Aperiodic multilayer interference coatings are of particular interest for a variety of hard x-ray applications, including target diagnostics, astrophysics, high energy physics and free-electron lasers. Such applications require large field of view along with the highest achievable photon efficiency for their optical components, pushing reflective multilayer coatings to their limits. This work investigates the design, experimental performance, modeling and optimization of high-reflectance aperiodic multilayers. Multilayer design starts with the implementation of an analytical method developed in the literature, which calculates the most efficient coating, featuring the highest achievable reflectivity with the least number of layers. A numerical optimization step is added for smoothing of high-frequency "ripples" or to comply with any specific requirement in terms of spectral or angular response. The design process also includes material-dependent specificities (e.g. typical roughness, interlayer formation) which are often crucial for accurate prediction of actual coating performance. We applied this method to develop novel high-reflectance broadband multilayers at 17.4 keV (Mo Kα emission line), working at angles of grazing incidence up to 0.6 degrees. The design methods employed in this work are presented, as well as the results obtained for a few multilayer systems, including Mo/Si, W/Si and W/SiC.
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.
At the Lawrence Livermore National Laboratory (LLNL) in collaboration with the Linac Coherent Light Source (LCLS) we are developing a mirror-based delay line for x-rays (MEL-X) to enable x-ray pump/x-ray probe experiments at Free Electron Lasers (XFELs). The goal of this project is the development and deployment of a proof-of-principle delay line featuring coated x-ray optics. The four-mirror design of the MEL-X is motivated by the need for ease of alignment and use. In order to simplify the overlap of the pump and the probe beam after each delay time change, a scheme involving super-polished rails and mirror-to-motor decoupling has been adopted. The MEL-X, used in combination with a bright pulsed source like LCLS, features a capability for a high intensity pump beam. Its Iridium coating allows it to work at hard x-ray energies all the way up to 9 keV, with a probe beam transmission of 35% up to 8keV, and 14% at 9keV. The delay time can be tailored to each particular experiment, with a nominal range of 70 - 350 fs for this prototype. The MEL-X, combined with established techniques such as x-ray diffraction, absorption or emission, could provide new insights on ultra-fast transitions in highly excited states of matter.
The Lawrence Livermore National Laboratory (LLNL) has been developing a novel X-ray imager for the National Ignition Facility (NIF) utilizing Kirkpatrick-Baez (KB) mirror geometry. A fully assembled mirror pack contains four KB optic pairs featuring cylindrical mirrors with custom-designed multilayer coatings. Multiple interchangeable mirror packs have been commissioned for various experimental campaigns, with high spatial resolution (< 5 μm) at the center of the field of view and 12× magnification.
Tight tolerances on the grazing angles of the X-ray mirrors require precision alignment and assembly of each component via a coordinate measuring machine, and a comprehensive off-line calibration of the four KB channels at X-ray wavelengths. The main goals of the calibration campaign are to measure the performance of the multilayer, validate the assembly procedure by measuring the as-built spatial resolution and determine the best object to mirror pack distance (drive depth) of the microscope for fielding at NIF. We report on the results of this effort on the first fully assembled NIF KB X-ray imager.
At the Lawrence Livermore National Laboratory (LLNL) we have engineered a silicon prototype sample that can be used to reflect focused hard x-ray photons at high intensities in back-scattering geometry.1 Our work is motivated by the need for an all-x-ray pump-and-probe capability at X-ray Free Electron Lasers (XFELs) such as the Linac Coherent Light Source (LCSL) at SLAC. In the first phase of our project, we exposed silicon single crystal to the LCLS beam, and quantitatively studied the x-ray induced damage as a function of x-ray fluence. The damage we observed is extensive at fluences typical of pump-and-probe experiments. The conclusions drawn from our data allowed us to design and manufacture a silicon mirror that can limit the local damage, and reflect the incident beam before its single crystal structure is destroyed. In the second phase of this project we tested this prototype back-reflector at the LCLS. Preliminary results suggest that the new mirror geometry yields reproducible Bragg reflectivity at high x-ray fluences, promising a path forward for silicon single crystals as x-ray back-reflectors.
