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Femtosecond OPCPAs from UV toshort-wave IR wavelengths forultrafast dynamics experiments fromcondensed matter to atoms,molecules, and clusters

Buß, J. H., Schulz, M., Grguraš, I., Golz, T., Prandolini, M.,et al.

J. H. Buß, M. Schulz, I. Grguraš, T. Golz, M. J. Prandolini, R. Riedel,"Femtosecond OPCPAs from UV to short-wave IR wavelengths for ultrafastdynamics experiments from condensed matter to atoms, molecules, andclusters," Proc. SPIE 11278, Ultrafast Phenomena and Nanophotonics XXIV,112781A (27 February 2020); doi: 10.1117/12.2551488

Event: SPIE OPTO, 2020, San Francisco, California, United States

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Femtosecond OPCPAs from UV to short-wave IRwavelengths for ultrafast dynamics experiments fromcondensed matter to atoms, molecules and clusters

J. H. Buÿa, M. Schulza, I. Grgura²a, T. Golza, M. J. Prandolinia,b, and R. Riedela

aClass 5 Photonics GmbH, Notkestr. 85, 22607 Hamburg, GermanybInstitut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761

Hamburg, Germany

ABSTRACT

Ultrafast dynamics experiments in physics, chemistry and biology, in condensed matter or gas phase requiredi�erent types of lasers. We present various OPCPAs as a powerful toolbox to deliver femtosecond pulses fromthe XUV to THz spectral region. A synchronized pump-probe laser has been realized, generating visible andnear-IR pump pulses (350�1300 nm) and UV pulses (250 � 350 nm) operating at 4 MHz. Furthermore, a 1.55 µmwavelength OPCPA has been demonstrated for e�cient THz-generation. For XUV generation in the waterwindow, an OPCPA is presented, delivering millijoule-level, femtosecond pulses from 1.7 � 2.2 µm with up to 100W average power.

Keywords: optical parametric chirped-pulse ampli�er, nonlinear optics, deep ultraviolet laser, infrared laser,water window, high harmonic generation, terahertz generation

1. INTRODUCTION

Research laboratories focused on ultrafast phenomena require robust and �exible laser systems in order to drive avariety of experiments. Previously, laser sources from x-ray to THz were driven from Ti:Sapphire lasers at 800 nmwith limited spectral bandwidth (Fourier limited pulse of ca. 20 fs), and more importantly limited power levelsand repetition rates. Commonly available systems were achieving a few microjoules at 100 kHz repetition rate,or a few millijoules at 1 kHz repetition rate. For many applications high repetition rates in the MHz range areneeded to increase the signal-to-noise ratio. At the same time, driving nonlinear interactions requires su�cientpulse energies up to the millijoule range. The recent advances in Yb-doped solid-state Lasers brought a newgeneration of high-power scienti�c lasers to the community, however, also at limited spectral bandwidth (Fourier-limited pulses of 200�2000 fs).1�3 Optical parametric chirped-pulse ampli�cation (OPCPA) together with bulkcrystal white-light-generation (WLG) opens up the possibility for high power lasers with average power wellabove 100W, repetition rates of 100 kHz in the millijoule-range or up to 10 MHz in the microjoule-range.4�8

Further, a wide wavelength range is accessible from ultraviolet to infrared, making OPCPA an ideal technologyfor realizing complex pump-probe architectures, including secondary sources, such as terahertz and extremeultraviolet high-harmonic generation (HHG).

In this paper, various laser architectures based on OPCPA will be presented with exemplary experimentaluse cases.

2. OPCPA SYSTEMS FOR ADVANCED SPECTROSCOPIC APPLICATIONS

The investigation of ultrafast dynamics in condensed matter and interfaces allows to understand complex materialsystems and fundamental interfacial chemical processes,9 leading potentially to the development of new functionalmaterials. The commonly applied methods range between spectroscopic techniques and surface microscopy,e.g. ultrafast photo-electron emission microscopy and spectroscopy (PEEMS) or time-resolved angular-resolvedphoto-emission electron spectroscopy (trARPES).10�13 In this section, two OPCPA systems are presented toserve the requirements of multi-dimensional pump-probe techniques.

Further author information: (Send correspondence to R. R.)R. R.: E-mail: robert.riedel@class5photonics.com, Telephone: +49 40 22863165-21

Ultrafast Phenomena and Nanophotonics XXIV, edited by Markus Betz,Abdulhakem Y. Elezzabi, Proc. of SPIE Vol. 11278, 112781A · © 2020 SPIE

CCC code: 0277-786X/20/$21 · doi: 10.1117/12.2551488

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2.1 Gap-Less Tunable OPCPA between 250 nm and 1300 nm

A compact OPCPA system has been developed to provide two synchronized outputs that can be individuallyspectrally tuned at a repetition rate of 4MHz (Fig. 1). The complete system was realized in a single housing ona footprint of 800× 1200mm, including the pump laser (Coherent Monaco-1035-80-60). Output Ch3 is tunablebetween 650 and 950 nm (fundamental wavelength), with a second-harmonic generation (SHG) stage providingwavelengths between 325 and 475 nm, and a third-harmonic generation stage providing UV wavelengths between250 and 316 nm. Output Ch2 is tunable between 650 and 1300 nm, with corresponding SHG output between325 � 650 nm in order to achieve gap-less excitation or probing photon energies. The corresponding spectra areshown in Fig. 2A and B.

