Interaction of short-wavelength free-electron laser (FEL) beams with matter is undoubtedly a subject to extensive investigation in last decade. During the interaction various exotic states of matter, such as warm dense matter, may exist for a split second. Prior to irreversible damage or ablative removal of the target material, complicated electronic processes at the atomic level occur. As energetic photons impact the target, electrons from inner atomic shells are almost instantly photo-ionized, which may, in some special cases, cause bond weakening, even breaking of the covalent bonds, subsequently result to so-called non-thermal melting. The subject of our research is ablative damage to lead tungstate (PbWO4) induced by focused short-wavelength FEL pulses at different photon energies. Post-mortem analysis of complex damage patterns using the Raman spectroscopy, atomic-force (AFM) and Nomarski (DIC) microscopy confirms an existence of non-thermal melting induced by high-energy photons in the ionic monocrystalline target. Results obtained at Linac Coherent Light Source (LCLS), Free-electron in Hamburg (FLASH), and SPring-8 Compact SASE Source (SCSS) are presented in this Paper.
The recent commissioning of a X-ray free-electron laser triggered an extensive research in the area of X-ray ablation of
high-Z, high-density materials. Such compounds should be used to shorten an effective attenuation length for obtaining
clean ablation imprints required for the focused beam analysis. Compounds of lead (Z=82) represent the materials of first
choice. In this contribution, single-shot ablation thresholds are reported for PbWO4 and PbI2 exposed to ultra-short
pulses of extreme ultraviolet radiation and X-rays at FLASH and LCLS facilities, respectively. Interestingly, the
threshold reaches only 0.11 mJ/cm2 at 1.55 nm in lead tungstate although a value of 0.4 J/cm2 is expected according to
the wavelength dependence of an attenuation length and the threshold value determined in the XUV spectral region, i.e.,
79 mJ/cm2 at a FEL wavelength of 13.5 nm. Mechanisms of ablation processes are discussed to explain this discrepancy.
Lead iodide shows at 1.55 nm significantly lower ablation threshold than tungstate although an attenuation length of the
radiation is in both materials quite the same. Lower thermal and radiation stability of PbI2 is responsible for this finding.
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, Si3N4, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate)
- PMMA, stainless steel, etc.) to an irradiance of 1013 W/cm2 and 1016 W/cm2, 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 1013 W/cm2. 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 1016 W/cm2, the OE
spectra are again dominated by the atomic lines. However, a weak emission of Al+ and Al2+ was registered as well. The
abundance ratio of Al/Al+ 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).
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/cm2. 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 1013 W/cm2.
A first explanation of the physics mechanism leading to damage is given.
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.
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.
Irradiation experiments were conducted at Prague Asterix Laser System (PALS) with the Ne-like zinc soft x-ray laser
(SXRL) at 21.2 nm (58.5 eV) delivering up to 4 mJ (~4 x 1014 photons), 100-ps pulses in a narrowly collimated beam.
The SXRL beam was focused using a 1 inch diameter off-axis parabolic mirror (f = 253 mm at 14 degrees) with a Mo:Si
multilayer coating (R = 30% at 21 nm) placed 2825 mm from the SXRL. The diameter of the SXRL beam incident on
the mirror was about 11 mm. Ablation experiments with a gradually attenuated beam were performed to determine the
single-shot damage threshold of various materials. In this case, the sample was positioned at the tightest focus of the
SXRL whose pulse energy was attenuated by aluminum filters of various thickness to adjust the fluence. Both the focal
spot area and single-shot damage threshold were determined from the plot of damaged surface areas as a function of a
pulse energy logarithm to dete. For PMMA, the focal spot area and the ablation threshold inferred from the data are
Sfoc = (1172±230) μm2 and Fth = (1.25±0.4) J/cm2, respectively. Inorganic materials have thresholds significantly higher
than organic polymers, e.g., amorphous and monocrystalline silicon gave values 2.5 J/cm2 and 4.2 J/cm2, respectively.
For prospective SASE FEL optical elements, the SiC coating is of great interest. Its damage threshold is of 20 J/cm2, i.e.,
slightly lower than that of monocrystalline silicon. The thresholds determined with the 100-ps pulses from plasma-based,
quasi-steady state SXRL are significantly higher than the thresholds obtained for 20-fs pulses provided by the SXR freeelectron
laser in Hamburg. There is a difference in PMMA thresholds of two orders of magnitude for these two sources.
Kinetic Boltzmann equations are used to model the ionization and expansion dynamics of xenon clusters irradiated with short, intense VUV pulses from free-electron-laser (FEL). This unified model
includes all of the predominant interactions that contribute to the cluster dynamics induced by this radiation. The dependence of the evolution dynamics on cluster size and pulse fluence is investigated.
It is found that the highly charged ions observed in the experiments are mainly due to Coulomb explosion of the outer shell of the cluster while ions formed in the interior of the cluster predominantly
recombine with plasma electrons. As a result, a large fraction of neutral atoms is formed within the core, the proportion depending on the cluster size. The predictions of ion charge distribution,
average ion charge and average energy absorbed per ion made with our model are found to be in good agreement with the experimental data. To our knowledge, our model is the first and only one
that gives a full and quantitatively accurate description of all of the experimental data collected from irradiated atomic clusters at 100 nm photon wavelength.
X-ray free-electron lasers generate ultrashort and very intense x-ray radiation in the wavelength domain reaching from
the VUV (100 nm and shorter) all the way to the hard x-ray domain (typically 0.1 nm). FEL radiation features extreme
brilliance, ultrashort pulse duration, and high peak power. Superconducting accelerators provide furthermore the
possibility to accelerate a large number of electron bunches during a single radio-frequency pulse. Likewise the total
number of x-ray pulses available for the experiments is increased leading to a significantly higher average brilliance.
FEL light sources, and those based on super-conducting accelerator technology, are therefore considered to provide a
new quality of short wavelength radiation if compared to existing x-ray sources. The high intensity and the high
repetition rate lead to new requirements for x-ray optics in terms of peak and average power. Values for peak and
average power are presented in relation to the proposed realization of the photon beamlines at the European XFEL
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.