Mirror Compressor

While propagation through the gas-filled hollow-core fiber introduces the desired spectralbroadening, it also introduces a significant amount of dispersion resulting in a positivelychirped pulse in the time domain. Positive chirp is a condition that occurs when lowerfrequency spectral components travel through a medium at higher velocities than higherfrequency components. This results in a pulse that has the spectral components spread outalong the time axis. In order to achieve the lowest pulse duration allowed by the uncertaintyprinciple, the temporal phases of the various frequency components must be aligned. Inthis apparatus this is done through the use of a series of negatively chirped multilayermirrors. These chirped mirrors are designed such the penetration depth into the surface ofthe mirror is a function of the wavelength of the light [76]. For a positively chirped laser pulse,negatively chirped mirrors allow the low frequency side of the pulse to penetrate further intothe surface and thus have a longer round-trip reflection time. Higher frequencies penetrateless deep into the surface and have a faster reflection time. By calculating the amountof chirp on the input pulse, a chirped mirror compressor can be designed to balance thefrequency components to a near Fourier transform-limited pulse duration. Figure 2.9 showsa second-order autocorrelation trace of the laser pulse after chirped mirror compression. Asimulated fit to the data gives a FWHM pulse duration of 6.5 fs. The deviation from the


FWHM = 6.5 fs

Measured

Fit

Au

toc

orr

ela

tio

n S

ign

al

Time Delay (fs)

0 2010-20 -10

1.0

0.0

0.2

0.4

0.6

0.8

Figure 2.9: A measured autocorrelation trace of the few-cycle laser beam after spectralbroadening in the Ne filled hollow-core fiber and temporal recompression in the chirped mir-ror compressor. The black circles are experimentally measured points while the red line isthe calculated fit to the measured data. The corresponding FWHM laser pulse duration is6.5 fs. The deviation from the fit in the wings of the pulse results from higher order phaseterms that manifest as pre- or post-pulses. These are not well characterized by autocorrela-tion but could be characterized using a pulse-reconstruction technique such as SPIDER orFROG.

fit in the wings of the pulse results from higher order phase terms that manifest as pre- orpost-pulses. These are not well characterized by autocorrelation but could be characterizedusing a pulse-reconstruction technique such as SPIDER or FROG [77, 78].

2.6 Vacuum System

Many portions of this apparatus are housed inside a high vacuum system with a base pressuremaintained below 1 × 10−6 Torr. A schematic of the vacuum chamber system is shown inFigure 2.10. The chamber consists of four regions: high-harmonic generation chamber, differ-ential pumping and filter chambers, experimental chamber and XUV spectrometer chamber.The high harmonic generation setup will be described in detail in Section 2.7, but briefly itconsists of a nickel tube gas cell with holes drilled in two sides for the laser beam to passthrough. Because of the high pressure required for efficient production of high harmonics,


CCD

XUV Spectrometer

High Harmonic Generation

Time-of-flight

spectrometer

Differential Pumping

Figure 2.10: Schematic of the vacuum chamber

high gas loads result. Therefore the chamber is pumped with a 2000 L/s turbomolecularpump backed by an oil-free scroll pump. A metal baffle with a hole drilled for the laser topass through separates this chamber from the differential pumping stage.

Differential pumping is used to decrease the pressure inside the vacuum chamber fromthe high pressure inside the HHG chamber to the low pressure necessary in the experimentalchamber. By separating the regions of high and low pressure with small apertures and bypumping the differential volume, the experimental chamber can be effectively isolated fromthe high gas load in the HHG chamber. This region is pumped by two 200 L/s turbomolecularpumps backed by oil-free scroll pumps. The differential pumping stage also contains linearvacuum feedthroughs on which a variety of filters for spectral selection and beam separationcan be mounted.

The third region is the experimental chamber. In this chamber the main photoelectronspectroscopy experiments are performed using the IR and XUV laser pulses and the time-of-flight electron spectrometer. The laser pulses are reflected and focused at near-normalincidence using a combination of a gold coated mirror and an XUV multilayer mirror (Section2.9). The mirror is mounted on a multi-axis picomotor stage to allow for precise controlover pointing of the laser beam while remaining under high vacuum conditions. The desiredsample is mounted on a three-axis translation stage with encoded picomotors for reproduciblepositioning of the sample with respect to the focus of the laser beam. The reflected laserlight from the sample is directed through a window to outside of the chamber and used toimage the sample surface to allow for exact positioning of the laser focus with respect to thesample surface. Photoelectrons emitted from the sample are detected normal to the surfaceby a linear time-of-flight electron spectrometer described in detail in Section 2.10. Thischamber is pumped by a 200 L/S turbomolecular pump backed by an oil-free scroll pump.

The fourth region is a homebuilt XUV spectrometer used for measuring the spectrum


Figure 2.11: Typical HHG cutoff spectrum generated by an 800 nm laser pulse in Ne gas.This spectrum was generated by laser pulses with an unlocked carrier-envelope phase.

of light created in the high harmonic generation process. It consists of a grating/slit and aliquid nitrogen cooled backlight CCD camera. It is described in detail in Section 2.11.

2.7 High Harmonic Generation

The primary tool used in generating isolated attosecond XUV pulses is a technique knownas high harmonic generation (HHG). While the mechanisms have been described in detail inSection 1.2.1, the experimental details as implemented in this apparatus will be describedhere. A typical HHG spectrum measured from neon gas is shown in Figure 2.11. A concave,silver coated spherical mirror with a focal length of 0.5 m is used to focus the few-cycle IRlaser beam into a thin gas cell flowing with Ne gas. The focal spot size is ∼50 µm 1/e2 radiusand the peak intensity is up to 1× 1015 W/cm2. The gas cell consists of a nickel tube with3.2 mm outer diameter and an inner diameter of 1.2 mm. Two holes approximately 0.5 mmin diameter have been drilled into opposite sides of the tube to allow the laser beam to passthrough. In order to maintain the desired gas pressure inside of the tube, these holes are


covered with Teflon tape and the laser is allowed to burn through the tape. This provides forthe smallest possible holes for the laser to pass through while still maintaining gas pressureinside of the cell. A typical tape job will last for 1-4 weeks depending on frequency of use.After this time, small changes in the laser pointing will broaden the hole to a larger diameterand decrease HHG efficiency and the cell must be re-taped.

Throughout these experiments, neon gas is used as the HHG generation medium. Thisprovides for a cutoff region around 90-100 eV. The gas cell pressure is a critical parametertowards achieving the optimum phase matching conditions for HHG. The pressure used inthis apparatus varies as the laser-drilled holes broaden, but it is typically around 150 Torrof Ne gas. Another critical parameter for phase matching conditions is the position of thegas cell with respect to the laser focus. In order to preferentially select the short electronrecombination trajectories the entrance to the gas cell is placed approximately 3 mm afterthe laser focus. In optimal generation conditions, an average XUV photon flux of ≈ 1010

photons/second is estimated within the bandwidth of the molybdenum/silicon multilayerXUV mirror (described in Section 2.9). The photon flux estimate is an approximation basedon known detector parameters such as the number of electrons necessary to generate a singlecount, the number of electrons generated per photon and the quantum efficiency of thedetector. The transmission spectrum of the Zr filters (Section ) and the reflectivity of theXUV mirror must also be accounted for.

For the generation of isolated attosecond pulses there is one additional parameter thatmust be considered beyond normal HHG - the carrier-envelope phase. As described inSection 1.2.2, a laser pulse with an unlocked CEP will generate a train of attosecond pulsesthat appear as a spectrum of discrete harmonics in the frequency domain. By locking theCEP and generating harmonics with a cosine pulse, the high energy region of the HHGspectrum can consist of a single isolated attosecond pulse, resulting in a continuum in thefrequency domain. Figure 2.12 shows two HHG spectra generated with CEP locked laserpulses at different relative values of the CEP. Because it is not straightforward to measurethe absolute CEP of the laser pulse, possible production of an isolated attosecond pulse istypically gauged by checking for spectral continuum at the high energy side of the HHGspectrum. The two spectra shown in Figure 2.12 are shifted by π phase relative to eachother. In the spectrum collected at 0 relative CEP (red), discrete harmonics are observed.In the spectrum collected at π relative CEP (black), a continuum is obtained around 95 eV.This continuum can then be spectrally selected to obtain an isolated attosecond pulse.

Experimentally, there are many parameters to be tweaked in order to obtain the optimalconditions for generating an isolated attosecond pulse with HHG. For fine controlling thepulse duration, a pair of glass wedges are inserted on a translation stage before the chirpedmirror compressor. The amount of glass inserted into the beam path is adjusted to achievethe best continuum in the HHG spectrum. In addition to the pulse duration, the gas pressureinside the hollow-core fiber and the gas pressure inside the HHG gas cell are scanned forbest HHG continuum on a routine basis.


0 Relative CEP Relative CEP

Photon Energy (eV)

No

rma

lize

d H

HG

In

ten

sit

y

70

1.0

11010090800.0

0.2

0.4

0.6

0.8

Figure 2.12: Generation of a continuum in the HHG cutoff region by using a CEP locked800 nm laser pulse. The two spectra are taken at zero and π relative CEP. The continuumat π relative phase is indicative of an isolated attosecond pulse in that energy region.


