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HRS Program
FLS2010 WorkshopMarch 4th, 2010
HHG based Seed Generation for X-FELs
Franz X. Kärtner, William S. Graves and David E. Monctonand WIFEL Team
Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology
Cambridge, MA, USA
2
AcknowledgementStudents:
Ch.-J. Lai, A. Benedick, S.-W. Huang, S. BhardwajA. Siddiqui, V. GkortsasB. Putnam, Li-Jin Chen
Research Scientists: K.-H. Hong, J. Moses
Postdocs and Visitors:
G. Cirmi (Politecnico Milano, Rocca Foundation)
A. Gordon (Technion, Israel)O. Muecke (Techn. Univ. Vienna)E. Falcao (Pernambuco, Brazil)
3
Outline Required Seed Power Levels
Single Pass Efficiencies in High Harmonic Generation
Wavelength Scaling of HHG
Seed Generation for High Repetition Rate FELS
A High Average Power HHG Source for 13.5 nmpumped by 515 nm Lasers (SHG of 1030nm), where powerful Yb-doped lasers exist
4
Required Seed Power Levels
Direct Seeding: 100 kW (30fs) 3 nJ
Seeding for HGHG: 100 MW (30fs) 3 µJ
Push direct seed wavelength as short as possible. How does efficiency scale?
Repetition rate determines drive power:
Efficiency determines required drive pulse energy
Efficiency: 10-6 Pulse energy 3 mJ // 3 J
Rep. Rate 1kHz / 10MHz Power: 3W / 30kW // 3 kW / 30MW
5
High Harmonic Generation
212XUV pI mxω = +
Corkum, 1993 Cutoff formula
ħωmax = Ip+3.17 Up
Elec
tric
Fie
ld, P
ositi
on
Time
Ionization
Three-Step Model Trajectories
6
Wavelength Scaling of HHG Efficiency
20
max 23.174pE
Iωω
= +1m e= = =
Atomic units Field amplitude
Drive pulse frequency
0EIncrease intensity
ω Decrease frequency(increase wavelength)
Ionization potential
What is the impact on HHG conversion efficiency?1.) Single-Atom Response2.) Gas properties3.) Phase matching
[ ])(1)1(
1)(
),(20236.0 0
24
)1(4
223/160
2250
cutoff
N
cutoffcutoff
recp tbEwNE
LkgaIκβ
ββ
σ
ωη +
−−
ΩΩ
∆=
−
HHG efficiency for N-cycle flat top pulseCutoff
( ) 92/9250
50 ~~/ −−Ω λω pcutoff UE
200 400 600 8001E-5
1E-4
1E-3
0.01
0.1
1
σ (a
.u.)
EnergyHHG (eV)
He Ne Ar Kr Xe
200 400 600 800
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
a rec
EnergyHHG (eV)
He Ne Ar Kr Xe
E. L. Falcão et al., Opt. Expr. 17, 11217 (June, 2009).
8
HHG Efficiency into Single Harmonic
800-nm (Xe)
400-nm driver (He)
• Conversion efficiency very sensitive todrive wavelength andinteraction parameters
800-nm driver (He)
9
Experimental HHG Setup
800-nm Ti:S amplifier(1 kHz, 7 mJ)
HHG chamber
Telescope &Beam delivery
Beam input port
Beam transport
Pulsed nozzle
Soft-X-ray spectrometer
30 40 50 60 700
300
600
900
HeHH In
tens
ity (A
rb. U
.)
