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