Ultrafast coherent diffraction using soft and hard X-rays is actually revolutionizing imaging science thanks to new sources recently available. This powerful technique extends standard X-ray diffraction towards imaging of non-crystalline objects and leads actually to a strong impact in physics, chemistry and biology. New ultrashort pulses recently available hold the promise of watching matter evolving with unprecedented space and time resolution. Femtosecond coherent and intense radiation in the soft X-ray (λ = 10-40 nm) is currently produced in our laboratory, from highly non linear frequency conversion (high harmonic generation). A high intensity UV-X coherent beam is obtained using a loose focusing geometry, which allows coupling a very high amount of Ti:Sapphire laser system energy in the HHG process. Using a long gas cell and a long focal length lens, the emitting volume can be increased by orders of magnitude compared to standard HHG set-ups. This approach, allows reaching up to 1x1011 photons per shot for the 25th harmonic (λ=32nm). We have already demonstrated nanoscale imaging in a single shot mode reaching 70 nm spatial resolution and 20 femtoseconds snapshot . We then implemented a recently proposed holographic technique using extended references. This technique, easy to implement, allows a direct non iterative image reconstruction. In the single shot regime, we demonstrated a spatial resolution of 110nm .This opens fascinating perspectives in imaging dynamical phenomena to be spread over a large scientific community. I will present recent results in the investigation of femtosecond phase spin-reversals of magnetic nano-domains . Finally, I will report on recent development on noise sensitivity of the technique and perspectives in attosecond coherent imaging .
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The recent development of intense sources in the XUV range (10-100 nm), such as X-ray laser, Free Electron Laser and
High order Harmonics (HoH), allows the study of high flux processes and ultra-fast dynamics in various domains.
At the SLIC facility of CEA-Saclay, we have built a gas-harmonic beamline to investigate the interaction of intense
XUV pulse with solids. High Harmonics of an IR laser (Ti:Sa at 800 nm, 35 fs, 13 mJ/pulse, 1 kHz) are generated in a
rare gas cell (Xe). The useful XUV range (40-60 nm) is selected with metallic filters. The harmonic beam is focused with
a parabolic mirror to a 10 μm focal spot on sample, leading to a fluence per shot of up to 1 mJ/cm2 (within a typical 10 fs
Studies aimed at understanding the damaging mechanisms caused by XUV irradiation on surface of various samples by
systematically varying of fluence and exposure time.
For PMMA irradiated in the desorption regime (fluence/shot ≤ 0.2 mJ/cm2), the surface presents craters whose profile
depends on the dose (Grey [Gy] = 1 J/kg). The crater evolution proceeds from the competition between two main
degradation processes, that is chain scission and cross linking. Namely, at low dose (≤ 1 GGy) polymer chain scission is
followed by the blow up of the volatile, molecular fragments, forming the crater. At high dose (> 10 GGy) the broken
chain-ends, in the near-surface layer of the remaining material, recombine by cross-linking, opposing desorption by
In a recent experiment at LCLS FEL facility, PMMA was irradiated at high fluence; the cross-linking signature was
identified from Raman spectroscopy. A kinetic model could be adapted for interpreting these original and very promising
The new XUV sources, which deliver spatially coherent pulses of high peak power, allow to study elementary
processes in the light/solid interaction in the high intensity regime (⩾1011W/cm2). Here, we report two
studies which have used high-order laser harmonics (HH) generated in gas as the excitation source. Firstly, we
have investigated the dynamics of electron relaxation in the wide gap CdWO4 dielectric crystal, an efficient
scintillator material, using time-resolved luminescence spectroscopy. The kinetics decay of luminescence shows
evidence of non radiative relaxation of the self-trapped excitons at the &mgr;s damage to surfaces of poly(methyl
methacrylate) - PMMA, induced by a multi-shot XUV-irradiation (1 kHz reprate) for given fluence, below
damage threshold range of ≈mJ/cm2. The main processes participating in the surface modification, polymer
chain scission followed by the blow up of the volatile, molecular fragments and cross-linking in the near-surface
layer of remaining material, are tentatively identified and associated to, crater formation for short-time exposure
(< 1min) and surface hardening for long-time exposure (⩾1min).