The Kirkpatrick Baez Optic (KBO) diagnostic designed for the National Ignition Facility (NIF) requires very
precise alignment between four pairs of mirrors that make up four x-ray imaging channels. Furthermore, the
overlapping image axis of the four pairs must be aligned to within a 50 μm radius of the NIF target center. In
order to achieve this the diagnostic utilizes a telescoping snout that when extended, locates the mirrors at the
end of a Diagnostic Load Package (DLP), cantilevered more than three meters out from its bolted connection
points. Discussed in this paper are the structural challenges and the mechanical design solutions that were
implemented to achieve the ±50 μm pointing accuracy. During an Inertial Confinement Fusion (ICF) experiment,
the KBO diagnostic will be 117 mm away from the extremely high impulse, target implosion shock wave, which
requires a unique approach to protecting the sensitive optics which will also be discussed.
This manuscript presents an overview of recent work performed on x-ray optics development, metrology and calibration for the Soft X-ray Research (SXR) and the Coherent X-ray Imaging (CXI) instruments at the Linac Coherent Light Source (LCLS) free-electron laser. We also present results on the first LCLS exposures of boron carbide (B4C)-coated samples at photon energies near the carbon K edge and discuss relevant analysis and implications for future experiments.
N. Loh, Dmitri Starodub, Lukas Lomb, Christina Hampton, Andrew Martin, Raymond Sierra, Anton Barty, Andrew Aquila, Joachim Schulz, Jan Steinbrener, Robert Shoeman, Stephan Kassemeyer, Christoph Bostedt, John Bozek, Sascha Epp, Benjamin Erk, Robert Hartmann, Daniel Rolles, Artem Rudenko, Benedikt Rudek, Lutz Foucar, Nils Kimmel, Georg Weidenspointner, Günther Hauser, Peter Holl, Emanuele Pedersoli, MengNing Liang, Mark Hunter, Lars Gumprecht, Nicola Coppola, Cornelia Wunderer, Heinz Graafsman, Filipe R. N. Maia, Tomas Ekeberg, Max Hantke, Holger Fleckenstein, Helmut Hirsemann, Karol Nass, Thomas White, Herbert Tobias, George Farquar, W. Henry Benner, Stefan Hau-Riege, Christian Reich, Andreas Hartmann, Heike Soltau, Stefano Marchesini, Sasa Bajt, Miriam Barthelmess, Lothar Strueder, Joachim Ullrich, Philip Bucksbaum, Keith Hodgson, Mathias Frank, Ilme Schlichting, Henry Chapman, Michael Bogan
Profiling structured beams produced by X-ray free-electron lasers (FELs) is crucial to both maximizing signal intensity for weakly scattering targets and interpreting their scattering patterns. Earlier ablative imprint studies describe how to infer the X-ray beam profile from the damage that an attenuated beam inflicts on a substrate. However, the beams in-situ profile is not directly accessible with imprint studies because the damage profile could be different from the actual beam profile. On the other hand, although a Shack-Hartmann sensor is capable of in-situ profiling, its lenses may be quickly damaged at the intense focus of hard X-ray FEL beams. We describe a new approach that probes the in-situ morphology of the intense FEL focus. By studying the translations in diffraction patterns from an ensemble of randomly injected sub-micron latex spheres, we were able to determine the non-Gaussian nature of the intense FEL beam at the Linac Coherent Light Source (SLAC National Laboratory) near the FEL focus. We discuss an experimental application of such a beam-profiling technique, and the limitations we need to overcome before it can be widely applied.
