KEYWORDS: Physics, Stanford Linear Collider, Free electron lasers, Lead, Photons, States of matter, Materials processing, Electrons, Analytical research, Raman spectroscopy
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.
Degradation of room temperature operation of TlBr radiation detectors with time is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. Scanning Auger electron spectroscopy (AES) in combination with sputter depth profiling was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage and create a TlBr1-xClx surface layer prior to metal contact deposition. Auger compositional depth profiling results reveal non-equilibrium interfacial diffusion after device operation in both air and N2 at ambient temperature. These results improve our understanding of contact/device degradation versus operating environment for further enhancing radiation detector performance.
TlBr radiation detector operation degrades with time at room temperature and is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. X-ray photoemission spectroscopy (XPS) was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage prior to Mo or Pt contact deposition. High-resolution photoemission measurements on the Tl 4f, Br 3d, Cl 2p, Mo 3d and Pt 4f core lines were used to evaluate surface chemistry and non-equilibrium interfacial diffusion. Results indicate that anion substitution at the TlBr surface due to the HCl etch forms TlBr1-xClx with consequent formation of a shallow heterojunction. In addition, a reduction of Tl1+ to Tl0 is observed at the metal contacts after device operation in both air and N2 at ambient temperature. Understanding contact/device degradation versus operating environment is useful for improving radiation detector performance.
Device-grade TlBr was subjected to various chemical treatments used in room temperature radiation detector fabrication
to determine the resulting surface composition and electronic structure. Samples of as polished TlBr were treated
separately with 2%Br:MeOH, 10%HF, 10%HCl and 96%SOCl2 solutions. High-resolution photoemission measurements
on the valence band electronic structure and Tl 4f, Br 3d, Cl 2p and S 2p core lines were used to evaluate surface
chemistry. Results suggest anion substitution at the surface with subsequent shallow heterojunction formation. Surface
chemistry and valence band electronic structure were further correlated with the goal of optimizing the long-term
stability and radiation response.
Chemical vapor deposition (CVD) has been used for the production of fused silica optics in high power laser
applications. However, relatively little is known about the ultraviolet (UV) laser damage threshold of CVD films
and how they relate to intrinsic defects produced during deposition. We present a study relating structural and
electronic defects in CVD films to the 355 nm pulsed laser damage threshold as a function of post-deposition
annealing temperature (THT). Plasma-enhanced CVD, based on SiH4/N2O under oxygen-rich conditions, was used
to deposit 1.5, 3.1 and 6.4 μm thick films on etched SiO2 substrates. Rapid annealing was performed using a
scanned CO2 laser beam up to THT~2100 K. The films were then characterized using X-ray photoemission
spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and photoluminescence (PL). A gradual
transition in the damage threshold of annealed films was observed at THT up to 1600 K, correlating with a decrease
in NB silanol and broadband PL emission. An additional sharp transition in damage threshold also occurs at ~1850
K indicating substrate annealing. Based on our results, a mechanism for damage-related defect annealing is
proposed, and the potential of using high-THT CVD SiO2 to mitigate optical damage is also discussed.
TlBr is a material of interest for use in room temperature gamma ray detector applications due to is wide bandgap 2.7 eV
and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of 1.3 % at 662 keV,
demonstrating the potential of this material system. However, these detectors are known to polarize using conventional
configurations, limiting their use. Continued improvement of room temperature, high-resolution gamma ray detectors
based on TlBr requires further understanding of the degradation mechanisms. While high quality material is a critical
starting point for excellent detector performance, we show that the room temperature stability of planar TlBr gamma
spectrometers can be significantly enhanced by treatment with both hydrofluoric and hydrochloric acid. By
incorporating F or Cl into the surface of TlBr, current instabilities are eliminated and the longer term current of the
detectors remains unchanged. 241Am spectra are also shown to be more stable for extended periods; detectors have been
held at 2000 V/cm for 52 days with less than 10% degradation in peak centroid position. In addition, evidence for the
long term degradation mechanism being related to the contact metal is presented.
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 >1012 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 1018 W/cm2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be 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, 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).
