Papers by Author: Kenta Miura

Abstract: A radio-photoluminescence (RPL) dosimeter with a copper-ion luminescent center was fabricated to evaluate its response in ionized particle detection. A focused proton microbeam with varying energies up to 3 MeV and heavy ions of 490 MeV osmium (Os) were employed along with X-rays to evaluate its performance in micrometer-scale radiation monitoring. The response to ionized particles was evaluated under focused proton beam irradiation where the peak wavelength differed from that obtained under X-ray irradiation. Two peaks were observed under Os irradiation where the secondary-generated particles and photons have a significant effect on the dosimeter. The results suggest that the fabricated RPL dosimeter with copper luminescence center could be used to estimate the irradiation effect of primary ionized particles separately from the effects of secondary particles, photons, and environmental background radiation.

Abstract: A real-time position detection system is developed for measuring heavy ions with low fluence and energy of several hundred MeV, which are generated from an azimuthally varying field (AVF) cyclotron accelerator. We investigate the photoluminescence of α-Al₂O₃ single crystals implanted with Eu (Al₂O₃:Eu), which is used in the detection system. The Al₂O₃:Eu scintillators with a fluence of 3.0 × 10¹⁶ cm⁻² are annealed at 500–900°C. The annealing conditions required for the Al₂O₃:Eu scintillators to obtain the maximum luminescence are 0.5 h at 600°C. The scintillator is placed on the AVF cyclotron target stage under atmospheric pressure and is irradiated by 260-MeV Ne. An inverted confocal microscope with a ×10 objective lens is positioned behind the Al₂O₃:Eu scintillator, and the luminescent images during ion irradiation are obtained by a position-sensitive camera unit with a 512 × 512 pixel electron multiplying charge-coupled device. The images indicate that our online measurement system has a sufficient spatial resolution, since the luminous diameter induced by irradiation with 190 ions /s is almost the same as that of the microbeam.

Abstract: Microstructure patterning in single-crystal chemical vapor deposition (SC-CVD) diamond films was investigated to establish a direct patterning procedure by using proton beam writing with a focused microbeam of protons. Test microstructural patterns on SC-CVD diamond with different total fluences from 0.5 to 10 pC/μm² were formed with a probe diameter of approximately 1 μm, a maximum scanning area of 800 × 800 μm, and a beam current of up to 100 pA. The most intensively irradiated pattern with beam fluence of 10 pC/μm² was visualized with both transmission and reflection optical microscopy. The other patterns created with lower beam fluences were characterized by the changes in reflective index and were visible only in the transmission microscopy images. The depth of patterns was precisely controlled by changing proton energies in range of 0.75–3 MeV. The structures in depth from 5 to 45 μm were successfully observed in optical microscope images.

Abstract: Enhancement of optical transmittance of polymer-dispersed liquid crystal (PDLC) cells was investigated by introducing a modification into the substrates. Surface treatment by rubbing was performed on both network-type and droplet-type PDLCs to investigate the effect on the optical transmittance of the cells. Differences in the transmittance of the PDLC in the polymer matrix were observed by introducing rubbing effect on network-type PDLC but not on droplet-type PDLC. These phenomena might be used to distinguish the application field of both types of PDLC cells by enhancing optical transmittance and scattering properties.

Abstract: We demonstrated a PMMA-based thermo-optic switch consisting of a Mach-Zehnder (MZ) type waveguide drawn by proton beam writing (PBW) and working at λ = 1.55 μm. The MZ waveguide was drawn by symmetrically coupling two Y junctions with a core width of 8 μm and a branching angle of 2°. A Ti thin-film heater and Al electrodes were formed on the surface of the MZ waveguide using conventional photolithography and wet-etching processes. An ON/OFF ratio of 9.0 dB and a switching power of 43.9 mW were obtained from the sample. The switching power is lower than for conventional commercial silica-based switches.

Abstract: Nonlinear light propagation in a dye-doped liquid crystal (LC) was investigated experimentally. A laser beam with wavelength far from the absorption peak of the material was coupled into an LC cell with homogeneous alignment, and the propagation in the cell was observed. When the polarization azimuth of the incident light was orthogonal to the orientation direction of the LC, soliton-like propagation was obtained for milliwatts of light power in spite of the low absorption. We clarified that the observed nonlinearity is due principally to the photothermal effect enhanced by the dye.

Abstract: In this paper, we designed and analyzed an efficient taper structure to couple light into and out of photonic crystal waveguides (PhCWs) fabricated by Si-ion implantation and electron beam lithography. The coupling structure employs the gentle refractive-index distribution produced in the SiO₂ layer by Si-ion implantation. A taper structure is designed for effective coupling of transverse electric (TE) polarized light (λ = 1.55 μm) into a submicron size PhCW consisting of triangular lattice of air holes (lattice constant, a = 0.666 μm, radius of air holes, r = 0.232 μm, waveguide width, W1 ~ 0.7 μm). The influence of the taper length on the transmission characteristics is investigated. Efficiency in excess of 95% is demonstrated using the finite-difference time-domain and beam propagation methods. This is important for their practical applications in photonic integrated circuits.

Abstract: Proton beam writing (PBW) has recently attracted much attention as a next-generation microfabrication technology. This is a direct-drawing technique and does not need any masks to transfer micropatterns to sample surfaces. In our previous work, we demonstrated the first single-mode straight-line and Y-junction PMMA-based waveguides fabricated using PBW and working at λ = 1.55 μm. In this work, we fabricated the first PMMA-based Mach-Zehnder waveguides for the wavelength utilizing PBW in order to construct thermo-optic switches.

Abstract: The visible-light emission properties of erbium (Er)-doped tantalum-oxide films prepared by co-sputtering were evaluated. Photoluminescence (PL) peaks at wavelengths of 550 and 670 nm could be observed from these films by ultraviolet-laser excitation after annealing. The dependence of Er³⁺ concentration, annealing temperature, and annealing time on the PL peak intensities were investigated. Such light-emitting sputtered films can be useful as high-index materials of photonic crystals that can be applied to novel active devices.

Abstract: Proton beam writing (PBW) has attracted much attention recently as a next-generation micro-fabrication technology. It is a direct-drawing technique and does not need any masks to transfer micro-patterns to sample surfaces. In addition, the refractive index of a poly (methyl methacrylate) (PMMA) can be increased by proton-beam irradiation. In this study, we fabricated the first 1.5-μm-band single-mode, straight-line waveguides and Y-junction waveguides consisting of PMMA layers using the PBW technique.