250 - 316 nm

< 50 fs

325 - 475 nm

< 50 fs

650 - 950 nm

< 30 fs

350 - 650 nm

< 50 fs

650 - 1300 nm

< 30 fs

1030 nm

300 fs

1 - 4 MHz

60 W

Figure 1. Schematic diagram of the 4MHz OPCPA system with gap-less seeder (Ch1, WD Seeder 650�1300 nm), andtwo individually tunable outputs (Ch2 and Ch3).

The outputs achieve up to 3W of average power in the fundamental outputs, 0.9W in the SHG, and 0.3Win the THG with pulse durations between 30 � 50 fs. Each fundamental output exceeds a pulse energy of 2µJover the entire tuning range at 1MHz repetition rate and more than 0.5 µJ for OPA1 and 0.2µJ for OPA2at 4MHz repetition rate. The SHG for both OPAs reaches e�ciencies of more than 20%, the THG of OPA2reaches a total conversion e�ciency of more than 5% of fundamental power. The tunable outputs are designedfor < 30 fs Fourier-limited pulse duration over the whole wavelength range. The pulse compression is achievedin a chirped-mirror array, supporting 650 nm to 950 nm, and 950 nm to 1300 nm spectral bandwidth at a groupdelay dispersion of GDD = −100 fs2.

2.2 Pump-Probe System from THz to XUV

A high-power, femtosecond laser system is developed, spanning an ultra wide frequency range from Terahertz(THz) to extreme-ultraviolet (XUV). The system can be used for long-wavelength excitation of carrier dynamicsand short-wavelength (XUV) probing of excited photoelectrons, with su�cient probing photon energy of 21.6 eVto cover the full Brillouin-zone of a solid-state material.12 It is based on a 350 W Yb:YAG picosecond laser(Amphos GmbH), which pumps three synchronized outputs: a high-�eld THz generation source, two individuallytunable optical parametric chirped-pulse ampli�ers (OPCPA 1+2, 3), a second- and fourth-harmonic generation(SHG and FHG) source, and a high-harmonic generation source (HHG), see Fig 3.

The pump laser is an Yb:YAG InnoSlab Laser system (Amphos3000) operating at 3.5mJ pulse energy at100 kHz repetition rate. The pulse duration is < 600 fs, which is ideal for OPCPA pumping and THz generation inlithium niobate.14 For the THz-driver, 2mJ of the pump output are directly provided using a beam splitter. TheHHG driver consists of a white-light generation (WLG) driven dual-stage OPCPA with wavelength tunabilityfrom 700 nm to 900 nm. In order to achieve a �nal HHG bandwidth of < 80meV at 21.7 eV and at su�cientlyshort pulse duration,15 the tunable outputs are designed for < 30 fs Fourier-limited pulse duration, as describedin Sec. 2.1. At a pump pulse energy of > 1.4 mJ, > 100 µJ of OPCPA output energy can be generated.

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A

B

Figure 2. Fundamental ampli�ed spectra (sub 30 fs) and corresponding second harmonic (SHG) and third-harmonic(THG) spectra spectra of A: Output 1, and B: Output 2.

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Yb:YAGInnoSlab

1030 nm3.5 mJ100 kHz600 fs

THzsource

XUV (HHG)source

UV (FHG)source

OPCPA1 + 2

OPCPA 3

SHG

2 mJ

1.4 mJ

0.1 mJ

> 100 µJ700 - 900 nm

< 30 fs

> 10 µJ700 - 900 nm

< 30 fs

> 30 µJ350 - 450 nm

< 30 fs

> 10 ph/s21.7 eV nm

80 meV

¹³

100 THz1 GV/cm²

100 nJ200 - 225 nm

< 50 fs

Figure 3. Schematic diagram of the 100 kHz Pump-probe system for THz pumping and XUV probing. The three outputsare operated in parallel.

The target wavelength for XUV (HHG) is about 57 nm. As even harmonics are forbidden for HHG pumpedat linear polarization (57 nm would be the 14th harmonic of 800 nm), the second harmonic (SHG) is generatedfrom the driver pulse in β-BBO, covering a HHG driver range of 350�450 nm.