Figure 2.13: Calculated transmission of a 200 nm thick Zr foil filter. For use with the XUVspectrometer, two such filters are used in-line.

2.8 Metal filters

Linear translation feedthroughs in the differential pumping region allow spectral filters tobe placed in the optical path. In this apparatus, two types of filter are commonly used.For use when only the HHG spectrum is desired and no IR light should be present, two200 nm thick Zr filters, 5 mm in diameter, are mounted in-line with each other. Figure2.13 shows the calculated transmission spectrum for one of these filters. The other filterused is a single 200 nm thick Zr filter, 2.2 mm in diameter, mounted on a 1 inch aluminumcoated pellicle, shown in Figure 2.14. The Zr filter has the same transmission as shown inFigure 2.13, transmitting only the XUV light, while the aluminum coated pellicle has partialtransmission of the IR light (ranging from 0.1% to 10% transmission, depending on the filterused and the desired amount of IR light). This filter is used to spatially separate the IRlaser light and the XUV HHG light in order to subsequently introduce a time delay betweenthem and perform a pump-probe experiment.

2.9 XUV Optics

In order to select only the portion of the HHG spectrum that has an isolated attosecondpulse, special optics designed to reflect a narrow spectral range in the XUV must be used.In this case multilayered mirrors optimized for reflection near 93 eV have been designed andprovided by the Center for X-Ray Optics (CXRO) at Lawrence Berkeley National Laboratory.Multilayer XUV mirrors work on the principle of interference [79]. The material type, numberand spacing of layers are optimized such that each interface backscatters some light and only


15 mm

Figure 2.14: 200 nm thick Zr filter mounted on an aluminum coated pellicle. This filterserves to separate the copropagating XUV and IR light into an inner beam and an outerbeam so that a time delay can be introduced between the two.

Figure 2.15: Multilayer Mo/Si XUV mirror reflectivity shown compared to the HHG con-tinuum. The mirror reflectivity is designed to spectrally select only the continuum region ofHHG, leaving an isolated attosecond pulse after reflection.


5-Axis stage

Fiber alignerOuter mirror mount

Micrometer/

translation stage

Piezo

Inner Mirror

Figure 2.16: Schematic of the cored-mirror used in this apparatus. The central portion ofthe mirror has a multilayer XUV coating and can be moved in the beam axis independentlyfrom the outer mirror by a piezo-translation stage. The outer mirror (not shown here forclarity) is gold coated and can be moved in the x- and y-directions by picomotors to allowfor precise spatial overlap between the inner and outer beams.

the desired wavelengths will constructively interfere. In the case of HHG with neon gas,molybdenum/silicon multilayer mirrors are made to reflect around a central energy of 93 eVwith a 4 eV FWHM bandwidth. Figure 2.15 shows the spectral reflectivity of the multilayermirror used in this apparatus compared to the CEP-locked HHG spectrum. The maximummeasured reflectivity is 70%. The reflectivity is clearly centered in the continuum region ofthe HHG spectrum. This allows for only the energy region with an isolated attosecond pulseto be reflected.

In this apparatus the multilayer mirror serves not only as a spectral filter, but also tofocus the XUV and IR light onto the sample and to introduce the pump-probe time delaybetween the two pulses. For this purpose a special cored-mirror geometry is used. A concavespherical mirror substrate with a 10 cm focal length and a surface figure of λ/10 (measuredat 633 nm) or better has a 3 mm diameter core cut out of the center. The core is retained


to give two separate mirror substrates, the 3 mm diameter center piece and the outer mirrorwith the ∼3.5 mm hole in the center. The center portion is then coated with the multilayerMo/Si XUV coating and the outer portion is given a high reflectivity gold coating. Byattaching the center mirror to a piezeoelectric controlled translation stage and reinserting itinto the outer mirror, the mirror will separately reflect the colinear XUV attosecond pulsefrom the center portion and the IR pulse from the outer portion. It also introduces a timedelay between the two pulses by moving the inner mirror along the beam axis (z-direction)with respect to the outer. A schematic of the mirror design is shown in Figure 2.16. Thepiezo stage in this apparatus has a minimum step size of 15 nm, corresponding to a minimumtime step of 5.2 fs (twice the minimum step size because reflected light covers the distancetwice). The outer mirror can be moved in the x- and y-directions by picomotors to allow forprecise spatial overlap between the inner and outer beams.

One major cause of experimental challenge regarding the cored mirror is achieving in-terferometric stability between the inner and outer mirror on the order of 15 nm. Evensmall vibrations in the apparatus, introduced by the turbomolecular pumps for example,can lead to a complete loss of timing information at the necessary temporal resolution. Thisstability can be checked by illuminating the inner and outer mirrors simultaneously with aCW red He/Ne laser and observing the interference pattern created. In order for attosecondexperiments to be possible there must be stable fringe contrast. If the fringes are visiblyshifting large amounts or blurring out entirely, steps must be take to reduce vibrations inthe cored-mirror design. It is important to use sturdy mirror mounts and translation stages,but also to implement measures such as placing the chamber on vibration damping rubberfoam or using vibration isolating bellows between the turbomoleuclar pumps and the vacuumchamber.

2.10 Time-of-Flight Electron Spectrometry

2.10.1 TOF Spectrometer

The primary method of experimental detection used throughout this dissertation is time-of-flight (TOF) electron spectroscopy. For this purpose a home built TOF electron spectrometerwas constructed. The principle of TOF spectroscopy is rather simple and relies only on basicclassical physics. By using a defined source of particle emission, in this case photoelectronsionized by the laser pulse, one can measure the length of time taken to travel to a detectora known distance from the source. By knowing the particle charge, mass and the distanceto the detector, the kinetic energy of the particle can easily be determined.

In this apparatus one of the most simple versions of a TOF electron spectrometer isused. A 0.59 m long µ-metal tube with an inner diameter of 45 cm is placed as close to thelaser-surface interaction region as possible. µ-metal is a material with a very high magneticpermeability that allows it to effectively shield an enclosed region from the Earth’s magnetic


field. It is important to use such a material for photoelectron spectroscopy because verylight, charged particles such as electrons will have their trajectories significantly deflected bythe Earth’s magnetic field over the length of the TOF spectrometer. Such a deflection willlengthen the time-of-flight and cause a falsely low kinetic energy measurement. The tube isgrounded to ensure field-free electron flight. At the end of the tube a micro-channel plate(MCP) detector is mounted. MCPs are glass plates with a large array of small channelspermeating them. The interior of the channels are coated with a low work function materialso that the impact of a particle will release a cascade of electrons. These electrons impingeon an anode at the rear of the microchannel plate and create a measurable electric current.In order to obtain easily measurable signal levels, multiple microchannel plates and electronaccelerating voltages are typically used. In this apparatus a Chevron stack configuration isused. In this configuration two microchannel plates are mounted on top of each other withtheir channels angled such that the cascade of electrons from the first plate easily flows intothe channels of the second plate. A grounded grid is placed in front of the first plate inorder to provide a flat field while still allowing the electrons to pass through. The plates arethen held at varying positive voltages to drive the electrons through to the anode. In thisapparatus the front of the first plate is held at 160 V, the back of the first plate and thefront of the second plate at 960 V, the back of the second plate at 1760 V, and the anode at1920 V. A voltage dividing box is used to easily provide these potentials using only a single4000 V input. A capacitive coupling box is used to obtain a voltage signal from the anodethrough a standard 50 Ω BNC connector without risking exposure to the high voltage beingapplied to the anode. A circuit diagram of the entire MCP apparatus shown in Figure 2.17.

2.10.2 Signal analysis

The signal from the MCP is routed through an analog constant fraction discriminator tocorrect for timing errors introduced by the MCP pulse height distribution and then countedby a digital multi-channel scaler installed in a PC. The Fast Comtec multi-channel scalerhas a bin size of 500 ps. The spectrometer resolution is determined by the length of theflight tube, the electron kinetic energy and the bin size of the multi-channel scaler. Figure2.18 shows a plot of the TOF kinetic energy resolution versus electron kinetic energy overthe range of electrons typically detected in this apparatus. Data is collected via a varietyof commercial programs and self-made Labview routines and data analysis is typically doneusing self-made Matlab routines. Data collection times vary depending on the particularexperiment, but for most samples each step is integrated for 60000 laser pulses (≈ 1 s) orless.


Figure 2.17: Circuit diagram of the MCP apparatus used in TOF detection. Providedcourtesy of Jordan TOF Products, Inc.


Figure 2.18: Kinetic energy resolution of the TOF electron spectrometer as a function ofelectron kinetic energy. For valence electrons emitted directly by the HHG produced XUVpulse, around 90 eV, the energy resolution is 0.9 eV. The plot only accounts for the instrumentresolution and does not include the bandwidth of the XUV pulse or any other contributionsto the final experimental resolution.

