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/cm<sup>2</sup> (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
We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ~15fs pulse of containing >10<sup>12</sup> photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 10<sup>18</sup> W/cm<sup>2</sup>. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given.
We exposed standard Mo/Si multilayer coatings, optimized for 13.5 nm radiation to the intense femtosecond XUV
radiation at the FLASH free electron laser facility at intensities below and above the multilayer ablation threshold. The
interaction process was studied in-situ with reflectometry and time resolved optical microscopy, and ex-situ with optical
microscopy (Nomarski), atomic force microscopy and high resolution transmission electron microscopy. From analysis
of the size of the observed craters as a function of the pulse energy the threshold for irreversible damage of the multilayer
could be determined to be 45 mJ/cm<sup>2</sup>. The damage occurs on a longer time scale than the XUV pulse and even above the
damage threshold XUV reflectance has been observed showing no measurable loss up to a power density of 10<sup>13</sup> W/cm<sup>2</sup>.
A first explanation of the physics mechanism leading to damage is given.
The beam of Free-Electron Laser in Hamburg (FLASH) tuned at either 32.5 nm or 13.7 nm was focused by a grazing
incidence elliptical mirror and an off-axis parabolic mirror coated by Si/Mo multilayer on 20-micron and 1-micron spot,
respectively. The grazing incidence and normal incidence focusing of ~10-fs pulses carrying an energy of 10 μJ lead at
the surface of various solids (Si, Al, Ti, Ta, Si<sub>3</sub>N<sub>4</sub>, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate)
- PMMA, stainless steel, etc.) to an irradiance of 10<sup>13</sup> W/cm<sup>2</sup> and 10<sup>16</sup> W/cm<sup>2</sup>, respectively. The optical emission of the
plasmas produced under these conditions was registered by grating (1200 lines/mm and/or 150 lines/mm) spectrometer
MS257 (Oriel) equipped with iCCD head (iStar 720, Andor). Surprisingly, only lines belonging to the neutral atoms
were observed at intensities around 10<sup>13</sup> W/cm<sup>2</sup>. No lines of atomic ions have been identified in UV-vis spectra emitted
from the plasmas formed by the FLASH beam focused in a 20-micron spot. At intensities around 10<sup>16</sup> W/cm<sup>2</sup>, the OE
spectra are again dominated by the atomic lines. However, a weak emission of Al<sup>+</sup> and Al<sup>2+</sup> was registered as well. The
abundance ratio of Al/Al<sup>+</sup> should be at least 100. The plasma is really cold, an excitation temperature equivalent to 0.8 eV was found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also
registered, both from the plasmas (typical is for carbon; there were no spectral lines) and the scintillators (on Ce:YAG
crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer).
Single shot radiation damage of bulk silicon induced by ultrashort XUV pulses was studied.
The sample was chosen because it is broadly used in XUV optics and detectors where
radiation damage is a key issue. It was irradiated at FLASH facility in Hamburg, which
provides intense femtosecond pulses at 32.5 nm wavelength. The permanent structural
modifications of the surfaces exposed to single shots were characterized by means of phase
contrast optical microscopy and atomic force microscopy. Mechanisms of different, intensity
dependent stages of the surface damage are described.
Ultra-fast soft x-ray lasers have opened a new area of laser-matter interactions which in most cases differ from the well
understood interaction of UV-vis radiation with solid targets. The photon energy >30eV essentially exceeds the width of
band gap in any known material and excites the electrons from the deep atomic and valence levels directly to the
conduction band. Both thermal and non-thermal phenomena can occur in such a material being caused by electron
thermalization and bond breaking, respectively. We report the first observation of non-thermal single-shot soft x-ray
laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate) - PMMA.
Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7nm, the samples have been
investigated by an atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by
the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze singleshot
imprints in PMMA. An intermediate regime of materials removal has been found, confirming the model predictions.
We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32nm as
a proof of an efficient material removal in the desorption regime.
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 (⩾10<sup>11</sup><i>W/cm</i><sup>2</sup>). 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 <i>CdWO</i><sub>4</sub> 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 <i>kHz</i> reprate) for given fluence, below
damage threshold range of ≈<i>mJ/cm</i><sup>2</sup>. 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
(< 1<i>min</i>) and surface hardening for long-time exposure (⩾1<i>min</i>).
An advanced time integrated method has been developed for soft X-ray pulsed laser beam characterization. A technique
based on poly (methyl methacrylate) - PMMA laser induced ablation has been used for beam investigations of soft X-ray
laser sources like FLASH (Free-electron LASer in Hamburg; formerly known as VUV FEL and/or TTF2 FEL) and
plasma-based Ne-like Zn laser performed at PALS (Prague Asterix Laser System). For the interaction experiments reported here, the FLASH system provided ultra-short pulses (~10-fs) of 21.7-nm radiation. The PMMA ablation was
also induced by plasma-based Ne-like Zn soft X-ray laser pumped by NIR beams at the PALS facility. This quasi-steady-state
(QSS) soft X-ray laser provides 100-ps pulses of 21.2-nm radiation, i.e. at a wavelength very close to that of
FLASH but with about 5,000 times longer pulses. In both cases, the PMMA samples were irradiated by a single shot
with a focused beam under normal incidence conditions. Characteristics of ablated craters obtained with AFM (Atomic
Force Microscope) and Nomarski microscopes were utilized for profile reconstruction and diameter determination of the
focused laser beams ablating the PMMA surface.
Single ≤1 kJ pulses from a high-power laser are focused into molecular gases to create large laser sparks. This provides a unique way to mimic the chemical effects of high-energy-density events in planetary atmospheres (cometary impact, lightning) matching the natural energy-density, its spatio-temporal evolution and plasma-volume scaling of such events in a fully-controlled laboratory environment. Some chemical reactions initiated by laser-induced dielectric breakdown (LIDB) in both pure molecular gases and mixtures related to the chemical evolution of the Earth's early atmosphere were studied. Most of the experiments were carried out in a static gas cell. However, an initial series of experiments was also performed with a gas-puff target placed within a vacuum interaction chamber. Under these dynamic conditions the hot core of a laser spark can be directly investigated.