Atsushi Shibukawa

Electron Irradiation Damage in Radiation-Resistant InP Solar Cells

In this paper it is shown that InP solar cell is more radiation-resistant than Si and GaAs solar cells. 1 MeV electron irradiation damages in InP solar cells with AM 1.5 conversion efficiencies exceeding 16.5% are examined. Minority carrier diffusion length and carrier concentration studies for defects induced by 1 MeV electron irradiation in InP solar cells are carried out, and the correlation between the measured defect parameters and the performance characteristics of the electron-irradiated InP solar cells are elucidated. InP solar cells with higher carrier concentration substrate are found to be more radiation resistant, which is due to the superior radiation-resistance for the high carrier concentration InP substrate.

Optical Studies of CaS:Eu, Sm Infrared Stimulable Phosphors

The photoluminescence (PL) and infrared stimulated luminescence (ISL) spectra of CaS:Eu,Sm infrared stimulable phosphors (ISPs) are studied. In addition, the concentration dependence of ISL intensity is examined. Sm3+- and Eu2+-related structures are found in both the PL excitation and emission spectra. Two types of Sm3+ are found, one of which exhibits strong emission at around 650 nm and the other, weak emission compared to the other emissions at around 565 nm and 605 nm. These are assigned to an asymmetric and a symmetric site, respectively. ISL excitation spectra coincide with the Eu2+ 4f7 to 4f65d1 and 4f66s1 transitions and range from 220 to 650 nm. ISL emission spectra coincide with the Eu2+ 4f65d1 to 4f7 transition and range from 550 to 750 nm. ISL stimulation spectra range from 0.8 µm to 1.7 µm and are thought to indicate the Sm2+ ion transition from 4f6 to 4f55d1. The maximum ISL intensity is obtained from a sample with Eu and Sm concentrations of 500 and 130 ppm, respectively.

Optical TE–TM mode conversion in double epitaxial garnet waveguide

TE-TM mode conversion was examined in two-layer epitaxial garnet films, with (Sc, Ga) substituted YIG films on a GGG substrate. Mode conversion efficiency of 96% was observed in a single-mode waveguide over a 4.0-mm propagation distance at 1.15-microm wavelength. Mode conversion was increased by 2.0 X 10(-4) optical anisotropy arising from the photoelastic effect of a 0.03% lattice mismatch between the guiding layer and the substrate. A 45 degrees polarization rotator becomes possible because the TE and TM mode degeneracy is nearly complete. A novel waveguide optical isolator using the 45 degrees rotator is proposed.

Completely Bi-Substituted Iron Garnet (BIG) Films Prepared by Electron Cyclotron Resonance (ECR) Sputtering

Completely Bi-substituted iron garnet (BIG) films were epitaxially synthesized on garnet substrates by electron cyclotron resonance (ECR) sputtering. The critical temperature for the epitaxial growth, crystallinities, and Faraday rotation θF of BIG films depend on the O2 gas flow rate. With an optimum flow rate of 7 sccm, single-crystal BIG films were obtained at temperatures above 390°C, and θF achieved its largest value of 8.4×104 deg/cm (λ=0.633 µm). The minimum ferromagnetic resonance (FMR) linewidth ΔH was 70 Oe; this is the smallest value ever reported. The uniaxial perpendicular magnetic anisotropy is mainly caused by the elastic deformation induced by the thermal expansion mismatch between the film and the substrate.

Structure and Lattice Deformation of Ce-Substituted Yttrium Iron Garnet Film Prepared by RF Sputtering

We investigated the structure and lattice deformation of CeY2Fe5-δO12-Z(Ce1YIG) film prepared by RF sputtering. We found that the lattice constant parallel to the film plane a// is equal to the lattice constant of the substrate as. This shows that this Ce1YIG film belongs to Region I in the Besser model. Some Ce1YIG films become a double-layer structure as the lattice constant mismatch || Δa|| increases. This change in the film structure caused by || Δa|| is very different from that of LPE film. In a Faraday rotation θF spectrum of single-layer films deposited on GCGMZG and GGG substrates, the θF dispersion around 800 nm is shifted to the longer wavelength side. We consider this θF shift around 800 nm to be induced by the difference in the film deformation.