0 500 1000 1500 2000 25000

100

200

300

400

Time of Flight (ns)

Ele

ctr

on

co

un

ts

Figure 2.19: A sample TOF photoelectron spectrum collected from ionization of a goldnanopillar sample by the few-cycle IR laser and integrated over 60000 laser pulses.


0 500 1000 1500 2000 25000

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Time of Flight (ns)

Ele

ctr

on

Co

un

ts

Figure 2.20: Raw data for a TOF photoelectron spectrum collected from ionization of aW(110) crystal by HHG produced ≈93 eV photons and integrated over 500000 laser pulses.The peak at 104 ns is caused by scattered photons and can be used for calibrating thespectrometer time zero. To the right of the photon peak a sharp peak from XUV emittedphotoelectrons can be seen, followed by a large low energy electron background.

2.10.3 Sample TOF data

Figure 2.19 shows a typical TOF electron spectrum collected after irradiation of a goldnanopillar sample with the few-cycle IR laser pulse. The spectrum is shown as raw time-of-flight data, integrated over 60000 laser pulses. The total number of counts is 313005,giving an average of 5217 electrons detected per second. It is important to subtract off thetime-zero before converting the time-of-flight to a kinetic energy scale. The time zero resultsfrom slight timing discrepancies between the 1 kHz trigger signal from the laser and the timeat which the pulse arrives in the interaction region and releases photoelectrons. This timeis determined by both the optical path length from the laser to the interaction region andthe delay of electrical signals in the various connections. It is most easily determined byobserving the photon peak, which results from photons scattering off of the target surfaceand arriving at the detector near-instantaneously. It can be difficult to observe the photonpeak with ionization by the few-cycle laser pulse alone, but typically it is very easy to observewhen irradiating the surface with HHG radiation.

Figure 2.20 shows a typical TOF electron spectrum collected by ionization of a W(110)


crystal using HHG generated XUV pulses (not an isolated attosecond pulse) centered at93 eV. The spectrum is integrated over 500000 laser pulses with an average of 3311 electronsdetected per second. The large low energy (long flight time) background is the result ofelectron kinetic energy loss via inelastic electron scattering inside the metal surface (thisresults from the fact that the XUV light will penetrate ≈3 nm into the surface [80] whilethe electron mean free path is only 0.4 nm at 90 eV kinetic energy [81]). This issue will bediscussed further in Chapter 4. The small photon peak can be seen at 104 ns and is usedfor calibration of time zero. In order to convert the time-of-flight axis to a kinetic energyaxis a transformation has to be applied to not only the independent axis but also to thedependent axis. This correction to the dependent axis is called the Jacobian and resultsfrom the non-uniform bin width of the scaler card in the kinetic energy domain.

The procedure for converting the independent axis is straightforward:

Eke =1

2mv2 =

1

2m

(d

t

)2

(2.1)

where d is the distance from the interaction region to the MCP detector and t is the electronflight time minus time zero. For the Jacobian correction to the dependent axis the followingcondition must be fulfilled:

D(t) dt = D(Eke) dEke

⇒ D(Eke) = D(t)dt

dEke(2.2)

where D(t) is the experimentally collected photoelectron counts as a function of time-of-flight and D(Eke) is the Jacobian corrected intensity corresponding to the kinetic energybins, therefore:

t =d√m√2E− 1

2ke

dt

dEke= −d

√m

2√

2E− 3

2ke

⇒ D(Eke) = −d√m

2√

2E− 3

2ke D(t) (2.3)

The intensity scale for the corrected kinetic energy spectrum can be brought back to physicalunits by multiplying the dependent axis by the correct ratio to make the integrated areaunder the spectrum equal to the integrated area under the time-of-flight spectrum. Figure2.21 shows the data from Figure 2.20 converted to kinetic energy. The peak centered atapproximately 86 eV results from valence band photoemission by the XUV pulse whilethe large low kinetic energy signal is the result of electrons scattered within the metal, asmentioned above. The peak is very broad due to both the broad bandwidth of the XUVmirror (4 eV FWHM) and the bandwidth of the tungsten conduction band, which spansalmost 10 eV [82].


0 20 40 60 80 1000

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Kinetic Energy (eV)

Ele

ctr

on

Co

un

ts/S

ec

on

d

Figure 2.21: The TOF data from Figure 2.20 after conversion to a kinetic energy scale andcorrection for the Jacobian. The peak centered at ≈86 eV is from XUV-induced valenceband photoemission while the large low energy signal is the result of inelastically scatteredelectrons within the metal.


CCD

Figure 2.22: Schematic of the XUV spectrometer.

2.11 XUV Spectrometer

The final component of the experimental apparatus is a homebuilt XUV spectrometer. Aschematic is shown in Figure 2.22. The spectrometer consists of a slit and transmissiongrating that are used to disperse the light onto a backlit, liquid nitrogen cooled CCD camera(Princeton Instruments XO Pixis 400B). The slit width is 500 µm and the transmission grat-ing consists of free standing 100 nm thick Si3N4 bars with a line density of 10000 lines/mm.Direct transmission through the grating (zero order) can be used to measure the spatialprofile of the HHG beam. Figure 2.23 shows the measured HHG beam profile generatedwith Ne gas. The 1/e2 diameter is shown for x- and y-cross sections measured at the centerof the beam. The beam is slightly asymmetric and slightly non-Gaussian.

The CCD chip is not wide enough to capture the fully dispersed spectrum if placeddirectly in line with the grating, so the camera is mounted on a flexible bellows that allowsfor the camera to be moved laterally to the position of the first order dispersion. This positioncan be easily calculated using the diffraction equation, λ/d = sin(θ), where λ is the XUVwavelength and d is the grating spacing. For 93 eV light the angle is 7.6. The distancebetween the grating and the CCD camera is 35 cm, resulting in a lateral CCD displacementof 4.7 cm. The spectrometer resolution is not usually considered for this apparatus since it isonly used as a diagnostic, however resolution is better than necessary to observe individualharmonic orders spaced 3 eV apart at around 90 eV. The CCD camera reads out in twodimensions, but it is often convenient to look at the spectrum integrated along the vertical(non-diffracted) axis in order to see total photon flux as a function of photon energy. Figure2.24 shows the raw two dimensional HHG spectrum measured at first order. The absorptionfrom the Si L-edge in the grating can be seen as a rapid drop in transmission at 100 eV.This edge can be used along with the Al L-edge around 73 eV to calibrate the photon energy


−5 −4 −3 −2 −1 0 1 2 3 4 5

−3

−2

−1

0

1

2

3

mm

mm

6.86 mm

5.4

0 m

m5

.40

mm

Figure 2.23: Measured zero order transmission of the HHG radiation generated in Ne gas,averaged over 10 scans with 1 s integration each scan. 1/e2 diameters of the x- and y-crosssections are measured as 6.86 mm and 5.4 mm, respectively. The white dashed lines showthe positions at which the line-outs were measured.


Photon Energy (eV)

mm

70 75 80 85 90 95 100

−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.5

Figure 2.24: First order dispersion spectrum of HHG XUV radiation generated in Ne gas,averaged over 10 scans with 1 s integration each scan.

scale of the spectrometer.

49

Chapter 3

Surface plasmon assisted electronacceleration in photoemission fromgold nanopillars

3.1 Introduction

Among the most intriguing phenomena in nanoscale materials today is the coherent elec-tronic excitation in metals known as the surface plasmon resonance (SPR). The SPR is acollective oscillation of conduction band electrons that typically occurs at optical frequenciesin noble metals [46]. For a short amount of time, these electrons oscillate in phase and cre-ate a strongly enhanced electric field at the surface of the metal/vacuum or metal/dielectricinterface [2, 1]. SPRs have enormous potential for applications in medicine, communica-tions, and electronics [6, 7], most of which take advantage of the strongly enhanced electricfield created by the plasmon at the metal surface. Techniques such as surface-enhancedRaman spectroscopy exploit this near field enhancement to allow the sensitive spectroscopicdetection of single molecules [8].

One area of significant interest is the plasmon response to excitation by high intensity,ultrafast laser pulses. Lasers that generate such pulses are becoming increasingly commonand have opened the door to studying new regimes of light/matter interactions. The goalof the present work is to investigate the interactions of laser-ionized photoelectrons withlocalized surface plasmon electric fields excited in a lithographically prepared nanostructuredarray. The use of a nanostructured surface is advantageous because the SPRs are exciteddirectly by ultrafast laser pulses without requiring special excitation geometries often used instudies of plasmon enhanced photoemission from flat gold surfaces [58, 59, 60, 61, 62, 63, 64]or extremely sharp metal tips [66, 67]. By measuring photoelectron kinetic energy spectraand electron yields as a function of laser excitation intensity, we observe photoelectron kineticenergies tens of eV higher than expected based on the laser excitation intensity. A classical

CHAPTER 3. SURFACE PLASMON ELECTRON ACCELERATION 50

CM

TOF

HCF

~30 fs

~7 fs

Ti:Sapphire, 1 kHz,

800 μJ, 30 fs

Figure 3.1: Schematic of the experimental apparatus. 30 fs FWHM, 800 µJ laser pulses arespectrally broadened in a gas-filled hollow-core fiber (HCF) and temporally compressed to≈7 fs FWHM with a series of multilayer chirped mirrors (CM). The laser is focused ontothe sample surface and photoelectrons are detected using a linear time-of-flight spectrometer(TOF).

electron acceleration calculation is used to model the data and to determine the average fieldenhancement from the nanostructures. Implications for possible studies of plasmon-enhancedattosecond photoelectron streaking are also briefly discussed.

