Ultrafast XUV Coherent Diffractive Imaging Xunyou GE, CEA Saclay Director : Hamed Merdji

Outline Coherent Diffractive Imaging (CDI) Optimisation of the high order harmonic beam line at CEA Saclay Holography with Extended Reference by Autocorrelation Linear Differential Operator (HERALDO) Laser modal filter using a hollow core fiber Pump-probe experiment of magnetic sample in preparation

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

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

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

The Harmonic beamline user chamber HHG chamber laser only 5m!! optics chamber spectrometer

H25 ( =32 nm) ph/shot ~ 0.25 µJ Laser parameters: 800 nm ~15 mJ 60 fs 20 Hz Focal length 5.65m IR antireflective mirror 150 nm thick Al filter 92% fringe contrast Young 100µm (15% of beam) H25 (32 nm) spectral linewidth λ/Δλ ~ 150 temporal duration ~ 20 fs Off axis parabolic mirror (multilayer coating) CCD camera ~ 8 cm gas cell (Argon) At the source: 2x10 9 photons/shot Spot size= 5*5  m² On sample: The Harmonic beamline for Coherent Diffraction Imaging

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

Optimization of the High order Harmonic Generation 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 nm RMS = = /7 The beam quality is twice diffraction limited.

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

Improved reconstruction quality Spatial resolution = 78 nm = 2.5 λ In single shot (20 fs) 1μm1μm Diffraction pattern in single shot (20 fs) Test Object MEB image 1μm1μm

HERALDO Object with a slit reference Autocorrelation Reconstruction FT Linear Differential Operator Soft X-ray holography with extended reference by autocorrelation linear differential operator (HERALDO) is a more general approach to Fourier transform holography (FTH). 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

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

Laser modal filter using a hollow core fiber Ti:saphire 140mJ, 200 ps compressor Focal lens CCD Parabolic mirror sample Gas cell afocal Hollow core fiber (in vacuum) lens ~40 mJ 60 fs Lens (focal length=750 mm) Fiber (in vacuum) length=30 cm, φ=250μm 140mJ ps duration 100 mJ ps duration + compressor Laser modal filter : Reconstructed laser profile before injection in the fiber Reconstructed laser profile filtered by the fiber A quasi EH11 mode of IR laser Energy transmission = 70 % m m

Laser modal filter using a hollow core fiber Without modal filter With modal filter Laser energy focused into cell 15 mJ 8 mJ Harmonic 32 nm 2x10 9 2x10 9 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. without fiber with fiber m m Preliminary results : we doubled the harmonic generation efficiency. Reconstruction of the laser focus spot in the gas cell (~30mJ/shot) 25th Harmonic beam profile in far field

Static imaging already done at 1.59 nm (Co L 3 edge) (Stanciu et al., PRL (2007)) UV polarizer Can we watch the magnetic domains change on a fs time scale? Circularly polarized “Imaging” ≈ 59eV (Co M edge)/53 eV (Fe M edge) Circularly polarized IR for all optical switching Eisebitt et al., Nature (2004) Pump-probe experiment in preparation

Merci