This PDF file contains the front matter associated with SPIE Proceedings Volume 9511, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.
X-ray free-electron lasers (XFELs) that utilize intense and ultra-short pulse X-rays may damage optical elements. We investigated the damage fluence thresholds of optical materials by using an XFEL focusing beam that had a power density sufficient to induce ablation phenomena. The 1 μm focusing beams with 5.5 keV and/or 10 keV photon energies were produced at the XFEL facility SACLA (SPring-8 Angstrom Compact free electron LAser). Test samples were irradiated with the focusing beams under normal and/or grazing incidence conditions. The samples were uncoated Si, synthetic silica glass (SiO2), and metal (Rh, Pt)-coated substrates, which are often used as X-ray mirror materials.
We investigate damage formation on the surface of fused silica by two femtosecond laser pulses, a tightly focused 266 nm (UV) pulse followed by a loosely focused 800 nm (IR) pulse. We show that the damage size is determined by the UV pulse, and only a small fraction of the normal UV damage threshold energy is needed to cause damage when combined with the properly delayed IR pulse. Our results, analyzed with a rate equation model, suggest that the UV pulse generates seed electrons through multiphoton absorption and the IR pulse utilizes these electrons to cause damage by avalanche ionization. By tuning such parameters like pulse energy, time delay, IR pulse duration and polarization, we further demonstrate that damage profile can be controlled.
The photon density profile of an X-ray free-electron laser (XFEL) beam at the focal position is a critical parameter for serial femtosecond crystallography (SFX), but is difficult to measure because of the destructive power of the beam. A novel high intensity radiation induced phasing method (HIRIP) has been proposed as a general experimental approach for protein structure determination, but has proved to be sensitive to variations of the X-ray intensity, with uniform incident fluence desired for best performance. Here we show that experimental SFX data collected at the nano-focus chamber of the Coherent X-ray Imaging end-station at the Linac Coherent Light Source using crystals with a limited size distribution suggests an average profile of the X-ray beam that has a large variation of intensity. We propose a new method to improve the quality of high fluence data for HI-RIP, by identifying and removing diffraction patterns from crystals exposed to the low intensity region of the beam. The method requires crystals of average size comparable to the width of the focal spot.
Silicon irradiated with an ultrashort laser pulse can experience two competing damage processes: the ultrafast ’nonthermal melting’ or the picoseconds ‘thermal melting’. The first one occurs if the density of excited electrons within the conduction band overcomes a certain threshold value, which leads to modification of the atomic potential energy surface and triggers a phase transition. The second one heats a material due to the electron-ion (electron-phonon) coupling, which in case of atomic temperature exceeding melting temperature also induces a phase transition. Our recently developed code XTANT (X-ray-induced Thermal And Nonthermal Transition; N. Medvedev et. al, Phys. Rev. B 91 (2015) 054113), can model both effects simultaneously. Nonadiabatic electron-ion coupling is treated within tight binding molecular dynamics model beyond the Born-Oppenheimer approximation. Two different channels of phase transition emerge at different irradiation dose: thermal melting of silicon into low-density-liquid phase occurs for deposited energies above ~0.65 eV/atom; nonthermal melting into high-density liquid takes place for doses higher than ~0.9 eV/atom. Here we discuss in detail electronic processes during such phase transitions. Evolution of the electronic structure is presented.
This paper deals with prediction of extreme ultraviolet (XUV) laser ablation of lithium fluoride at nanosecond timescales. Material properties of lithium fluoride were determined based on bibliographic survey. These data are necessary for theoretical estimation of surface removal rate in relevance to XUV laser desorption/ablation process. Parameters of XUV radiation pulses generated by the Prague capillary-discharge laser (CDL) desktop system were assumed in this context. Prediction of ablation curve and threshold laser fluence for lithium fluoride was performed employing XUV-ABLATOR code. Quasi-random sampling approach was used for evaluating its predictive capabilities in the means of variance and stability of model outputs in expected range of uncertainties. These results were compared to experimental data observed previously.
The detail characteristics of a compact laser-plasma X-ray source, dedicated for application in soft X-ray contact microscopy is presented in the paper. The source is based on a double-stream gas puff target, irradiated with nanosecond laser pulses from a commercial Nd:YAG laser. The use of the gas puff target makes possible to produce soft X-ray radiation in the “water window” region without target debris production. Details of the characterization measurements and optimization of the source are presented and discussed.
Characterization of a free-electron laser (FEL) pulse can be done with a pump–probe scheme, using an FEL pump and a visible light probe on an optically transparent solid-state target. With such experimental scheme, pulse duration can be monitored on a shot-to-shot basis. It relies on the changes in optical properties induced by the FEL excitation of electrons. Here we analyze effects of different cross sections used in the modeling of electron kinetics. XCASCADE, a Monte Carlo toolkit for modeling x-ray-induced electron cascades (N. Medvedev, Appl. Phys. B 118 (2015) 417), is used for this purpose. Two different cross sections are compared: atomic BEB model vs complex-dielectric function formalism that accounts for collective effects in solids. It is shown that for photon and electron energies above a few tens of eV, the both models coincide very closely. For lower energies in the VUV regime, the difference in the cross sections become more significant, nevertheless producing qualitatively similar electron kinetics and increase in the density of excited electrons.
Impacts of low energy He+ ions on reflectivity and stability of EUV multilayers is investigated in this work. Combination of X-ray reflectivity, grazing incidence EUV reflectivity near Silicon edge, and theoretical ion irradiation damage analysis can explain the degradation of ML performances. It is found that MLs irradiation of 4 keV helium ions degrades reflectivity performances with much more impact on grazing incidence mirrors. The proposed method can also regain changes in optical properties due to the irradiations of low energy ions.