3.2 Experimental

3.2.1 Apparatus

The measurements use a few-cycle femtosecond, visible-infrared laser pulse to excite a SPRin a lithographically prepared gold nanopillar sample and to simultaneously ionize photoelec-trons from the sample surface. Photoelectron kinetic energies are measured as a function ofexcitation intensity using a linear time-of-flight (TOF) electron spectrometer. A schematicof the experimental apparatus is shown in Figure 3.1.

The apparatus consists of a Femtolasers Femtopower Compact Pro multi-pass amplifiedTi:Sapphire laser system that produces ≈30 fs full-width at half-maximum (FWHM), 800 µJlaser pulses at a repetition rate of 1 kHz. A 1 m long, 250 µm inner diameter hollow coreglass fiber filled with 1.9 Bar of Ne gas is used for spectral broadening through self-phasemodulation followed by temporal compression with a series of negatively chirped mirrorsto a pulse duration of ≈7 fs FWHM. The laser spectrum extends from 540 nm to 930 nm


a) b)250 nm

Inte

ns

ity

Wavelength (nm)

Figure 3.2: (a) Scanning electron microscope (SEM) image of the gold nanopillar array. (b)Dark-field scattering measurement of a single nanopillar from an identically prepared samplewith a larger pitch to allow for measurement of a single particle.

(1% level of intensity). The laser pulse is focused at grazing incidence, 75 to the samplesurface normal, using a near-normal incidence spherical mirror with a high reflectivity goldcoating and a 10 cm focal length to a spot size of approximately 60 µm (1/e2 diameter). Thegrazing angle stretches the spot size along the direction of propagation to ≈300 µm. Thesample is housed in vacuum at a pressure of < 5× 10−7 Torr. No steps were taken to cleanthe sample surface. Emitted photoelectrons are detected normal to the sample surface bythe field-free TOF electron spectrometer using a micro-channel plate (MCP) detector. Theacceptance angle of the spectrometer is 2 and the total flight length is 0.59 m. Signal pulsesfrom the MCP are processed by an analog constant-fraction discriminator to correct for theMCP pulse height distribution and then counted with a digital multi-channel scaler witha 500 ps resolution. The energy resolution of the TOF spectrometer varies with electronkinetic energy, ranging from ≈11 meV for 5 eV electrons to ≈1 eV for 100 eV electrons.At the count rates present in this experiment, the probability of missing an electron countduring the detection electronics pulse-pair resolution dead-time ranges from 0.2% at thelowest count rates to 10% at the highest count rates. p-Polarized light is used throughoutthis experiment to excite SPRs normal to the sample surface and parallel to the TOF axis.

3.2.2 Nanopillar Sample

The sample investigated consists of free-standing gold nanopillars attached to a 10 nm thickbinding layer of chromium. A surface consisting of 12 nm of gold on top of 10 nm of chromiumwas coated with 300 nm of photoresist and then patterned using electron-beam lithography.


The exposed photoresist was chemically removed and gold was then electroplated onto thesurface. Finally, the unexposed photoresist was chemically removed and ion sputtering wasused to remove the tops of the pillars and the Au plating base layer, leaving free-standing goldpillars on top of a conductive thin film of chromium. Individual nanopillars are cylindricalwith a diameter of 100 nm and a height of 285 nm and are arranged in a cubic lattice with250 nm pitch. The tops of the pillars become partially rounded during the plasma etchingstep. Figure 3.2a is a scanning electron microscope (SEM) image taken at 29.0 from thesurface normal. The nanopillar shape and aspect ratio were chosen such that the SPRalong the long axis of the nanopillar (parallel to the TOF axis) is resonant within the laserbandwidth. Dark-field scattering measurements of individual nanopillars from a sample withlarger pitch (4 µm) but otherwise prepared identically show a broadband plasmon resonance(Figure 3.2b), centered near 700 nm, that is well overlapped with the laser bandwidth.

Control experiments were performed on a commercially available 50 nm thick gold filmcoated onto a Si 〈111〉 wafer (Ted Pella #16012-G). The flatness of the Si wafer allows for agold surface with only ≈2.5 nm root-mean-squared surface roughness, which is measured byatomic force microscopy. Because of a mismatch in momentum, laser photons cannot coupleto a surface plasmon wave in a flat gold surface unless special excitation geometries such asthe Kretschmann configuration are used[1, 55]. Under the experimental geometry presentedhere, no coupling should occur and plasmon enhanced effects should not be observed fromthis sample. Independent measurements of the damage threshold of the gold surface weremade by raising the laser intensity to the point where photoelectron spectra collected atlower intensities were no longer reproducible. Measurements presented here are collectedbelow the damage threshold.

3.3 Results and Discussion

3.3.1 Photoelectron Spectra

In order to determine the interaction of ionized photoelectrons with the surface plasmonfield, photoelectron kinetic energy spectra are recorded as a function of the excitation laserintensity. The few-cycle laser pulse is used to excite the plasmon resonance and simul-taneously inject photoelectrons by multiphoton photoemission into the enhanced plasmonelectric field. The work function of polycrystalline gold ranges from 4.7 eV to 5.2 eV [83].The broadband laser pulse has < 7×10−3 intensity in the spectral range below 527 nm (halfof 4.7 eV), therefore photoemission should require at least three laser photons to eject anelectron into the continuum, even at the high energy side of the laser bandwidth. The exci-tation intensity is varied using a variable neutral density (ND) filter, the dispersion of whichis pre-compensated by chirped mirrors. The laser pulse energy is measured at each intensitystep and is used along with the pulse duration and the measured focal spot size to determinethe intensity. Scanning the variable ND filter does not produce a detectable change in pulse


a) b)

Figure 3.3: (a) Photoelectron kinetic energy spectra taken from a flat gold surface as afunction of excitation intensity. (b) Photoelectron spectra taken from the gold nanopillarsat the same intensities as (a). Strong acceleration of photoelectrons to high kinetic energiesis indicative of photoelectron emission in the presence of plasmon-enhanced electric fields.Because of the inability of photons to directly excite a SPR in flat gold, a minimal increasein kinetic energy is present in (a).

duration, which is monitored by second-order interferometric autocorrelation. By decreasingor increasing the excitation intensity, the plasmon electron oscillation in the nanopillars canbe driven more weakly or more strongly, respectively. We expect this change in field strengthto result in varying degrees of acceleration experienced by photoelectrons injected into theplasmon field.

Figure 3.3a shows a series of photoelectron spectra taken from the reference flat gold sam-ple as a function of excitation intensity. Each spectrum is integrated over 60000 laser pulses.As previously noted, a plasmon oscillation cannot be directly excited on the flat gold surfaceby the laser because of the momentum mismatch between the laser photons and the surfaceplasmon resonance, therefore there should be no field enhancement. Figure 3.3b shows asimilar series of photoelectron spectra taken from the gold nanopillar sample. As the laserintensity is increased, the maximum kinetic energy measured increases substantially and anincreasingly strong secondary peak is formed between 10 eV and 40 eV. The missed electroncounts due to the detector pulse-pair resolution do not significantly alter the shape of thespectral distribution when corrected for. The dramatically increased electron kinetic energywith increasing excitation intensity in the nanopillars compared to the minimal increase atthe same intensities in the flat gold spectra strongly suggests an enhanced plasmon-field-based acceleration mechanism. To further investigate the details of this mechanism weconsider both the ionization and the acceleration processes in the following sections.


a) b)

3α I α I

2

Figure 3.4: Log-log plot showing the total number of detected photoelectrons as a functionof excitation intensity, I, for (a) the flat gold surface and (b) the gold nanopillar sample.While the flat gold surface demonstrates the expected third order multiphoton dependence,only a second-order dependence is observed in emission from the nanopillars.

