Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 MHz repetition rate

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm². Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm²) and liquid state (54 mJ/cm²), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes

Damage threshold of coating materials on x-ray mirror for x-ray free electron laser

We evaluated the damage threshold of coating materials such as Mo, Ru, Rh, W, and Pt on Si substrates, and that of uncoated Si substrate, for mirror optics of X-ray free electron lasers (XFELs). Focused 1 μm (full width at half maximum) XFEL pulses with the energies of 5.5 and 10 keV, generated by the SPring-8 angstrom compact free electron laser (SACLA), were irradiated under the grazing incidence condition. The damage thresholds were evaluated by in situ measurements of X-ray reflectivity degradation during irradiation by multiple pulses. The measured damage fluences below the critical angles were sufficiently high compared with the unfocused SACLA beam fluence. Rh coating was adopted for two mirror systems of SACLA. One system was a beamline transport mirror system that was partially coated with Rh for optional utilization of a pink beam in the photon energy range of more than 20 keV. The other was an improved version of the 1 μm focusing mirror system, and no damage was observed after one year of operation.Takahisa Koyama, Hirokatsu Yumoto, Takanori Miura, Kensuke Tono, Tadashi Togashi, Yuichi Inubushi, Tetsuo Katayama, Jangwoo Kim, Satoshi Matsuyama, Makina Yabashi, Kazuto Yamauchi, and Haruhiko Ohashi, "Damage threshold of coating materials on x-ray mirror for x-ray free electron laser", Review of Scientific Instruments 87, 051801 (2016) https://doi.org/10.1063/1.4950723

Spontaneous Pattern Formation in Photoelectrodeposited Semiconductor Films

The ability to manipulate matter with ever-increasing precision has enabled the fabrication of nanoscale structures with unprecedented utility. Scalable patterning technologies have dramatically transformed diverse application spaces such as computing and photonics, in part due to diminishing cost per unit area. The work in this thesis presents a template-free, bottom-up technique based on photoelectrodeposition which allows the direct fabrication of periodically nanostructured thin films of semiconductor material over large areas. First, we examine the effects of wavelength, polarization and incidence angle of illumination on the film morphology. We develop an understanding of the pattern formation to be the result of interference of light scattered across the surface of the growing interface. We also examine the morphological effects of more complex illumination conditions. For example, when deposited under two different illumination wavelengths, the period of patterned films self-optimizes to concentrate light absorption to the tips of the nanostructures . Additionally, we find that the relative polarization angles and phases of two illumination sources can be tuned to produce film morphologies ranging from isotropic mesh-type patterns to orthogonally arranged, intersecting lamellar structures with independent periodicities. We deepen our understanding of these observations by building a probabilistic computational model that correlates the local light absorption with a local growth probability at the interface of the film with few material parameters. We find that this model is able to reproduce experimentally observed morphological features for all illumination conditions investigated in this work. Through Fourier analysis, we find quantitative agreement between the simulated and experimental periods. Separately, we use electrodynamic simulations on idealized lamellar structures to understand the effect of two coincident illumination sources on the spatial absorption profile.</p