Andrew Martin, Jakob Andreasson, Andrew Aquila, Saša Bajt, Thomas R. Barends, Miriam Barthelmess, Anton Barty, W. Henry Benner, Christoph Bostedt, John Bozek, Phillip Bucksbaum, Carl Caleman, Nicola Coppola, Daniel DePonte, Tomas Ekeberg, Sascha Epp, Benjamin Erk, George Farquar, Holger Fleckenstein, Lutz Foucar, Matthias Frank, Lars Gumprecht, Christina Hampton, Max Hantke, Andreas Hartmann, Elisabeth Hartmann, Robert Hartmann, Stephan Hau-Riege, Günther Hauser, Peter Holl, André Hoemke, Olof Jönsson, Stephan Kassemeyer, Nils Kimmel, Maya Kiskinova, Faton Krasniqi, Jacek Krzywinski, Mengning Liang, Ne-Te Duane Loh, Lukas Lomb, Filipe R. N. Maia, Stefano Marchesini, Marc Messerschmidt, Karol Nass, Duško Odic, Emanuele Pedersoli, Christian Reich, Daniel Rolles, Benedikt Rudek, Artem Rudenko, Carlo Schmidt, Joachim Schultz, M. Marvin Seibert, Robert Shoeman, Raymond Sierra, Heike Soltau, Dmitri Starodub, Jan Steinbrener, Francesco Stellato, Lothar Strüder, Martin Svenda, Herbert Tobias, Joachim Ullrich, Georg Weidenspointner, Daniel Westphal, Thomas White, Garth Williams, Janos Hajdu, Ilme Schlichting, Michael Bogan, Henry Chapman
Results of coherent diffractive imaging experiments performed with soft X-rays (1-2 keV) at the Linac Coherent
Light Source are presented. Both organic and inorganic nano-sized objects were injected into the XFEL beam
as an aerosol focused with an aerodynamic lens. The high intensity and femtosecond duration of X-ray pulses
produced by the Linac Coherent Light Source allow structural information to be recorded by X-ray diffraction
before the particle is destroyed. Images were formed by using iterative methods to phase single shot diffraction
patterns. Strategies for improving the reconstruction methods have been developed. This technique opens
up exciting opportunities for biological imaging, allowing structure determination without freezing, staining or
crystallization.
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.
This manuscript presents a first study of the contamination observed on some of the x-ray mirrors for the Linac Coherent
Light Source (LCLS) free-electron laser, the implications to the mirror lifetime properties and an evaluation of candidate
techniques towards successful recovery of these B4C- and SiC-coated mirrors. Initial experimental results and plans for
upcoming mirror recovery experiments are discussed. A summary of experimentally determined FEL damage thresholds
of B4C and SiC materials is also given, and their wavelength dependence is discussed.
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).
The development and properties of reflective coatings for the x-ray offset mirror systems of the Linac Coherent Light
Source (LCLS) free-electron laser (FEL) are discussed in this manuscript. The uniquely high instantaneous dose of the
LCLS FEL beam translates to strict limits in terms of materials choice, thus leading to an x-ray mirror design consisting
of a reflective coating deposited on a silicon substrate. Coherent wavefront preservation requirements for these mirrors
result in stringent surface figure and finish specifications. DC-magnetron sputtered B4C and SiC thin film coatings with
optimized stress, roughness and figure properties for the LCLS x-ray mirrors are presented. The evolution of
microstructure, morphology, and stress of these thin films versus deposition conditions is discussed. Experimental results
on the performance of these coatings with respect to FEL damage are also presented.
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.
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.
Short-pulse ultraviolet and x-ray free electron lasers of unprecedented peak brightness are in the process of
revolutionizing physics, chemistry, and biology. Optical components for these new light sources have to be able to
withstand exposure to the extremely high-fluence photon pulses. Whereas most optics have been designed to stay intact
for many pulses, it has also been suggested that single-pulse optics that function during the pulse but disintegrate on a
longer timescale, may be useful at higher fluences than multiple-pulse optics. In this paper we will review damage-resistant
single-pulse optics that recently have been demonstrated at the FLASH soft-x-ray laser facility at DESY,
including mirrors, apertures, and nanolenses. It was found that these objects stay intact for the duration of the 25-fs
FLASH pulse, even when exposed to fluences that exceed the melt damage threshold by fifty times or more. We present
a computational model for the FLASH laser-material interaction to analyze the extent to which the optics still function
during the pulse. Comparison to experimental results obtained at FLASH shows good quantitative agreement.
The development of defect-free reticle blanks is an important challenge facing the commercialization of extreme ultraviolet lithography (EUVL). The basis of an EUVL reticle are mask blanks consisting of a substrate and a reflective Mo/Si multilayer. Defects on the substrate or defects introduced during multilayer deposition can result in critical phase and amplitude defects. Amplitude- or phase-defect repair techniques are being developed with the goal to repair many of these defects. In this paper we discuss the selection of a capping layer for amplitude-defect repair, and report on experimental results of the reflectance variation over the amplitude-defect repair zone for different capping layers. Our results suggest that carbon and silicon carbide are the leading candidates for capping layer materials. We further performed a quantitative assessment of the yield improvement due to defect repair. We found that amplitude- and phase-defect repair have the potential to significantly improve mask blank yield, and that yield can be maximized by increasing the number of Mo/Si bilayers.
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