3.3.2 Total Electron Emission Scaling with Laser Intensity

To determine what effect the nanopillar SPR has on the photoelectron ionization process, ameasurement is made of the total number of electrons detected as a function of the excitationintensity. Figure 3.4 shows the integrated photoelectron yield versus laser excitation inten-sity, I, for both samples on a log-log scale. The error bars are determined by the probabilityof missing electron counts during the detection electronics pulse-pair resolution dead-time.The data are the same as is shown in Figure 3.3 but with additional data points that arenot displayed in Figure 3.3 for clarity. In both the nanopillar case and the flat gold case,a linear slope fits the observed trend, suggesting multiphoton ionization where the slope ofsuch a fit results in a nth order intensity dependence, where n is the number of photonsrequired to exceed the work function of the metal [62]. Figure 3.4a shows the measured pho-toionization intensity dependence for the flat gold surface. In the flat gold case the expectedI3 dependence is observed, indicating a three photon multiphoton ionization process andno plasmon enhancement. For the gold nanopillar sample, Figure 3.4b, an I2 dependenceis observed, corresponding to one fewer photon needed to exceed the work function thanexpected for multiphoton ionization with the laser pulse. Although the very weak intensityof the laser pulse below 527 nm could play some role, this is unlikely. An In−1 dependencehas been previously observed in multiphoton ionization from a gold surface in the presenceof coupled surface and interface plasmon waves [62] and in localized plasmon hot spots ona Cu surface [84]. The effect is attributed to the promotion of valence band electrons to anelectronically excited state via the SPR, giving the electrons an amount of energy, ~ωplasmon,


a) b)

Figure 3.5: (a) Spectra modeled from classical electron trajectory calculations (black lines)compared to the experimental data (symbols). Each trace is offset by one order of magnitudefrom the previous trace for clarity. In the model, multiphoton emission is followed by classicalacceleration in an enhanced field. An average field enhancement of 32 brings the model inclose agreement with the experimental data. The intensities shown in the legend are theenhanced intensity values, (I ∝ E2), used for the calculation. The experimental data is thesame as shown in Figure 3.3a. (b) The experimental data (symbols) compared to a range ofmodeled spectra calculated for average field enhancement factors from 25-39 (shaded areas).Each trace and shaded area is offset by two orders of magnitude from the previous trace forclarity.

above the Fermi level and permitting one fewer photon for multiphoton ionization. An over-all two-photon power dependence can occur, depending on the relative magnitudes of thecross sections for promotion to the excited state and ionization. When compared to the I3

dependence observed for the flat gold surface, this corroborates the concept that a plasmonenhancement occurs in the nanopillars and is responsible for the observed accelerations inFigure 3.3a.

3.3.3 Classical acceleration model

The acceleration of the electrons in the plasmon field is modeled using a one dimensionalclassical electron trajectory calculation. In this calculation 1000 electrons are released intothe enhanced electric field of an assumed 7 fs FWHM Gaussian laser pulse at every timestep, which are spaced by 4.8 as, resulting in a total of 7.5 million electrons. The frequencyof the enhanced field is assumed to be the same as the frequency of the laser pulse. A fullcalculation of multiphoton ionization from a metal surface is beyond the scope of this work;instead, initial electron kinetic energies are chosen randomly from a log-normal fit to the


photoelectron spectra from the flat gold surface shown in Figure 3.3b. The amplitude ofthe log-normal fit is the only parameter varied for initial spectra at the different intensityvalues. After the initial release of electrons into the enhanced electric field, the position andvelocity of each electron is calculated at each time step by integrating the classical equationsof motion for a charged particle in an electric field. The final electron velocity as a functionof ionization time, vf (ti), is described by:

vf (ti) = v(ti) +

∫ ∞ti

qE(t)

me

dt (3.1)

where v(ti) is the initial electron velocity, q is the elementary charge, me is the electron mass,and

E(t) =E0

σ√

2πe−(t−a)2/(2σ2)e−i(ωt+φ) (3.2)

where E0 is the peak value of the enhanced electric field, σ = FWHM/(2√

2 ln 2) with theFWHM laser pulse duration, a is the center of the Gaussian pulse, ω is the angular frequencyof the electric field and φ is the carrier-envelope-phase (CEP) of the exciting laser pulse.The final electron velocity distribution as a function of electron ejection time is weightedby a temporal emission probability ∝ I(t)2, corresponding to a second-order multiphotonionization process. Electrons with negative final velocity are excluded from the resultingspectrum because they would not reach the detector.

Because of the Gaussian spatial mode of the laser focus, not all ejected electrons willexperience the peak intensity value. To account for this, the spectra presented here areconstructed by integrating over individual spectra calculated at a range of intensity valuesover the spatial extent of the laser focus. The contributions from each spectrum are weightedaccording to the area illuminated by that intensity. In addition, these spectra are averaged for5 values of the CEP over a range of 2π. The model is constructed as if the emission were froma flat surface (without nanostructures) with a uniform enhancement over the spatial profileof the laser pulse. In reality, the nanostructured surface will have an inhomogeneous fieldenhancement and ejected electrons will experience different fields. Additionally, the degreeof coupling between localized plasmon modes is unknown at this time. The enhancementfactor considered below represents an average of all of these possible inhomogeneities. Thesingle free parameter in the calculation is therefore the average field enhancement factor dueto the nanopillars, which is adjusted to give agreement between the observed and modeledphotoelectron spectra. The same enhancement factor is applied to all of the spectra. Thelifetime of the plasmon oscillation, phase-lag between the plasmon field and the exciting laserfield, and surface recollision effects are not included in the calculation.

In order to directly compare the modeled spectra to all of the experimental data, theabsolute electron yields of all the modeled spectra were scaled by a single value. This value isthe ratio between the integrated photoelectron yield of the spectrum measured at the highestexcitation intensity and the integrated photoelectron yield of the spectrum calculated at the


highest enhanced intensity. This scaling places the modeled traces on the same absolute scaleas the measured data while preserving the relative scaling of the modeled spectra producedby the calculation.

Figure 3.5a shows the experimental nanopillar data (symbols) compared to the modeledspectra (black lines), where a field enhancement of 32 times the experimentally used fieldstrength is chosen. The uncertainty in the enhancement factor is estimated to be ±7 and isdetermined by qualitatively comparing spectra calculated at various enhancement values tothe experimental data. Figure 3.5b shows the experimental data (symbols) compared to arange of spectra modeled with enhancement factors ranging from 25 to 39 (shaded areas).

The calculated spectra qualitatively reproduce the main features of the experimentaldata. Post-ionization acceleration of photoelectrons in the enhanced electric field results inthe shifting of electrons from the initial kinetic energy distribution to higher kinetic ener-gies and the formation of a secondary maximum between 10 eV and 40 eV. The fact thatthe secondary maximum is stronger in the modeled spectra and offset by several eV fromthe experimental data may result from non-uniform acceleration of photoelectrons due tothe inhomogeneity of the enhanced field across the nanostructured surface. In additionto the electron kinetic energies, the relative electron yields of the spectra modeled usingsecond-order multiphoton emission at the enhanced excitation intensities match well to theexperimentally observed yields. When combined with the evidence for plasmon-assisted mul-tiphoton ionization described previously, the results of this model support a two step processof plasmon-enhanced multiphoton ionization followed by classical electron acceleration in aplasmon-enhanced field. In addition, surface rescattering effects are expected to contributehigher energy electrons to the spectra [85, 86], and their exclusion from this calculation mayresult in the failure of the model at the highest observed kinetic energies. Moreover, plasmondephasing rates, which may be dependent on the amplitude of the launching field, have notbeen accounted for and could account for deviations of the data from the model as a functionof intensity at higher electron kinetic energies. Since only an average enhancement factoris used, and because the highest kinetic energies derive from the highest field regions of thenanopillars, it is also not surprising that deviations at high kinetic energies are observed.Previous calculations of thin-film propagating plasmon-enhanced electron acceleration thatinclude a more detailed description of the surface plasmon field produce similar results asthis simple model [59, 68].

3.4 Conclusions

In conclusion, we observe photoelectron kinetic energies in photoemission from lithograph-ically prepared gold nanopillars that are consistent with electron acceleration in electricfields with average strengths between 25 and 39 times higher than the experimentally usedlaser field strengths. Reference measurements from a flat gold surface do not produce suchhigh electron kinetic energies at the same excitation intensities. The presence of a plasmon-


induced field enhancement is further supported by analysis of the excitation intensity de-pendence of the total electron emission yield. Multiphoton emission is observed for boththe flat gold surface and the nanopillar sample, with the expected three-photon ionizationprocess for the flat gold, yet a two-photon ionization process from the nanopillars indica-tive of plasmon-enhanced multiphoton ionization. Classical electron trajectory calculationssupport the concept that the electrons are first ionized via multiphoton ionization and thensubsequently accelerated in the enhanced electric field of the nanopillars to high kineticenergies.

These results provide the basis for the possibility of SPR-enhanced attosecond streakingfrom localized plasmon resonances in nanostructured surfaces. Such a concept has beenexplored theoretically for both nanostructures [87] and roughened metal surfaces [88]. Insuch an experiment, the attosecond streak camera scheme [41] would be modified to utilizethe plasmon-enhanced electric field as a probe instead of the intense few-cycle laser pulse.Electrons are emitted by an isolated attosecond pulse in the presence of a plasmon field thathas been excited by the femtosecond pump pulse. As the electrons are emitted they willthen be streaked by the sum of the plasmon field and the laser field. By taking advantageof the SPR field enhancement, it may be possible to reduce the contribution of the laserfield itself sufficiently that information on the SPR lifetime and dynamics of the oscillatingSPR electrons can be obtained directly from the streak trace. Given the observation of asignificant field enhancement and multi-eV streaking of photoelectrons presented here, weexpect that attosecond streaking studies of plasmon dynamics in metal nanostructures ispossible.

