An intense kHz and aberration-free two-colour high harmonic source for seeding FELs

1. Palaiseau - FRANCE 25/08/09 An intense kHz and aberration-free two-colour high harmonic source for seeding FELs from EUV to soft X-ray range G. LAMBERT guillaume.lambert@ensta.fr J. Gautier, C.P. Hauri, Ph. Zeitoun, C. Valentin, F. Tissandier, T. Marchenko,J.Ph. Goddet, M. Ribiere, G. Rey, A. Sardinha, M. Fajardo, G. Meynard and S. Sebban Thanks to Imagin Optics UMR 7639 http://loa.ensta.fr/

2. Motivations for seeding with High order Harmonics (HH) Goal: femtosecond, high repetition rate, fully coherent FEL light at short wavelengths with high stability and high tuneability in a short scale facility => Seeding with HH produced in gas Harmonics properties Already obtained -fs pulse duration -Full coherence -High repetition rate: kHz HH currently First MHz HH in xenon (J. Boullet et al. optics letters, 34, 1489 (2009)) To be improved -Intensity at short wavelengths: for instance: needed from 32 nm to 13 nm at sFLASH in 2010 -Tuneability (only odd HH): => need to considerably chirp the driving laser and/or change the gap of the undulator -Wavefront: diffraction limited beam (aberration-free) => need to drastically clip the amplified HH beam or use adaptive optics for HH -Stability of the shot to shot intensity Keep the simplicity of the classical HH setup

3. High order harmonics generated with a two-colour field ω Harmonics 2ω Noble gas Ti: Sa IR Filter ~1014 W/cm2 BBO crystal HHG Lens x WE=ex.Esinwt Ip Tunnel ionization t~T/4 Oscillation in the laser field t~3T/4 Radiating recombination t~T Initial state t~0 G. Lambert et al. to be published in New Journ. of Phys. Neon Theoretical and technical principles: X 25 ω +2ω Semi-classical model in three steps: -increase of the number of photons -double harmonic content even types: 2x(2n+1) from 2w 2x(2n) from the mixing -redshift:

4. General set-up Transmission grating (2000 lines/mm) CCD Camera Iris Spectrometer 1 dt ω Intensity (arb. un.) 0.5 2ω Δt2ω 0 -60 -30 0 30 60 Time (fs) BBO crystal, (100 mm or 250 mm thickness) Aluminium filters (200 nm thickness) Spherical multilayer mirror 800 nm 1 kHz < 7 mJ 35 fs Lens, f=1.5 m Calibrated XUV photodiode CCD Camera Grid Gas cell (4 to 7 mm long) Wavefront sensor Simple system but only relative control on the 2nd harmonic generation parameters (mainly the BBO thickness): -I2w/Iw (also depending on the laser intensity) -Temporal shift (dt): 18 fs every 100 mm -Δt2w/ Δtw (also depending on the laser chirp)

5. Harmonic spectra obtained with either ω or ω+2ω technique 104 103 102 101 He, ω He, ω +2ω Ne, ω Ne, ω +2ω Ar, ω Ar, ω +2ω Xe, ω Xe, ω +2ω 100 10 15 20 25 30 35 40 45 50 55 60 65 70 75 100 µm thick BBO crystal, and with the optimization parameters corresponded to ω: Eω=6 mJ, LC=7-9 mm and PG=30-35 mbar Al cut (x 0.5) (x 0.5) -Flat spectra (same intensity level for odd and even harmonics) (x 25) -Increase limited at high wavelengths due to an already relatively high efficiency for Xe and Ar Intensity (arb. un.) (x 100) => ω+2ω technique compensates the weak efficiency at short wavelengths Wavelength (nm) G. Lambert et al. to be published in New Journ. of Phys.

6. Optimization of the flux and of the wavefront (Ar gas) 50 40 Iω~3.9 1 I2ω~2.9 Iω~4.4 30 I2ω~5.4 Iω~0.8 I2ω~1 20 0.5 λ l/5 rms l/17 rms l/6 rms 10 0 35 40 45 50 55 60 0 -iris clipping technique: change the focusing geometry/energy clean the major part of the distortions in the outer part of the beam: l to l/6 rms Iωand I2ωin 1014 W.cm-2 H18 100 mm thick BBO crystal H14 Φ=40 mm -ω (LC=8 mm and PG=30 mbar) to ω+2ω (LC=4 mm and PG=16 mbar) Φ=40 mm Energy per pulse (nJ) -very high increase on 2x(2n+1) type of even harmonics (50 nJ) due to strong blue/IR and distortions limited: l/5 rms Φ=20 mm Φ=20 mm Iω~1.2 H22 -iris clipping: limited decrease of intensity But distortions about l/17 rms: First aberration-free high harmonic beam H19 H17 H21 H23 H25 H27 H15 30 Wavelength (nm) G. Lambert et al. submitted to EPL

7. Summary of the two-colour HH properties Already obtained -fs pulse duration -Full coherence -High repetition rate: kHz to MHz soon -Intensity at short wavelengths -Tuneability:both odd and even harmonics Use parametric amplifier (1.2-1.5 mm) -Wavefront: aberration-free beam -Simple system To be improved -Stability of the shot to shot intensity?

8. Thank you for your attention J. Gautier, C.P. Hauri, Ph. Zeitoun, C. Valentin, F. Tissandier, T. Marchenko, J.Ph. Goddet, M. Ribiere, G. Rey, A. Sardinha, M. Fajardo, G. Meynard and S. Sebban Imagin Optics LaserlabIntegrated Infrastructures Initiative RII-CT-2003-506350 TUIXS European project (Table top Ultra Intense XUV Sources) FP6 NEST-Adventure n.012843 UMR 7639 http://loa.ensta.fr/