Ultrafast XUV Coherent Diffractive Imaging
Xunyou GE, CEA SaclayDirector : 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
)( rfsample
CCD camera
mm
L
D
)( k
Phase lost
Measured diffracted intensity
2)( kfTF
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 :
L
Dr
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 imageSpatial resolution = 119 nm
Diffraction pattern in single shot (20 fs pulse duration)
Test objectMEB Image
3 µm
The Harmonic beamline
user chamber
HHG chamber
laser only 5m!!
optics chamber
spectrometer
H25 (l=32 nm)4.1010 ph/shot~ 0.25 µJ
Laser parameters:800 nm~15 mJ60 fs20 Hz Focal length 5.65m
IR antireflective mirror
150 nm thick Al filter
92% fringe contrast Young slit @ 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:
2x109 photons/shotSpot size= 5*5mm²
On sample:
The Harmonic beamline for Coherent Diffraction Imaging
15 20 25 30 35 40 450,00
0,05
0,10
Energie photons (eV)
sans filtre avec filtre Al (100 nm)
Sig
nal R
H [a
.u.]
9
H25
Ar
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
16 17 18 19 20 21 22 23 24 25 26 27 280
0.1
0.2
0.3
0.4
0.5
RMS function of the beam aperture(Gas pressure=8 mbar, Cell length=8cm)
16 17 18 19 20 21 22 23 24 25 26 27 280
0.1
0.2
0.3
0.4
0.5
RMS function of the beam aperture(Gas pressure=8 mbar, Cell length=8cm)
2 3 4 5 6 7 8 9 10 11 12 130.02
0.04
0.06
0.08
0.1
0.12
0.14
RMS function of the cell length(Beam aperture=21mm, Gas pressure=8 mbar)
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 beam quality is twice diffraction limited.
Front d’onde sur la grille HartmannRMS=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
Rawalignment
Optimizedalignment
With spatialfilter
Ø 3µm
Improved reconstruction quality
Spatial resolution = 78 nm = 2.5 λIn single shot (20 fs)
1μm
Diffraction pattern in single shot (20 fs)
Test ObjectMEB image
1μm
HERALDO
Object with a slit reference
Autocorrelation Reconstruction
FTLinear 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 reconstructionSpatial 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:saphire140mJ, 200 ps
compressor
Focal lens
CCD
Parabolic mirror
sam
ple
Gas cell afocal
Hollow core fiber (in vacuum)
lens
~40 mJ60 fs
Lens (focal length=750 mm)
Fiber (in vacuum)
length=30 cm, φ=250μm140mJps duration
100 mJps duration
+ compressorLaser 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 mJHarmonic 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.
without fiber with fiberm
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 BESSY @ 1.59 nm (Co L3 edge)
(Stanciu et al., PRL (2007))
UV polarizer
Can we watch the magnetic domainschange on a fs time scale?
Circularly polarized “Imaging” pulse @ ≈ 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