59

Chapter 4

Application of attosecond streaking tocondensed matter targets

4.1 Overview

The extension of attosecond streaking from the gas phase to condensed matter systems isdesirable due to the fact that ultrafast, correlated electron dynamics occur in many solids,from superconductors to plasmonic materials. Many of these systems are not completelyunderstood, and a more thorough understanding of the electron dynamics that lead to theirunique properties is important. As has been described previously in this dissertation, onesystem of particular interest is the surface plasmon resonance (SPR). Unfortunately, con-densed matter presents many challenges that are not present in the gas phase, and the directapplication of existing attosecond techniques is not always possible. This chapter will de-scribe the efforts to adapt attosecond technology to solids and the challenges and resultsassociated with that effort.

4.2 Previous experiments in the literature

Thus far, only one experiment using isolated attosecond pulses to probe electron dynamicsin a condensed matter material has been published [40]. In this experiment, Cavalieri andcoworkers used isolated attosecond pulses and the streak-field technique with 800 nm, IRlaser pulses to measure a time delay of 110 ± 70 as between the photoemission of electronsfrom the delocalized valence band of a W(110) single crystal and electrons from the 4f stateof the same. Figure 4.1 shows the streaking data from their experiment. Panel (a) shows thephotoelectron spectra collected at two different time delays between the XUV attosecondpulses and the IR laser pulses. The blue line is the raw TOF data collected far from zerodelay of the XUV and IR pulses (indicated by position (1) in panel (b)), where zero delay isdefine as temporal overlap of the maxima of the pulse envelopes. In this case negative time

CHAPTER 4. CONDENSED MATTER ATTOSECOND STREAKING 60

Figure 4.1: (a) Photoelectron spectra collected at different time delays between the XUVattosecond pulses and the IR laser pulses. The positions of the delays are shown as dashedlines in (b). Peaks from the tungsten valence band (83 eV) and 4f state (56 eV) can be seenin a spectrum taken far from zero time delay (blue line), and the Fermi level is denoted byEf . This same spectrum is also shown after subtraction of the large multiphoton backgroundemission and numerical smoothing (red line). These peaks broaden out from streaking bythe IR laser field in the spectrum measured at zero time-delay (black line). (b) The fullstreaking spectrogram after subtraction of the multiphoton background emission.


delays correspond to arrival of the IR pulses before the XUV pulses. Emission peaks from thevalence band (83 eV) and the 4f state (56 eV) can be clearly seen. In addition to these peaks,a large multiphoton background from the IR laser pulses can be seen starting at around 50 eVand rising rapidly to below 30 eV (this background is discussed further in Section 4.3.1).The red line is the same data after subtraction of the multiphoton signal and numericalsmoothing. The black line is data taken near zero delay time, corresponding to position(2) in panel (b), after multiphoton background subtraction and numerical smoothing. Whencompared to the peaks in the spectrum measured far from zero delay (position (1)), the peakscorresponding to the tungsten valence band and 4f state are clearly broadened (with theresulting decrease in amplitude) by the presence of the IR laser field. The streaking to higherkinetic energies is particularly clear from the increase in signal level above the Fermi level(Ef ). Figure 4.1 shows the full streaking spectrogram constructed from individual spectraintegrated for 60 s at each delay time and after subtraction of the multiphoton background.

Figure 4.2 shows the two streak traces from Figure 4.1b after cubic-spline interpolation.The conduction band is shown in the upper panel and the 4f state is shown in the lowerpanel. The region from ≈ 65 eV to ≈ 83 eV had been removed so that the streak tracescan be compared more directly. A small temporal shift between the two streak traces ishighlighted by the dashed white lines. This delay is quantified by performing center-of-massanalysis on the streak traces, shown in Figure 4.2b. These center-of-mass plots are used todetermine the delay of 110±70 as. The delay is suggested to originate from two factors. Thefirst is that the final state bands of the valence electrons are calculated, using a static bandstructure, to have stronger dispersion than those of the 4f electrons. This results in a lowereffective mass and thus a larger group velocity for the valence band electrons, ultimatelyleading to a faster escape time from the surface. The second factor is the fact that the 4felectrons have a longer inelastic mean free path (IMFP) than the valence electrons and thusoriginate ≈ 1 A deeper in the surface. The longer average distance of travel caused by theincreased IMFP and the slower velocity of the 4f electrons are said to result in the temporaldelay observed in the streak traces.

Treating photoemission in the presence of a streaking laser field from a complex condensedmatter band structure is extremely complicated, however, and three alternative theoreticaltreatments have already emerged. Kazansky and Echenique have expanded from the orig-inal calculation to a time-dependent calculation and found that the static band structureapproximation does not hold [89]. They conclude that the group velocity, which results frominterference of electron wavepackets scattered from atoms in the lattice, does not have timeto form in the attosecond timescale (because not enough scattering events occur) and thuscannot be the reason for the observed delay. Instead, they find that the time delay resultsmainly from the difference in the initial electronic states, localized for the 4f electrons anddelocalized for the valence band electrons. Zhang and Thumm use a similar model to thatof Kazansky [90], but they allow the few-cycle IR streak field to penetrate into the surface,which Kazansky does not. In the presence of the streak field, they find that the delay re-sults from interference between 4f electrons emitted from different atomic layers of the solid.


Figure 4.2: Streaking data from a W(110) crystal from Ref. [40]. (a) Streak traces followingcubic-spline interpolation of photoemission from the valence band (upper panel) and the4f state (lower panel) from a W(110) single crystal. A very small time delay between thetwo streak traces is highlighted by the dashed white lines. (b) Center-of-mass plots for thevalence and 4f streak traces in (a). The resulting delay is 110±70 as, where the error resultsfrom the calculation of the center-of-mass.


This interference does not occur for the valence band because it is considered completelydelocalized in their model. Finally, Lemell and coworkers used a classical transport theoryto model the results [91]. They find that the delay results from a combination of the largeremission depth of the 4f electrons and also slowing of some of the valence electrons by in-elastic scattering, resulting in final valence electron kinetic energies commensurate with thekinetic energy of the 4f electrons but at a delayed time. No consensus explanation has yetbeen formed.

4.3 Streaking results from a W(110) single crystal

In order to verify the production of isolated attosecond pulses by the apparatus constructedin this dissertation, streaking experiments from a W(110) single crystal are demonstratedand discussed below.

4.3.1 Photoelectron background emission

One of the primary challenges of XUV photoelectron spectroscopy from condensed mat-ter materials as compared to gas phase studies is the presence of a large photoelectronbackground. There are two major sources of photoelectron background present in theseexperiments: inelastically scattered electrons resulting from XUV ionization deeper in thematerial than the electron inelastic mean free path and multiphoton ionization by the few-cycle, 800 nm laser pulses.

The first source of background results from the fact that the ≈ 90 eV photons of theattosecond pulse will penetrate ≈ 3 nm into the surface (the distance is measured at thepoint where the intensity is 1/e of the initial intensity) of the condensed matter target(measured normal to the surface) [80]. Electrons will be generated all along the path of theXUV light, yet the electron escape depth (equivalent to the electron IMFP for detectionnormal to the surface) is only 0.4 nm for 90 eV kinetic energy electrons [81]. This value canbe determined from the so-called universal curve for electron IMFP, shown in Figure 4.3,which applies to photoemission from many different solids. Figure 4.4 shows a schematicdemonstrating the process. This results in a very large background of inelastically scatteredelectrons and a relatively small amount of direct photoemission. To calculate the percentageof direct photoemission as a function of the total emitted electrons, one must integrate overboth the ionization probability and the inelastic scattering probability. This results in theexpression:

I = I0λ

λ+ dp(4.1)

where I0 is the number of photons that penetrate the surface (this assumes that every photonwill produce one electron), λ is the inelastic mean free path and dp is the attenuation length


Figure 4.3: The universal curve for electron inelastic mean free path (IMFP) taken fromRef. [81]. For electrons with kinetic energy around 90 eV, the IMFP is only ≈ 0.4 nm.Because this length is shorter than the penetration depth of XUV radiation into the sample,a background of inelastically scattered electrons results.

~3 nm

~0.4 nm

1/e1XUV

Emitted

Electrons

z

e-

e- 1/e 1

z

Probability of

electron escape

without scattering15 deg

Figure 4.4: A schematic illustrating the source of the large inelastically scattered photoelec-tron background resulting from XUV photoemission. The 93 eV light penetrates ≈ 3 nmnormal to the surface (z-axis), releasing electrons all along the path of the light, while theIMFP is only ≈ 0.4 nm for 90 eV electron kinetic energy. Electrons released deeper thanthis have very little probability of escaping the surface without inelastically scattering.