A multilayer-coated 27-cm focal length parabola, optimized to reflect 13.5 nm wavelength at normal incidence,
was used in multiple FLASH experiments and focused the beam to a sub-micron beam size. The intensity of the beam
was measured indirectly from the depths of craters left by the FLASH beam on PMMA-coated substrates. Comparing
simulated and experimental shapes of the craters we found the best match for a wavefront error of 0.45 nm, or λ/30. We
further estimated that the FWHM of the focal spot was 350 nm and that the intensity in the focus was 1018 W/cm2. The
sub-micron FLASH beam provided extreme intensity conditions essential for warm dense matter experiments. The same
optic was used in multiple experiments and survived the beam. However, after the first measurements, which took place
over several days, the optical surface was contaminated. This contamination reduced the mirror reflectivity, which was
partially recovered by oxygen plasma cleaning. However, even the partially cleaned multilayer-coated optic is still
diffraction limited and can focus the beam in future experiments to a sub-micron beam size.
The layered anisotropic chalcogenide semiconductors GaSe and GaTe single crystals have been grown by a modified
vertical Bridgman technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te).
The crystals harvested from ingots of up to 10 cm length and up to 2" diameter, have been characterized by measuring
resistivity through current-voltage (I-V) characteristics and bulk carrier concentration and mobility through Hall effect
measurements. Micro-hardness, infrared microscopy, etching characteristics, low-temperature photoluminescence (PL)
and contact resistivity studies have also been performed to further characterize the grown crystals.
We present a review of recent development and applications of soft x-ray lasers, undertaken at the PALS Centre. The applications benefit from up to 10-mJ pulses at the wavelength of 21.2 nm. We describe the pumping regimes used to produce this soft x-ray laser, and outline its emission characteristics. A significant fraction of applications carried out using this device includes probing of dense plasmas produced by IR laser pulses and high-energy-density-in-matter experiments. Results obtained in these experiments are reviewed, including x-ray laser probing of dense plasmas, measurements of transmission of focused soft x-ray radiation at intensities of up to 1012 Wcm-2, measurements of IR laser ablation rates of thin foils, and probing high density plasmas by x-ray laser Thomson scattering
Recent experiments were carried out on the Prague Asterix Laser System (PALS) towards the
demonstration of a soft x-ray laser Thomson scattering diagnostic for a laser-produced exploding foil. The
Thomson probe utilized the Ne-like zinc x-ray laser which was
double-passed to deliver ~1 mJ of focused
energy at 21.2 nm wavelength and lasting ~100 ps. The plasma under study was heated single-sided using a
Gaussian 300-ps pulse of 438-nm light (3ω of the PALS iodine laser) at laser irradiances of 1013-1014 W
cm-2. Electron densities of
1020-1022 cm-3 and electron temperatures from 200 to 500 eV were probed at
0.5 or 1 ns after the peak of the heating pulse during the foil plasma expansion. A flat-field 1200 line mm-1
variable-spaced grating spectrometer with a cooled charge-coupled device readout viewed the plasma in the
forward direction at 30° with respect to the x-ray laser probe. We show results from plasmas generated
from ~1 μm thick targets of Al and polypropylene (C3H6). Numerical simulations of the Thomson
scattering cross-sections will be presented. These simulations show electron peaks in addition to a narrow
ion feature due to collective (incoherent) Thomson scattering. The electron features are shifted from the
frequency of the scattered radiation approximately by the electron plasma frequency ±ωpe and scale as ne1/2.
Multilayers are artificially layered structures that can be used to create optics and optical elements for a broad
range of x-ray wavelengths, or can be optimized for other applications. The development of next generation x-ray
sources (high brightness synchrotrons and x-ray free electron lasers) requires advances in x-ray optics. Newly
developed multilayer-based mirrors and optical elements enabled efficient band-pass filtering, focusing and time
resolved measurements in recent FLASH (Free Electron LASer in Hamburg) experiments. These experiments are
providing invaluable feedback on the response of the multilayer structures to high intensity, short pulsed x-ray sources.
This information is crucial to design optics for future x-ray free electron lasers and to benchmark computer codes that
simulate damage processes.