2.3 OPCPA system for THz-generation in organic crystals

The excitation of low energy modes in matter plays a key role in advanced material studies as described above.The laser-driven generation of high-�eld THz pulses has advanced signi�cantly during the previous decade.Various methods for high-�eld THz generation are proposed, tested, and re�ned in rapid succession, combininga broad range of state-of-the art driving lasers with numerous di�erent THz generation media - from organic toinorganic crystals to complex devices, antenna and multilayer structures, or gases, each favorable for di�erentparameter sets.16�18

A commonly used generation process for high-�eld THz-pulses is optical recti�cation, which is a second-ordernonlinear process (Schematic Fig. 4). It is induced via a second-order polarization within a nonlinear crystalwith nonlinear susceptibility χ(2): P (t) ∝ χ(2)A2(t) using an ultrashort pulse with �eld envelope A(t). The

electric �eld of the THz pulse is expressed as ETHz = ∂2P (t)∂t2 . For e�cient conversion, phase-matching needs

to be ful�lled between the group velocity ngr of the laser pulse and the phase velocity nTHz of the THz pulse.The most commonly used nonlinear crystal is LiNbO3 (LN). It features a high transmission in both the NIRand the THz range, and possesses a high nonlinear coe�cient of de�, LN = 168 pmV−1. However, the phase-mismatch is severe with ngr = 2.25 at 800 nm and nTHz = 4.96 at 2THz. Consequently, a tilted pulse-frontphase-matching approach must be applied,19 which introduces further complexity to the generation scheme.Alternatively, recently developed organic nonlinear crystals, such as DAST and DSTMS,20 feature also a hightransparency and a large nonlinear coe�cient. Using driver laser pulses centered at 1.55 µm allows for an easiercollinear phase-matching scheme with ngr = 2.07 at 1550 nm and nTHz = 2.25 at 2THz.

For e�cient THz generation in organic crystals, the White Dwarf OPCPA concept was extended to a tunable,high power laser system centered at a wavelength of 1.55µm. In addition, an optically synchronized compressedprobe pulse with pulse duration of < 15 fs at 850 nm, was made available as a second output channel (Fig. 5).This probe-pulse can be used, e.g. for electro-optical sampling in a THz set-up. This system can be pumpedby a standard industrial Yb-doped solid-state laser, for example, Yb:YAG Innoslab or thin-disk, or a Yb-doped�ber laser system (here: ActiveFiberSystems GmbH), and is scalable to hundreds of watts.8

A small fraction of the Yb-pump laser is used for WLG (Fig. 5) over a range from 650 nm to 1750 nm and canbe used to seed one (or more) optical parametric ampli�ers (OPA) using a beam splitter, providing stable andpassive synchronization for both channels. In this case, the one OPA channel (1) is used to potentially generateTHz in organic crystals with a tunable SWIR range from 1.45�1.65 µm (see example spectrum in Fig. 6D) a secondsynchronized channel (2) is compressed to less than 15 fs and is directly from the WLG centered at 850 nm. Thelarger fraction of the pump beam is split to pump two subsequent OPCPA stages to achieve a �nal pulse energy

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Figure 4. THz generation via optical recti�cation.

Pump laserYb:fiber

< 350 fs1030 nm> 200 µJ350 kHz

Stretcher

1030 nm

WLG

Compressor

WHITE DWARF OPCPA

OPA 2OPA 1

Compressor

Channel 11550 nm< 36 fs

Channel 2850 nm< 15 fs

Figure 5. Schematic diagram of the high-power synchronized dual-channel laser at 1550 nm: WLG - white light generation,OPA - optical parametric ampli�er.

of up to 31 µJ at 350 kHz repetition rate. Both channels are compressed in individual compressors, using anall-chirped-mirror set-up with glass wedges for dispersion �ne-tuning in the <15 fs output. The compressor doesnot require an active phase shaper, thus representing a compact and reliable component. The 1.55 µm output ischaracterized using second-harmonic frequency-resolved optical gating (SHG FROG), as shown in Fig. 6A andB. The achieved pulse duration is < 36 fs FWHM (Fig. 6C).