Photon Energy (eV)
0
4000
8000
12000 Ne
0
15000
30000
45000Ar
• Pulse energy of 0.94 mJ for all gases• Peak intensity: ~7.8x1014 W/cm2 (estimation)• Nozzle length: 2 mm
HHG spectra generated by 400-nm driver
Ar: 0.05 mbarNe: 0.3 mbarHe: 1 bar
Ar: 0.05 barNe: 0.3 barHe: 1 bar
Total HHG efficiency from 400-nm driver
• Conversion efficiency of up to 2x10-4 from He over Al window• “Good” agreement to analytic theory [1]
E. L. Falcão-Filho et al., Opt. Express 17, 11217 (June, 2009).
0.0 0.2 0.4 0.6 0.8 1.01E-6
1E-5
1E-4
1E-3
Al window (20-70 eV)
He Calculation with Ar Calculation with He
400-nm pulse energy (mJ)
ArNeCo
nver
sion
Effic
ienc
y
Efficiency per harmonic from 400-nm driver
• 8x10-5 at ~35 eV and 1x10-5 at ~60 eV for He• 6x10-5 at ~27 eV for Ar
Peak intensity: ~1.6x1015 W/cm2
40 50 60 70 80 90 100 1100
20
40
60
80
100
HH In
tens
ity (A
rb. U
.)
Photon Energy (eV)
40 50 60 70 80 90 100 1100
100
200
300
HH
Inte
nsity
(Arb
. U.)
Photon Energy (eV)
HHG spectra generated from 800-nm driver
He: 1 barEnergy: 2 mJ
Ne: 0.3 barEnergy: 2 mJ
Total HHG efficiency from 800-nm driver
• Conversion efficiency of up to 2x10-6 from He over Al and Zr window• Efficiency per harmonic is one-to-two-order-of-magnitudes lower.
Zr window(60-100 eV)
Al window(20-70 eV)
15
Comparison with previous results
800-nm driver (He)
400-nm driver (He)
• Conversion efficiency very sensitive to the driving wavelength • But predictable from our analytic theory that has shown a good agreement
to experimental results studied by 400-nm and 800-nm drivers.
In final OPA stage:• Yb:YAG pump replaces Nd:YLF• BBO replaces MgO:PPSLT
2-µm drive laser based on cryo-Yb:YAG pump laser
MgO:PPLN
DFGMgO:PPLN
OPA 1λ = 2.0 µm 140µJ
Si
2.5 mJ,30 fs
SuprasilMgO:PPSLT
OPA 2
30 mJ
BBO
OPA 3
1 mJ
800-nm OPCPA
seed 800-nm OPCPApump
Nd:YLF CPA systemCFBG, 2 YDFA, Nd:YLF regen amp +
2 Nd:YLF multipass amp, grating stretcher12 ps, 4 mJ @1kHz
Ti:Sapphireoscillator
Yb:YAG CPA systemCFBG, YDFA, Yb:YAG regen amp +
Yb:YAG multipass amp, grating stretcher15 ps, 30 mJ @1kHz
λ = 1.0 µm
AOPDF
2.2-µm drive wavelength extends HHG cutoff to 500 eV Conversion efficiency of 10-7-10-8
Best current water-window experimental result: 300 eV cutoff, η ~ 5x10-8, using multi-mJ 1.6-µm drive pulsesE. J. Takahashi et al., PRL 101, 253901 (2008).
Gaussian pulse, τFWHM = 6 cycles
Ne gas, p = 3 bar, L = 2.5 mm, w0 = 50 µm, E ~ 1 mJ
Theoretical Prediction
Simulation parameters:
High-flux, High Repetition Rate 13.5-nm
(~93 eV) EUV source
30 40 50 60 70 80 90 100 1100
2
4
6
8
10
Effic
ienc
y (1
0-6)
Energy (eV)0.6 0.7 0.8
1E-4
1E-3
0.01
0.1
1
10
Effic
ienc
y (1
0-6)
Pump Energy (mJ)
515 nm 400 nm
• With 515-nm drive pulses generated from SGH of powerful 1µm lasers
Efficiency into single harmonic: ~ 10-5
19
High Intensity Femtosecond Enhancement Cavities for High Repetition Rate FELs
Use enhancement cavity to scale efficiency to ~ 10-2
High-Power Enhancement Cavity
20
Requirements: optical beam access, high-intensity in interaction region, and low loss
1-MW intracavity power, 10 mJ, ~100 fs pulses circulating Cavity Finesse > 3000
15 cm
2.6 mm
patterned dielectric mirror
Confocal cavity for high-intensity Bessel-Gauss beams – Cavity shown enables 1000 TW/cm2
1000 TW/cm2
0.1 TW/cm2
21
Preliminary Cavity Demonstration
Single-mode HeNe source
Beam Expander
Pellicle
CCD
Photodiode
Polarizerλ/2
R=91%R=99%
20μm Piezo2μm Piezo
LPF PI
42 kHz
Lock-in Amp
First demonstration of cavity operation is carried out with CW laser. Also, axicon coupling optics excluded. Instead, collimated beam is used allowing measurement of intrinsic suppression of higher modes.