Light sources capable to deliver intense and ultrashort pulses in the VUV domain, based on free electron lasers or on the
high order harmonic generation have appeared recently. They bring the possibility to explore a new domain in the
field of laser matter interaction. Such sources are available in the visible or near IR range -specially at 800 nm, thanks to
Ti-Sa lasers - since more than ten years, and the interaction of femtosecond pulses with solids has been studied in great
details. In this paper we will discuss how the knowledge which has been acquired in the visible domain can be used for
the VUV studies. I will concentrate on the case of wide band gap dielectric materials (SiO2, MgO, Al2O3), and on the
intensity domain around breakdown and ablation threshold. This type of material is interesting not only because they are
involved in numerous applications, but above all because their band gap (Eg) lying in the range 6 to 10 eV, a clear
distinction can be made for what concern their interaction with visible (hνEg). We discuss here
two important aspects that must taken into account to understand the energy balance of the interaction. The first is the
energy distribution of photoexcited carriers, which are clearly different in the case of visible or VUV light.
Photoemission spectroscopy demonstrate that the distribution highly depends upon the incident intensity in the visible
and near IR, and can be "warmer" than the one observed by irradiation with VUV photon, despite their much larger
energies. The second important parameter is the excitation density achieved during the excitation. Experiments carried
out in the IR using the technique of time resolved interferometry allow to measure the density of electrons excited in the
conduction band at intensities above and below the optical breakdown threshold. The results show that in the process of
laser breakdown multiphoton excitation dominates the avalanche process for picosecond and subpicosecond pulses. The
simulations performed to interpret these measurements can be used to predict the damaging mechanism of wide band gap
dielectrics submitted to ultra intense VUV pulses.
This communication describes the research work plan that is under implementation at the SPARC FEL facility in the
framework of the DS4 EUROFEL programme. The main goal of the collaboration is to study and test the amplification
and the FEL harmonic generation process of an input seed signal obtained as higher order harmonics generated both in
crystals (400 nm and 266 nm) and in gases (266 nm, 160 nm, 114 nm). The SPARC FEL can be con-figured to test
several cascaded FEL layouts that will be briefly analysed.
The multi-mJ, 21-nm soft-x-ray laser at the PALS facility was focused on the surface of amorphous carbon (a-C) coating, developed for heavily loaded XUV/x-ray optical elements. AFM (Atomic Force Microscopy) images show 3-micrometer expansion of the irradiated material. Raman spectra, measured with an Ar+ laser microbeam in both irradiated and unirradiated areas, confirm a high degree of graphitization in the irradiated layer. In addition to this highfluence (~ 1 J/cm2), single-shot experiment, it was necessary to carry out an experiment to investigate consequences of prolonged XUV irradiation at relatively low fluence. High-order harmonic (HH) beam generated at the LUCA facility in CEA/Saclay Research Center was used as a source of short-wavelength radiation delivering high-energy photons on the surface at a low single-shot fluence but with high-average power. a-C irradiated at a low fluence, i.e., < 0.1 mJ/cm2 by many HH shots exhibits an expansion for several nanometers. Although it is less dramatic change of surface morphology than that due to single-hot x-ray-laser exposure even the observed nanometer-sized changes caused by the HH beam on a-C surface could influence reflectivity of a grazing incidence optical element. These results seem to be important for estimating damages to the surfaces of highly irradiated optical elements developed for guiding and focusing the ultraintense XUV/x-ray beams provided by new generation sources (i.e., VUV FEL and XFEL in Hamburg; LCLS in Stanford) because, up to now, only melting and vaporization, but not graphitization, have been taken into account.