0 20 40 60 80 1000

0.02

0.04

0.06

0.08

0.1

Kinetic Energy (eV)

Ele

ctr

on

Co

un

ts/S

ec

on

d

0 20 40 60 80 1000

0.02

0.04

0.06

0.08

0.1

Kinetic Energy (eV)

Ele

ctr

on

Co

un

ts/S

ec

on

d

a) b)

Detector saturation

Electrons emitted

without scattering

Figure 4.5: (a) Photoelectron kinetic energy spectrum from a W(110) single crystal surfaceionized by HHG generated XUV pulses centered at 93 eV. The peak centered at ≈ 86 eVis from electrons that escape from the surface without scattering (indicated approximatelyby grey shaded area), while the large low energy background results from electrons that areinelastically scattered within the metal. (b) Comparison of XUV only photoelectron emissionfrom W(110) (black line) to photoemission from the XUV plus the few-cycle, 800 nm laserpulses (red line). Instability in the HHG flux has led to a slight decrease in overall signalbetween the two measurements. The two pulses are positioned at zero time overlap andthe intensity of the 800 nm laser pulses is typical for a streaking experiment. The broadpeak centered at 34 eV is the result of multiphoton emission by the 800 nm laser pulsesand is saturating the detector below 34 eV (indicated by the arrow). In addition, increasedamplitude above ≈ 95 eV shows streaking of electrons from the valence band peak to higherkinetic energies.

of the light, measured normal to the surface. For the parameters given above, this results inonly 11% of total electron emission escaping the surface without inelastically scattering.

Figure 4.5a shows an example of ionization from a W(110) crystal surface using only theHHG generated XUV light centered at 93 eV. The spectrum is integrated over 500000 laserpulses with an average of 3311 electrons detected per second. The peak at approximately86 eV results from direct valence band photoemission by the XUV pulse while the large lowkinetic energy signal is the result of the inelastically scattered electrons. The integratednumber of photoelectron counts under a Lorentzian peak fit to the XUV peak is ≈ 6% ofthe total number of detected electrons.

The second major source of photoelectron background that is relevant for attosecondstreaking experiments from condensed matter is the multiphoton ionization induced by thefew-cycle, 800 nm laser pulses. While multiphoton background does exist in streaking fromgas-phase targets, it is usually not a problem because of the high ionization potentials of gasphase atoms and molecules. In comparison, most solids have relatively low work functions


that can permit a large amount of multiphoton ionization at typical streak-field intensities.For example, the ionization potential of Ne atoms is 21.6 eV, while the work function ofa W(110) crystal surface is only 5.5 eV [92]. This means that fourteen 1.55 eV, 800 nmphotons are needed to ionize a Ne atom while only four of the same photons are necessaryto ionize from the tungsten surface.

Figure 4.5b demonstrates the multiphoton background produced by the 800 nm, few-cyclelaser pulses in a typical streaking measurement. The black line is the same data as shownin Figure 4.5a, while the red line is measured with the 800 nm laser pulse overlapping withthe XUV pulse at zero time delay. Instability in the HHG flux has led to a slight decrease inoverall signal between the two measurements. The peak from direct XUV emission at 86 eVhas been broadened out to both lower and higher kinetic energies (the CEP is not lockedin this measurement, so electrons are streaked to both higher and lower kinetic energieswithin the integration period). The peak centered at 34 eV is the result of multiphotonionization from the few-cycle, 800 nm laser pulses. Multiphoton emission is high enoughthat the detector is saturated below 34 eV, indicated by the arrow in the figure. Saturationonly occurs when the 800 nm laser pulse is present (red line), not in the XUV only spectrum(black line).

4.3.2 Demonstration of attosecond streaking

Figure 4.6 shows the first demonstration of attosecond streaking from the apparatus devel-oped in this dissertation (data collected by Joseph Robinson, with whom the system wasjointly developed). The spectrogram is constructed from a series of photoelectron spectracollected from a W(110) single crystal at varying time delays between the attosecond XUVpulses and the IR laser pulses. Negative time delays represent time at which the attosecondXUV pulses arrive before the IR laser pulses. Each time step (200 as) is integrated over30000 laser pulses. The sample is mounted at the Brewster’s angle of 15 grazing incidenceangle in order to minimize the intensity of the reflected 800 nm laser beam [93]. The streak-field intensity is estimated to be 8×1011 W/cm2 based on measurements of the pulse energy,pulse duration and focal spot size. The data is normalized to the integrated electron yieldbetween 75 eV and 110 eV at each time step to account for fluctuations in the HHG yield.No smoothing or other processing has been applied.

In this spectrogram, only the valence band is clearly seen because the emission from the4f state is suppressed by surface contamination. The apparatus currently does not supportultra-high vacuum (UHV) conditions (typically described as< 10−10 Torr), therefore cleaningand preparation of the sample surface would have very little effect as the surface would bequickly contaminated again. In the supplemental material of Ref. [40], it is stated that,at a pressure of < 10−9 mbar, the 4f photoelectron peak is only present for ≈ 3 hoursafter heating in front of an oxygen doser to ≈ 1400 K followed by repeated flash heating toabove 2200 K. After this time the surface contamination becomes enough that 4f electronscannot effectively escape the surface without being inelastically scattered. New equipment


Figure 4.6: First demonstration of attosecond streaking from the apparatus developed in thisdissertation. The spectrogram is constructed from a series of photoelectron spectra collectedfrom a W(110) single crystal at varying time delays between the attosecond XUV and the IRlaser pulses. Negative time delays represent the time at which the attosecond XUV pulsesarrive before the IR laser pulses. Each time step (200 as) is integrated over 30000 laserpulses.


Figure 4.7: Spectral centroid analysis of the streaking trace presented in Figure 4.6. Thecentroid is calculated from data between 70 eV and 110 eV and clearly demonstrates thesub-optical-cycle resolution of the streaking spectrogram.

currently being constructed in the Kaindl laboratory will allow future experiments to achievethe proper surface conditions.

Spectral centroid analysis is used to more clearly visualize the modulation of the kineticenergy distribution. The centroid of the kinetic energy spectrum measured between 70 eVand 110 eV is shown in Figure 4.7 and is calculated according to:

Centroid =

∑110eV70eV EkC(Ek)∑110eV

70eV C(Ek)(4.2)

where Ek is the kinetic energy and C(Ek) is the corresponding number of electron counts ateach kinetic energy value. The centroid of the kinetic energy distribution is clearly modulatedas a function of the delay between the attosecond XUV pulse and the few-cycle IR pulse. Theexpected period for an 800 nm laser field is 2.66 fs, however the average peak-to-peak spacingin Figure 4.7 is 3.15 fs, corresponding to a wavelength of 945 nm. While this wavelength iscontained in the laser pulse bandwidth, the discrepancy is surprising. One possible sourceof error is spatial chirp. Longer wavelength light will diverge at a faster rate than shorterwavelength light. Due to the split mirror geometry, the outer portion of the beam is usedas the streak field and the inner portion is removed by the Zr filter. It is possible thata spatial chirp results in a streak-field wavelength that is longer than expected. Anotherpossible source of error is drift in the piezoelectric-driven translation stage that controls the


temporal delay between the XUV attosecond pulse and the few-cycle laser pulse. The effectof such a drift is unknown and requires further investigation.

One technique that has been useful in attosecond gas-phase experiments is the pulse re-construction algorithm known as frequency-resolved optical gating for complete reconstruc-tion of attosecond bursts (FROG-CRAB) [94]. The technique can be used to reconstructpulse duration and phase information for both the isolated attosecond pulses and the few-cycle streak field laser pulses used in a streaking experiment. Unfortunately, the centralmomentum approximation used in the FROG-CRAB technique fails if the electrons ejectedby the attosecond pulses are not ionized from a well-defined state. In the case of W(110),the valence band is nearly 10 eV wide [91, 82]. Beyond this, the strong background sig-nal from both the XUV-induced inelastic scattering and the IR-laser-induced multiphotonemission cause deterioration in the performance of the FROG-CRAB algorithm. Becauseof this, it will be desirable in future experiments to have the capability of performing gasphase measurements in the same apparatus as the condensed matter measurements. Withthis capability, the isolated attosecond pulse could be characterized in the gas phase andthen used for a condensed matter experiment.

4.4 Streaking results from amorphous Cr thin film

In addition to streaking from a tungsten single crystal surface, streaking from an amorphouschromium thin film has also been observed using the apparatus constructed in this disserta-tion. As mentioned above, the only previous isolated attosecond pulse measurement from acondensed matter surface used a single crystal surface with Brewster’s angle incidence of the800 nm laser pulses (minimizing the reflected beam). It was thus unknown how streakingfrom a strongly reflecting thin film may behave. Cavalieri has suggested that the phase-shifted reflection of the 800 nm laser pulses may affect the streaking results [95]. Here wehave shown that streaking from an amorphous thin film with a strongly reflected beam ispossible.

Figure 4.8 shows streak traces taken with isolated attosecond pulses from (a) W(110)and (b) a 10 nm thick film of amorphous Cr. The spectra have both been normalized forfluctuations in HHG yield and numerically smoothed. The two samples are mounted side-by-side in the vacuum chamber and the measurements were taken on the same day. Thesamples are both mounted at a 15 grazing incidence angle, which is the Brewster’s angle fortungsten but not for chromium. Each trace is constructed from a series of scans integratedfor 20000 laser pulses and with time steps of ≈ 200 as. Because of the noise present inthis experiment, spectral centroid analysis does not provide a clear picture of the streakingtrace. To more clearly visualize the photoelectron streaking in the two spectrograms, Figure4.9 shows the summation of the photoelectron yield of each streak trace between 93 eVand 110 eV at each point along the time axis. Sub-optical-cycle resolution of the streakinglaser field is clearly visible in both spectrograms. As opposed to Figure 4.7, the expected


a)

b)

Time Delay (fs)

Time Delay (fs)

Kin

eti

c E

ne

rgy

(e

V)

Kin

eti

c E

ne

rgy

(e

V)

Figure 4.8: Comparison of attosecond streaking from (a) a W(110) single crystal and (b) a10 nm thick amorphous chromium thin film. The 800 nm streak field intensity was not thesame in both measurements, which accounts for the different amounts of streaking in thekinetic energy domain.