22
Cavity Results With One Patterned Mirror
Pellicle (loss<1%)
CCD
R = 91% or 99%R = 99%
First cavity experiments done with single patterned mirror
Asymmetric modes seen, showing general structure of desired modes, but differing transverse profiles
Transverse profiles at cavity center
R=91%
R=99%
23
~30 modes with <1% loss
only 2 modes (superposition modes in each direction) with <1% loss, next
higher mode >5% loss
Cavity Results With One Patterned MirrorLo
ss
Mode
One Patterned Mirror
Loss
Mode
Two Patterned Mirrors
24
Thank YouNeeds large average power Yb-doped Lasers!
25
Analytical Bessel-Gauss Form of ModesThe cavity modes have been analyzed numerically with custom paraxial wave optics software package. They can also be understood from an analytical perspective as Bessel-Gauss beams.
Bessel-Gauss beam is a superposition of tilted Gaussian beams with wavevectors lying along the surface of a cone,
Tilted Gaussian Beam
26
Analytical Bessel-Gauss Form of ModesBessel-Gauss beams traversing paraxial optical systems transform with an ABCD matrix similar to a Gaussian beam. Bessel-Gauss beams can then be shown to be modes of the confocal resonator, and the dominant modes of our special cavity.
Bessel-Gauss Modified Bessel-Gauss
Numerically computed mode
Analytical Bessel-Gauss mode
Field profile at focus: numerical versus analytical
solution
Pump laser upgrade > 50 mJ, 2 kHz, 10 ps
(c) Yb:YAG 4-pass amplifier
fs, Yb-fiber oscillator
CFBG stretcher
λ/4F1029
PBS
1 mW 400 ps
λ/4λ/2
FI
Telescope
30-mW Yb-fiber preamplifier (1030 nm)
>40 W
Yb:YAG crystal
PCTFP
(b) Yb:YAG regenerative amplifier
TFP
FI
seedRegen output5 mJ@2 kHz
Fiber-coupled LD
λ/4
Telescope
(a) Fiber seed
>60 mJ@2 kHz
Telescope
10 ps, >50 mJ@2 kHz
(d) Multi-layer dielectric grating compressor
LN2 Dewar Yb:YAG crystals
Fiber-coupled pump laser
DM
λ/4
DML1 L2 L2 L1
TelescopeTFP
TFP
Telescope
27
28
SummarykW-class cryogenically cooled Yb:YAG ps-lasers are ideal for
Inverse Compton Scattering Sources (direct use)-> 2nd generation synchrotron like laboratory sources with
exceptional beam properties
micron sized source ideal for phase contrast imaging
fs-pulse durations ideal for time resolved x-ray diffraction
Pumping of few cycle OPCPAs covering the visible to MID IR range
Analytic HHG efficiency formulas and wavelength scaling
Development of few-cycle 2-µm OPCPA (200 µJ)
Initial results on 800 nm OPCPA
29
(a) For a 5-cycle-driver-pulse, ∆k = 0, L = 5 mm at 1 bar.
(b) Same as (a) includingplasma and neutral atom phase mismatching.
Efficiency Measurement using Calibrated XUV PhotodiodeAt 40eV, Al transmission = 30%, photodiode response = 4 electrons/photon
( )19
4030% 4 1.6 10 1000 30
ph phHH
I eV I JE pulseA−
×= =
× × × ×
photodiode response