3. OPCPA FOR ULTRAFAST DYNAMICS IN THE WATER WINDOW

Studying ultrafast dynamics in the gas-phase and atomic clusters often requires pulse energies on the millijoulescale. Furthermore, strong-�eld dynamics require intensities of at least 1014Wcm−2. High-harmonic generation(HHG) spectroscopy and associated photoionization spectroscopy in the soft-x-ray regime have been successfullyproven as a technique to resolve fundamental strong-�eld phenomena and electron dynamics on the attosecondscale.21,22 In addition, the short wavelengths allow for nanometer spatial resolution imaging.23 Recently, ad-vanced schemes have been developed to exploit overlapping light �elds in order to isolate di�erent degrees offreedom, such as chirality properties of matter.24 The majority of the measurements are subject to long integra-tion times due to the weak light-matter interaction cross-sections in the experimental target and the detectiontechnologies. This is specially true for coincidence measurements, where the number of interaction events mustbe kept clearly below 1 event per pulse. Studying those properties in organic and biological molecules is opti-mally addressed in the water transparency window above the carbon K-edge (284 eV) and the oxygen K-edge(543 eV). In order to extend the HHG cut-o� energy into this region requires longer driver wavelengths λ than thepreviously used 800 nm or 1030 nm, utilizing the wavelength-dependent cut-o� energy law Ecut ≤ IP + 3.17UP ,with the ionization potential IP , the ponderomotive potential UP ∝ λ2 × IL, and the laser intensity IL.

We present a high-power OPCPA system that delivers a driver wavelength of λ = 2µm at a pulse energy of2.0mJ, a repetition rate of 50 kHz, and a pulse duration of < 30 fs. The laser system consists of two di�erentpump lasers temporally locked to a common oscillator (PUMP and AUX in Fig. 7). First, a 30 µJ AUX-pulsegenerates a broadband (1.7�2.5 µm) seed spectrum. This seeder is a combination of a standard non-collinearWhite Dwarf OPCPA (WD-800 OPCPA) from Class 5 Photonics tailored to a bandwidth between 660nm and725nm in the visible and a BBO-based collinear di�erence frequency generation (DFG) stage seeded with 2 µJ

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A C

B D

Figure 6. Pulse characterization of the 1550 nm OPCPA using frequency-resolved optical gating (SHG-FROG), showingmeasured interferogram (A), reconstructed interferogram (B), reconstructed pulse envelope (blue) and temporal phase(red, C), and reconstructed (blue solid), measured (blue dashed) spectrum, and spectral phase (red, D).

of 1030nm light and pumped by the visible WD OPCPA output with 2�3 µJ. Subsequently, the generated200 nJ CEP-stable idler pulse centered at 2 µm is pre-amplifed to 1.5W using 20W of the main PUMP output(AMPHOS GmbH: 50 kHz, 16mJ, 1.3 ps). A typical pre-amplifed spectrum and beam pro�le is given in Fig. 8.

After the pre-amplifcation stage, two booster OPCPA stages amplify the signal to > 100W. The system alsocomprises a single-shot cross-correlator, which monitors the temporal drift of the two pump laser outputs. A PIDfeedback-loop allows for drift compensation using a linear piezo-stage (Fig. 7). The temporal pulse compressionis achieved in an array of 24 chirped-mirrors, optimized for group delay dispersion (GDD) and third-orderdispersion (TOD).

WHITE DWARF WD- 2 CEP OPCPA2 00

WLGOPA3DFG

Idler 1.7 - 2.5 µm

OPCPA 1-3Signal 2000 nm

delay

Compressor- 4800 fs²

StretcherNOPA

640 - 740 nm

Pump Laser

16 mJ50 kHz800 W

1030 nm AUX2.0 µm< 30 fs100 W

PUMP

Cross-Correlator

FPGA

SHG

SUPERNOVA SN-2 00 CEP OPCPA2

Figure 7. Schematic diagram of the 2 µm 2mJ 50 kHz OPCPA.

4. CONCLUSION AND OUTLOOK

In order to face the challenges for advanced and future ultrafast dynamics experiments from condensed matterto atoms, molecules and clusters, Class 5 Photonics GmbH utilizes and develops laser sources based on opticalparametric chirped-pulse ampli�cation combined with state-of-the-art high-power Yb-doped picosecond Lasers.

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Figure 8. Ampli�ed spectrum (left) and beam pro�le (right) of OPCPA1.

In this work, we reviewed recent results on di�erent system architectures: (i) A gap-Less Tunable OPCPAbetween 250 nm and 1300 nm, operating two parallel outputs at 4 MHz repetition rate and 30 fs pulse duration;(ii) a pump-probe system in the 100 µJ class at 30 fs pulse duration to drive two independent secondary sourcesat THz and XUV; (iii) an OPCPA system at 1.55 µm and 36 fs pulse duration for THz-generation in organiccrystals; and (iv) a high-power OPCPA centered at 2µm, featuring < 30 fs pulse duration at average power upto 100W for high-�ux HHG in the water window.

Future work will comprise more results at 100W average power and carrier-envelope measurements, andresults on mid-IR generation at 8µm and a high-�ux commercial HHG source at 21.7 eV in order to provide highrepetition rate ultrafast Laser sources over an extreme wavelengths range.

4.1 Acknowledgments

We acknowledge the contribution of all our customers, research collaborators, and industry partners. The highpower pump Lasers in this work have been provided by Coherent, Inc., ActiveFiberSystems GmbH and AmphosGmbH.

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