W(110) Cr Thin FIlm

a) b)

Figure 4.9: Summation of the photoelectron yield between 93 eV and 100 eV for (a) a W(110)single crystal surface and (b) a 10 nm thick amorphous chromium thin film. Sub-optical-cycleresolution is clearly visible in both spectrograms.

periodicity of 2.6 fs (corresponding to an 800 nm streak-field wavelength) is observed in bothstreak traces in Figure 4.9.

4.5 Surface plasmon enhanced attosecond spectroscopy

The contents of this section have been published in Chemical Physics Letters, 463 (2008)11-24, Ref. [87].

In this section, we propose and discuss a novel spectroscopic technique that directly ac-cesses light-induced potentials in nanoparticles and molecules with sub-cycle precision usinglaser light as a pump and the attosecond pulse as a probe. A laser pulse induces a microscopiccharge displacement in a quantum system and the time-delayed attosecond pulse ionizes anelectron that — on its way out — samples the charge-migration-induced local electric poten-tial of the initial bound-state orbital of the electron. One phenomenon of particular interest,chosen here as an example to illustrate the method, is the surface plasmon resonance (SPR)in metal nanoparticles. The SPR is a collective oscillation of the conduction band electronsthat exists at optical frequencies for the noble metals [46]. The period of these oscillationsis thus only a few femtoseconds, and the lifetime of the coherent motion is on the order of10 fs [50, 48, 47, 51]. By using the attosecond pulse as the probe, we will be able to directlyobserve the charge oscillation of the nanoparticle by measuring its surface potential in realtime.

Instead of using the electric field of the laser pulse to provide temporal resolution bystreaking, the laser-driven sub-cycle-oscillating dipole field of the surface plasmon in thevicinity of the particle is strong enough to lead to acceleration and deceleration of emittedelectrons. The SPR is excited using the optical laser pulse and the attosecond pulse intro-


duced at a variable time delay to ionize an electron near the nanoparticle surface. The kineticenergy of this electron measured far away from the particle is the sum of the excess energy(attosecond photon energy minus ionization potential) and the negative of the surface poten-tial (due to the negative electron charge) that results from the transiently-induced electricdipole of the particle. The soft x-ray attosecond pulse must have a sufficiently high photonenergy to ensure the initial kinetic energy of the electron is great enough that it will escapethe plasmon electric dipole field in less than one half cycle, preventing cycle-averaging of theelectron acceleration in the surrounding dipole field. By measuring the photoelectron energydistribution as a function of time delay between the attosecond pulse and the laser pulse, thetransient surface potentials, and thus the microscopic dipole response of the particle, will bemapped out with sub-femtosecond precision.

In the case of a spherical nanoparticle, the SPR can be well-modeled by use of the dipoleapproximation for a sphere embedded in a dielectric medium [96]. In this approximation,the surface potential is:

V = E · r cos θε− εmε+ 2εm

(4.3)

where E is the electric field strength of the driving pulse, r is the radius of the particle, θis the angle of emission with respect to the laser polarization, ε is the complex dielectricconstant of the sphere, and εm is the dielectric constant of the embedding medium. In thecalculation shown below, vacuum is assumed as the embedding medium, giving εm = 1. Ex-perimentally obtained frequency-dependent dielectric data for gold are used [97], resultingin a plasmon decay time of 9.3±0.9 fs. The particle size is chosen as r = 60 nm. The SPR isdriven on resonance with a 6 fs-FWHM Gaussian pulse centered at 530-nm wavelength withan intensity of 9.8×109 W/cm2 intensity, orders of magnitude less than typical streak fieldintensities required for atomic species [36, 41, 12], because of the large polarizability of thenanoparticle that enhances the electric field near its surface. The surface potential is calcu-lated as a function of time and a spherical emission probability distribution is considered forthe electrons. The resulting photoelectron spectrum is convoluted with a 3.6 eV FWHM, 500as transform-limited probe pulse to give a time-dependent photoelectron spectrum (modelsimulations shown in Fig. 4.5).

If the time-dependent field of the laser pulse is precisely known (which can, for instance,be accomplished by a conventional streak-field measurement), the full spectroscopic set ofamplitude and phase information is available for both the dipole response and the laserfield. After Fourier-transforming this data to the spectral domain, a full reconstruction ofthe frequency-dependent dipole response (resonance curve) is then possible by dividing thecomplex-valued spectrum of the dipole potential by the complex-valued spectrum of the laserfield.

One chemically attractive application of this spectroscopic technique is to probe the dy-namics of the SPR when different molecules are adsorbed on the surface of the particle. Intechniques such as surface-enhanced Raman spectroscopy, the amount of signal enhancementachieved is directly proportional to the coherence time of the plasmon oscillation [98, 15, 51].


+ +++ +

- - - - -

a)

Time (fs)

Ekin

− E

0 (

eV

)

−10 −5 10 15−20

−15

−10

−5

0

5

10

15

20

0 0.5 1

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2x 108

0 5

Ele

ctr

ic F

ield

Str

en

gth

(V

/m)

b)0 0.5 1

e-

TOF

vo

Fsurface

Figure 4.10: Simulation of a proposed measurement of field-induced attosecond time-resolveddipole potentials. a) Schematic of the dipolar charge distribution induced in a metal nanopar-ticle by the exciting laser field. Electrons are freed by the attosecond pulse with an initialvelocity v0 and sample the induced electric field, experiencing a force Fsurface, before they aredetected by a time-of-flight spectrometer with a small collection angle of 15. b) Simulatedtime-dependent photoelectron kinetic energy spectrum as a function of time-delay betweenthe attosecond and the 530-nm laser pulse, where E0 = v2

0/2 is the kinetic energy in theabsence of the plasmon excitation. A 6 fs laser pulse (white line) at 9.8×109W/cm2 inten-sity excites the plasmon which is then probed by a time-delayed 500 as pulse. A temporalbroadening of the dipole potential response function — mapped out by the intensity maximaof the photoelectron spectral distributions — compared to the driving pulse shows the finitelifetime of the plasmon resonance (sustained dipole oscillations at late times after the drivinglaser pulse is over). In addition to resolving the decay time of the plasmon resonance, indi-vidual plasmon oscillations are observed with sub-cycle resolution, permitting the possibilityto unravel nonlinear dynamics.


It is therefore of great interest to understand how adsorbed molecules will affect plasmondynamics due to dephasing effects such as chemical interface damping. Another importantapplication of this general spectroscopic method was recently proposed: combining pho-toemission electron microscopy (PEEM) with attosecond pulses to probe spatio-temporalsurface-plasmon dynamics in metal films [88].

While we focus on metal nanoparticles to introduce the principle of the technique, thesame measurement concept can potentially be extended — at higher intensities to com-pensate for the lower polarizability — to study collective or correlated electron dynamicsin smaller systems, such as molecules or atoms. It can be thought of as producing “light-induced chemical shifts” of inner-valence/core-electron energies by perturbing (displacing)the valence electronic orbitals by a laser electric field. From these energy shifts it will bepossible to extract information about the local polarizability and electron response (displace-ment of charge distribution) in molecules at specific atomic sites. Site selectivity is achievablesince the core-electron energies of constituent atoms and therefore the corresponding elec-tron kinetic energies after ionization in the probe step will be different for different atoms.Since laser electric fields can easily reach the scales of interatomic fields in molecules (leadingto the well-known chemical shifts of core-electron energies used in x-ray spectroscopy), weexpect the effects of light-induced chemical shifts to be of similar magnitude and thus easilymeasurable.

4.6 Conclusions

The results presented in this chapter have demonstrated the capability of the apparatusdescribed in this dissertation to produce isolated attosecond XUV pulses and to perform at-tosecond streaking experiments with them. Attosecond streaking from both a W(110) singlecrystal surface and from a 10 nm thick amorphous chromium thin film have been demon-strated. In addition, a new technique for surface-plasmon-resonance-enhanced attosecondstreaking has been proposed theoretically. Despite this progress, the capabilities of the ap-paratus are limited. Because of the inability to maintain UHV vacuum conditions and toadequately prepare sample surfaces, many surface science experiments will not be possible.Because of this, a new vacuum chamber has been designed by, and is currently being assem-bled by, other members of the Kaindl research group. This new design will support vacuumpressures < 10−10 Torr and will allow for a variety of sample preparation methods, includingion sputtering and annealing. This improvement to the system will allow for a broader rangeof investigations in the future.

75

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Plasmon enhanced photoelectron spectroscopy and the ... · Plasmon enhanced photoelectron spectroscopy and the generation of isolated ... binding potential of the atom’s valence