As the James Webb Space Telescope (JWST) pursues its observing journey, several thousands of icy-grain spectra are expected to be measured and analysed. The inventory of ices in particular, via the observations of background sources, is accessible for hundreds of lines of sight (LOSs) per molecular-cloud region, opening the possibility to add strong constraints on the solid phase chemistry in a vast domain of cloud densities. SynthIceSpec is a synthetic infrared (IR) spectrum generator that has been designed as a tool to support observing proposals and to test the outcome of chemical models. It is based on laboratory measurements of pure and mixed ices, where each vibrational component is fitted by a sum of Gaussian profiles. Given an initial ice chemical composition (either set by the user or the outputs of a chemical model), a full JWST spectrum is generated, to which the contribution of silicates; continuum, stellar photospheric absorption bands; and extinction law can be added. For the continuum, stellar photospheric models for a wide range of spectral types can be selected by the program, or, Spectral Energy Distribution (SEDs). We present a few use cases of SynthIceSpec: we probed the impact of dust temperature on CO2 ice formation using IR data and gas-grain modelling. Next, we used SynthIceSpec to explore the detectability of the main feature of CH3CN at 4.45 & micro;m in a cold core environment with the JWST, which was previously tentatively detected in YSOs. The detection thresholds we derive are reasonably low and observable, but identification is directly impacted by the photosphere absorptions that can greatly hinder identification. For some photostellar types, it could remain feasible. Coupled with the Estimated Time Calculator of the Space Telescope Science Institute, SynthIceSpec can be used to find the optimum observational setup for new observations.
The Megajoule Laser (LMJ) uses large transmission diffraction gratings composed of fused silica to focus high-energy laser beams at 351 nm. These gratings operate in the final stage of the laser beamline and must withstand high-energy nanosecond pulses. Increasing the laser-induced damage threshold (LIDT) of transmission gratings is a challenge that requires a detailed understanding of the damage process. Here we numerically and experimentally study the LIDT of silica-based transmission gratings as a function of the electric field intensities (EFI) occurring in the vicinity of the nanostructured surface of diffraction gratings. Damage tests are performed on different samples under varying polarizations and incident angles and compared with numerical EFI simulations. We show that the damage threshold is strongly correlated with the maximum EFI occurring inside the silica pillars of the grating. We evidence a linear relation between the LIDT and the EFI with parameters depending on the sample quality.
This letter proposes the design and measurement of a periodic metasurface that achieves anomalous reflection with reduced RCS in a given parasitic direction. A previous study proposed a semianalytical model to predict the RCS behaviour of such a metasurface. However, this first study did not include any experimental exploration to verify the theoretical results. To complete this study, this work presents an experimental validation of the proposed design, with a focus on manufacturing and measurement issues. The synthesis, design specifications, fabrication method and experimental setup are presented and discussed. Measurement results are also examined in detail, highlighting some limitations in metasurface RCS measurements. The proposed metasurface effectively achieves the predicted RCS level reduction in the considered parasitic direction. The agreement between simulation and experimental results demonstrates the accuracy of the modelling and the efficiency of the optimisation procedure.
Fused silica is a cornerstone material in photonics thanks to its outstanding optical, thermal and mechanical robustness. Its surface can be selectively functionalized with CO2 lasers, whose strong infrared absorption confines energy to a thin layer and enables precise thermal effects-from annealing to ablation and laser polishing. Here we quantify how CO2 laser parameters govern crater morphology and the surrounding heat-affected zone (HAZ). Pits were machined with a 180 & micro;m-diameter focused beam using single pulses from 10 & micro;s to 2 s across the evaporative regime. Optical profilometry coupled with quantitative phase microscopy maps both topography and subsurface refractive-index changes, providing a rapid, non-destructive assessment of the HAZ. We show that, for a fixed removal depth, crater aspect ratio and HAZ width can be tuned independently by modulating pulse width and power: short, high-fluence pulses minimize collateral damage and favor smooth profiles. A two-dimensional finite-element model reproduces the transient temperature field and corroborates the experimental trends. The resulting guidelines enable high-precision CO2 laser processing of fused silica while mitigating HAZ and associated thermo-mechanical stress, microcracking and debris- crucial for generation of advanced optical components.
Iron samples with varying initial grain sizes were submitted to dynamical compression using laser ramp loading. Electron backscatter diffraction mappings of crystal orientations were performed in their cross section and evidenced the emergence of reversion variants (i.e., grains not present in the initial microstructure induced by the loading-unloading cycle) whose sizes scale with the initial grain size. The analysis procedure of these variants combines the comparison of their orientations with theoretical predictions from a graph of transformation sequences and the characterization of their inclinations in terms of kinematical compatibility conditions. It appears that reversion variants can be alpha-Fe twins or alpha- epsilon-alpha ' variants formed either within epsilon-Fe grains or at their boundaries. The coexistence of two epsilon-Fe variants is identified as a condition for the formation of alpha- epsilon-alpha ' variants. Stress inhomogeneities induced by alpha-Fe twinning are also highlighted.
