arXiv subjects

Yoshiaki Nishijima

Publications and source records attributed to Yoshiaki Nishijima.

10 recordsLinked to original sources

arXiv2026

Nanoscale Surface Analysis of High Entropy Alloy

Nanoscale surface analysis of 1 micrometer thick high entropy alloys (HEAs) was carried out using nano-IR for hyperspectral imaging and single point spectroscopy in the 700-1700 1/cm spectral range. Nano-IR is based on the detection of scattered light from an oscillating metal coated nano-tip in one of the arms of the Fourier transform infrared spectrometer and has a resolution defined by the tip radius of the probe, 20 nm, regardless of the excitation wavelength. HEA CuPdAgPtAu showed an absorption and reflection increase at 900-1100 1/cm band, which is consistent with Drude-Lorenz modeling of permittivity, however, could also signify oxide formation as tested by X-ray photoelectron spectroscopy of CuPdAgPtAu and CrFeCoNiCuMo. Realization of polarization analysis for nano-IR nano-spectroscopy in the plane perpendicular to the sample's surface is discussed and modeled. The currently available modality of surface analysis with the excitation-detection mode of the p-pol. antenna can be extended to full 3D analysis of the orientational dependencies of local absorbance and refractive index.

physics.optics↗

arXiv2026

High Entropy Alloy under Shock Compression: Optical-Pump X-Ray-Probe

High entropy alloys (HEAs) are multi-principal-element alloys designed for tailorable mechanical performance and have been attracting significant engineering interest, yet their fundamental behaviour under extreme dynamic conditions, such as shock loading, remains unexplored. Here, we report laser-shock experiments on two different types of 1-micrometers-thick HEA microfilms, CuPdAgPtAu and CrFeCoNiCuMo, on 25-micrometers-thick black-Kapton ablator driven by a high intensity laser pulse (532 nm, 5 ns, 16 J, 0.5-mm diameter focal spot) and probed by an X-ray free electron laser (XFEL) pulse (12 keV, 7 fs). Time-resolved X-ray diffraction (XRD) shows the formation of a transient phase with a lattice compression up to 5.1% of the CuPdAgPtAu HEA along the (111) plane; this transient compressed phase existed for 0.3 ns. The impedance matching Hugoniot analysis estimated a shock pressure of 55 +/- 6 GPa in the HEA film, while Au- and Fe-based equations of state (EoS) modelling predict 80 GPa (0.8 MBar) at the free HEA surface. The free HEA surface reached maximum velocities of ~ 5 km/s as recorded from in situ monitoring with the velocity interferometry system for any reflector (VISAR) imaging. These initial HEA results show the suitability of HEA sample preparation and XFEL-based XRD characterisation under extreme shock loading, and are promising for experimental determination of the EoS of this emerging class of materials (beamtime proposal No.: 2024A8503 for a 6-hour preliminary experiment).

cond-mat.mtrl-sci↗

arXiv2024

Bessel-beam direct-write of the etch-mask in a nano-film of alumina for high-efficiency Si solar cells

Large surface area applications such as high-efficiency > 26% solar cells require surface patterning with 1-10 micrometers periodic patterns at high fidelity over 1-10 cm^2 areas (before up scaling to 1 m^2) to perform at, or exceed, the Lambertian (ray optics) limit of light trapping. Here we show a pathway to high-resolution sub-1 micrometer etch mask patterning by ablation using direct femtosecond laser writing performed at room conditions (without the need for a vacuum-based lithography approach). A Bessel beam was used to alleviate the required high surface tracking tolerance for ablation of 0.3-0.8 micrometer diameter holes in ~40 nm alumina Al2O3-mask at high writing speed, 7.5 cm/s; a patterning rate 1 cm^2 per 20 min. The plasma etching protocol was optimised for a zero-mesa formation of photonic crystal (PhC) trapping structures and smooth surfaces at the nanoscale level. Scaling up in area and throughput of the demonstrated approach is outlined.

physics.optics↗

arXiv2021

Beyond Lambertian light trapping for large-area silicon solar cells: fabrication methods

Light trapping photonic crystal (PhC) patterns on the surface of Si solar cells provides a novel opportunity to approach the theoretical efficiency limit of 32.3%, for light-to-electrical power conversion with a single junction cell. This is beyond the efficiency limit implied by the Lambertian limit of ray trapping 29%. The interference and slow light effects are harnessed for collecting light even at the long wavelengths near the Si band-gap. We compare two different methods for surface patterning, that can be extended to large area surface patterning: 1) laser direct write and 2) step-&-repeat 5-times reduction projection lithography. Large area throughput limitations of these methods are compared with the established electron beam lithography (EBL) route, which is conventionally utilised but much slower than the presented methods. Spectral characterisation of the PhC light trapping is compared for samples fabricated by different methods. Reflectance of Si etched via laser patterned mask was 7% at visible wavelengths and was comparable with Si patterned via EBL made mask. The later pattern showed a stronger absorbance than the Lambertian limit (M.-L. Hsieh et al., Sci. Rep. 10, 11857 (2020)).