Time-resolved soft x-ray photoelectron spectroscopy is used to probe the non-steady-state evolution of the valence band electronic structure of laser heated ultra-thin (50 nm) metal foils and bulk semiconductors. Single-shot soft x-ray laser induced time-of-flight photoelectron spectroscopy with picosecond time resolution was used in combination with optical measurements of the disassembly dynamics that have shown the existence of a metastable liquid phase in fs-laser heated metal foils persisting 4-5 ps. This metastable phase is studied using a 527 nm wavelength 400 fs laser pulse containing 0.3 - 2.5 mJ laser energy focused in a large 500 × 700 μm2 spot to create heated conditions of 0.2 - 1.8 × 1012 W cm-2 intensity. The unique LLNL COMET compact tabletop soft x-ray laser source provided the necessary high photon flux, highly monoenergetic, picosecond pulse duration, and coherence for observing the evolution of changes in the valence band electronic structure of laser heated metals and semiconductors with picosecond time resolution. This work demonstrates the continuing development of a powerful new technique for probing reaction dynamics and changes of local order on surfaces on their fundamental timescales including phenomena such as non-thermal melting, chemical bond formation, intermediate reaction steps, and the existence of transient reaction products.
We present optical constants derived from synchrotron reflectance measurements of iridium-coated X-ray witness mirrors over 0.05-12 keV, relevant to the Chandra X-ray Observatory effective area calibration. In particular we present for the first time analysis of measurements taken at the Advanced Light Source Beamline 6.3.2 over 50-1000 eV, Chandra's lower-energy range. Refinements to the currently tabulated iridium optical constants (B. L. Henke et al., At. Data Nucl. Data Tables 54, 181-343, 1993 and on the Web at http://www-cxro.lbl.gov/optical_constants/) will become important as the low-energy calibration of Chandra's X-ray detectors and gratings are further improved, and as possible contaminants on the Chandra mirror assembly are considered in the refinement of the in-flight Ir absorption edge depths. The goal of this work has been to provide an improved tabulation of the Ir optical constants over the full range of Chandra using a self-consistent mirror model, including metallic layers, interface roughness, contaminating overlayer, and substrate. The low-energy data present us with a considerable challenge in the modeling of the overlayer composition, as the K-absorption features of C, O, and N are likely to be present in the ~10A overlayer. The haphazard contamination and chemical shifts may significantly affect optical constants attributed to this overlayer, which will distort the iridium optical constants derived. Furthermore, the witness mirror contamination may be considerably different from that deposited on the flight optics. The more complex modeling required to deal with low-energy effects must reduce to the simpler model applied at the higher energies, which has successfully derived optical constants for iridium in the higher energy range, including the iridium M-edges, already used in the Chandra calibration. We present our current results, and the state of our modeling and analysis, and our approach to a self-consistent tabulation.
X-ray laser induced time-of-flight photoelectron spectroscopy has been used to probe the core-level and valence band electronic structure of room-temperature bulk materials with picosecond time resolution. The LLNL COMET compact tabletop x-ray laser source provides the necessary high photon flux, high energy, monochromaticity, picosecond pulse duration, and coherence for probing ultrafast changes in the chemical and electronic structure of these materials. Valence band and core-level spectra were recorded for transition metal surfaces. In situ sputter etching with Ar ions at 30° incidence will be implemented to improve the surface purity and consequently increase core-level and valence-band photoemission intensity. This work demonstrates a powerful new technique for probing reaction dynamics and for probing changes of local order on surfaces on their fundamental timescales. Future work will include the study of fundamental phenomena such as non-thermal melting, chemical bond formation, intermediate reaction steps, and the existence of transient reaction products.
We have investigated the surface degradation of bare and sol-gel coated deuterated potassium dihydrogen phosphate (DKDP) crystals when exposed to 351-nm laser pulses in atmospheric air and nitrogen and at pressures ranging from atmospheric down to 10-5 Torr vacuum. Optical microscopy, surface topography, surface chemical analyses, 351-nm pumped photoemission maps, and photometry results have been used to characterize these samples. We report the occurrence of two potentially linked surface degradation phenomena: the development of increased photoemission and the development of unacceptable surface roughening in the region exposed to the beam in vacuum. We note no degradation for surfaces exposed in air or nitrogen at pressures exceeding 1 Torr. Diamond-turned DKDP surfaces show a ubiquitous, low-intensity photoemission signature before exposure to any laser fluence. The observed reduction of this emission signal as a function of operating pressure and accumulated laser energy when crystals are exposed to 351-nm laser pulses in air can be correlated with the removal of surface carbon.
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