Proton Irradiation Damage in GaAs Single Crystals Examined for Solar Cells

In this study, high-energy proton (0.5–2 MeV) irradiation damage in GaAs solar cells and single crystals was compared with 60Co γ-ray irradiation damage. The minority carrier diffusion length in proton-irradiated GaAs single crystals can be evaluated from the photovoltaic effect of solar cells. The damage rate in GaAs due to proton irradiation equivalent to that due to 60Co γ-ray irradiation was determined experimentally. The dependence of the damage rate on the proton energy and changes in the solar cell spectral reponse show that high-energy protons above 1 MeV produce uniform damage in a GaAs solar cell active layer, and most low-energy proton irradiation damage is confined within the surface layer. The behavior of proton irradiation damage in GaAs on annealing was also examined.

Al0.2Ga0.8As p+‐n junction solar cells grown by molecular beam epitaxy

Al0.2Ga0.8As p+‐n junction solar cells were fabricated by molecular beam epitaxy (MBE) and the relationship between cell properties and growth conditions was examined. It was found that growth temperature strongly influenced the minority carrier diffusion length in cell layers. At a growth temperature of 700 °C, minority carrier diffusion length was much improved and a high conversion efficiency of 12.9% (1 sun AM1.5, for an active area) was obtained.

Al/sub 0. 2/Ga/sub 0. 8/As p/sup +/-n junction solar cells grown by molecular beam epitaxy

Al0.2Ga0.8As p+‐n junction solar cells were fabricated by molecular beam epitaxy (MBE) and the relationship between cell properties and growth conditions was examined. It was found that growth temperature strongly influenced the minority carrier diffusion length in cell layers. At a growth temperature of 700 °C, minority carrier diffusion length was much improved and a high conversion efficiency of 12.9% (1 sun AM1.5, for an active area) was obtained.

Spatial cross modulation method using a random diffuser and phase-only spatial light modulator for constructing arbitrary complex fields

We propose a spatial cross modulation method using a random diffuser and a phase-only spatial light modulator (SLM), by which arbitrary complex-amplitude fields can be generated with higher spatial resolution and diffraction efficiency than off-axis and double-phase computer-generated holograms. Our method encodes the original complex object as a phase-only diffusion image by scattering the complex object using a random diffuser. In addition, all incoming light to the SLM is consumed for a single diffraction order, making a diffraction efficiency of more than 90% possible. This method can be applied for holographic data storage, three-dimensional displays, and other such applications.

Wavefront shaping with disorder-engineered metasurfaces

Recently, wavefront shaping with disordered media has demonstrated optical manipulation capabilities beyond those of conventional optics, including extended volume, aberration-free focusing and subwavelength focusing. However, translating these capabilities to useful applications has remained challenging as the input–output characteristics of the disordered media (P variables) need to be exhaustively determined via O(P) measurements. Here, we propose a paradigm shift where the disorder is specifically designed so its exact input–output characteristics are known a priori and can be used with only a few alignment steps. We implement this concept with a disorder-engineered metasurface, which exhibits additional unique features for wavefront shaping such as a large optical memory effect range in combination with a wide angular scattering range, excellent stability, and a tailorable angular scattering profile. Using this designed metasurface with wavefront shaping, we demonstrate high numerical aperture (NA > 0.5) focusing and fluorescence imaging with an estimated ~2.2 × 108 addressable points in an ~8 mm field of view.Using designer-disordered metasurfaces, optical input–output characteristics, which are typically difficult to obtain, can be known a priori. The approach is used for wavefront shaping, high-numerical-aperture focusing and fluorescence imaging.