physics.optics↗

arXiv2020

Tilted black-Si: ~0.45 form-birefringence from sub-wavelength needles

The self-organised conical needles produced by plasma etching of silicon (Si), known as black silicon (b-Si), create a form-birefringent surface texture when etching of Si orientated at angles of $θ_i < 50 - 70^\circ$ (angle between the Si surface and vertical plasma E-field). The height of the needles in the form-birefringent region following 15~min etching was $d\sim 200$ nm and had a 100 $μ$m width of the optical retardance/birefringence, characterised using polariscopy. The height of the b-Si needles corresponds closely to the skin-depth of Si $\simλ/4$ for the visible spectral range. Reflection-type polariscope with a voltage-controlled liquid-crystal retarder is proposed to directly measure the retardance $Δn\times d/λ\approx 0.15$ of the region with tilted b-Si needles. The quantified form birefringence of $Δn = - 0.45$ over $λ= 400-700$~nm spectral window was obtained. Such high values of $Δn$ at visible wavelengths can only be observed in the most birefringence calcite or barium borate as well as in liquid crystals. The replication of b-Si into Ni-shim with high fidelity was also demonstrated and can be used for imprinting of the b-Si nanopattern into other materials.

physics.app-ph↗

arXiv2020

Lithography-free Kirchhoff's Metasurfaces

Lithography-free metasurfaces composed of a nano-layered stack of materials are attractive not only due to their optical properties but also by virtue of fabrication simplicity and cost reduction of devices based on such structures. We demonstrate a multi-layer metasurface with engineered electromagnetic absorption in the mid-infrared (MIR) wavelength range. Characterisation of thin SiO$_2$ and Si films sandwiched between two Au layers by way of experimental absorption and thermal radiation measurements as well as finite difference time domain (FDTD) numerical simulations is presented. Comparison of experimental and simulation data of optical properties of multilayer metasurfaces show guidelines for the absorber/emitter applications.

physics.optics↗

arXiv2019

Dielectric cross-shaped resonator based metasurface for vortex beam generation in Mid-IR and THz wavelengths

Metasurfaces are engineered thin surfaces comprising two dimensional (2D) arrays of sub-wavelength spaced and sub-wavelength sized resonators. Metasurfaces can locally manipulate the amplitude, phase and polarization of light with high spatial resolution. In this study, we report numerical and experimental results of a vortex-beam-generating metasurface fabricated specifically for infrared (IR) and terahertz (THz) wavelengths. The designed metasurface consisted of a 2D array of dielectric cross-shaped resonators with spatially varying length, thereby providing desired spatially varying phase shift to the incident light. The metasurface was found to be insensitive to polarization of incident light. The dimensions of the cross-resonators were calculated using rigorous finite difference time domain (FDTD) analysis. The spectral scalability via physical scaling of meta resonators was demonstrated using two vortex generating optical elements operating at 8.8~$μ$m (IR) and 0.78~THz (Terahertz). The vortex beam generated in the mid-IR spectral range was imaged using FTIR imaging miscroscope equipped with a focal plane array (FPA) detector. This design could be used for efficient wavefront shaping as well as various optical imaging applications in mid-IR spectral range, where polarization insensitivity is desired.

physics.optics↗

arXiv2018

Kirchhoff's metasurfaces

Thermo-optical properties of the nanodisc and metal hole array plasmonic perfect absorber (PPA) metasurfaces were designed and characterised at midinfrared wavelengths. Both, light emitter and detector systems are highly thought after for the future sensor networks in the internet-of-things for various spectral domains. Reciprocity of the absorbance and emittance is shown experimentally, i.e., the PPAs are following Kirchhoff's law where the patterns exhibiting a strong optical absorption were found enhanced thermal radiation. Design principles and scaling for photo-thermal conversion are discussed. The highest efficiency of light-to-heat and heat-to-radiation were obtained for the Au-Si-Au structures.

physics.optics↗

arXiv2017

Optical readout of hydrogen storage in films of Au and Pd

For hydrogen sensor and storage applications, films of Au and Pd were (i) co-sputtered at different rates or (ii) deposited in a sequential layer-by-layer fashion on a cover glass. Peculiarities of hydrogen uptake and release were optically monitored using 1.3 micrometers wavelength light. Increase of optical transmission was observed for hydrogenated Pd-rich films of 10-30 nm thickness. Up to a three times slower hydrogen release took place as compared with the hydrogen uptake. Composition ratio of Au:Pd and thermal treatment of films provided control over the optical extinction changes and hydrogen uptake/release time constants. Higher uptake and release rates were observed in the annealed Au:Pd films as compared to those deposited at room temperature and were faster for the Au-richer films. Three main parameters relevant for sensors: sensitivity, selectivity, stability (reproducibility) are discussed together with the hydrogenation mechanism in Au:Pd alloys.

cond-mat.mes-hall↗

arXiv2016

Au-Ag-Cu nano-alloys: tailoring of permittivity

Precious metal alloys enables new possibilities to tailor materials for specific optical functions. Here we present a systematic study of the effects of a nanoscale alloying on the permittivity of Au-Ag-Cu metals at 38 different atomic mixing ratios. The permittivity was measured and analyzed numerically by applying the Drude model. X-ray diffraction (XRD) revealed the face centered cubic lattice of the alloys. Both, optical spectra and XRD results point towards an equivalent composition-dependent electron scattering behavior. Correlation between the fundamental structural parameters of alloys and the resulting optical properties is elucidated. Plasmonic properties of the Au-Ag-Cu alloy nanoparticles were investigated by numerical simulations. Guidelines for designing plasmonic response of nano- structures and their patterns are presented from the material science perspective.

cond-mat.mtrl-sci↗