Ultrafast XUV Coherent D iffractive I maging

1. Ultrafast XUV Coherent Diffractive Imaging Xunyou GE, CEA Saclay Director : HamedMerdji

2. Outline • Coherent Diffractive Imaging (CDI) • Optimisation of the highorderharmonicbeam line at CEA Saclay • Holography with Extended Reference by AutocorrelationLinearDifferentialOperator (HERALDO) • Laser modal filterusing a hollowcorefiber • Pump-probe experiment of magnetic sample in preparation

3. Phase lost Coherent Diffractive Imaging : LenslessImaging CCD camera Can the phase be recovered? Measured diffracted intensity L Use a phase retrieval iterative algorithm to “guess” the phase D no high quality optics for imaging! Short wavelength mm sample The spatial resolutionislimited by the diffractive angle and the wavelength : • Short wavelength (in XUV domain) • Coherent beam • High SNR • Ultrashort pulse duration Free Electron Laser Source requirements: High Order Laser Harmonics

4. TF Support Constraint 2 : autocorrelation pattern The reconstruction should be inside the autocorrelation pattern of the object. To retrieve the phase : two constraints Constraint 1 : The diffraction pattern intensity The module square of the FT of the reconstruction should be equal to the measured intensity.

5. Recent demonstration at CEA Saclay The music note, AttoPhysique SPAM CEA, PRL 2009 3 µm Reconstruction image Spatial resolution = 119 nm Test object MEB Image Diffraction pattern in single shot (20 fs pulse duration) Ravasio et al. PRL 2009

6. user chamber The Harmonicbeamline spectrometer optics chamber HHG chamber only 5m!! laser

7. The Harmonicbeamline for Coherent Diffraction Imaging Off axis parabolic mirror (multilayer coating) Laser parameters: 800 nm ~15 mJ 60 fs 20 Hz At the source: H25 (l=32 nm) 4.1010ph/shot ~ 0.25 µJ 150 nm thick Al filter Focal length 5.65m CCD camera IR antireflective mirror ~ 8cm gascell (Argon) On sample: H25 (32 nm) spectral linewidth λ/Δλ ~ 150 temporal duration ~ 20 fs 2x109 photons/shot Spot size= 5*5mm² 92% fringe contrast Young slit @ 100µm (15% of beam)

8. Optimization of the High orderHarmonicGeneration • XUV wave front sensor • - Signal intensity • - Wave front profile • Aberration coefficients • Reconstruction of the focus spot Reconstructed profile of the harmonic source IR pump laser Pupil of IR laser =24 mm GasCell Mirror -0,20 0 0,20 (mm) Non optimized source Al filters Hartmann grid Pupil of IR laser=21 mm CCD -0,05 0 0,05 (mm) Optimized source

9. Optimization of the High orderHarmonicGeneration The optimized value range for each parameter: - Laser energy focalised in the cell = 15 mJ - Beam aperture = 20~21 mm - Gas pressure = 8~9 mbar - Cell length = 5~8 cm Wave front @32 nm RMS = 0.113l = l/7 The beamqualityistwice diffraction limited.

10. Optimization of the parabola 0,44λ Raw alignment -0,44λ CCD IR pump laser -7,5 0 7,5 (mm) RMS=0.177λ (λ=32nm) Front d’onde sur la grille Hartmann Gascell Hartmann grid Optimized alignment Mirror Al filters Objet test Focus spot of the parabola With spatial filter Off axis parabola Ø 3µm

11. Improved reconstruction quality 1μm 1μm Test Object MEB image Diffraction pattern in single shot (20 fs) Spatial resolution = 78 nm = 2.5 λ In single shot (20 fs)

12. HERALDO Soft X-ray holography with extended reference by autocorrelation linear differential operator (HERALDO) is a more general approach to Fourier transform holography (FTH). LinearDifferentialOperator FT Reconstruction Object with a slit reference Autocorrelation • Fourier transform holography => The phase is encoded in the interferential • fringes of the hologram • Extended reference => increase the interferential fringes visibility of the • hologram • The reconstruction process => non-iterative and non ambiguous

13. HERALDO : experimentresults Test object Reconstruction in vertical direction 3,5 µm Coherent averaging Reconstruction process Final reconstruction Spatial resolution = 105 nm In single shot (20 fs) (Gauthier et al., PRL 2010) Diffraction pattern in single shot (20 fs) Reconstruction in horizontal direction

14. Laser modal filterusing a hollowcorefiber 100 mJ psduration + compressor Fiber (in vacuum) length=30 cm, φ=250μm 140mJ psduration Lens (focal length=750 mm) Hollowcorefiber (in vacuum) Parabolicmirror compressor Laser modal filter: afocal ~40 mJ 60 fs Ti:saphire 140mJ, 200 ps sample lens CCD Gascell Focal lens m m Reconstructed laser profile filtered by the fiber A quasi EH11 mode of IR laser Energy transmission = 70 % Reconstructed laser profile before injection in the fiber

15. Laser modal filterusinga hollowcorefiber Reconstruction of the laser focus spot in the gascell (~30mJ/shot) Preliminaryresults : wedoubled the harmonicgenerationefficiency. withfiber withoutfiber m m 25th Harmonicbeam profile in far field Without modal filter With modal filter Laser energy focused into cell 15 mJ 8 mJ Harmonic photons @ 32 nm 2x109 2x109 Focus spot of the parabola elliptical shape, sometimes 2 or 3 spots one quasi circular spot Pulse stability not stable stable We are now working on the optimization of the HHG process with the modal filter.

16. Pump-probe experiment in preparation Static imaging already done at BESSY @ 1.59 nm (Co L3 edge) Can we watch the magnetic domains change on a fs time scale? Circularly polarized “Imaging” pulse @ ≈ 59eV (Co M edge)/53 eV (Fe M edge) Eisebitt et al., Nature (2004) UV polarizer Circularly polarized IR for all optical switching (Stanciu et al., PRL (2007))

17. Merci