Hiroshi Toyota

Fabrication of Microcone Array for Antireflection Structured Surface Using Metal Dotted Pattern

An antireflection surface with sub-wavelength structure has been successfully fabricated on a fused silica substrate. The fabricated antireflection structured surface consists of a microcone array of fused silica with a period shorter than the wavelengths of visible light. The microcone array is made by a reactive ion etching method using fluorocarbon plasma. A microdisk array of chromium thin film, formed by an electron-beam lithography and lift-off process, is used as the etching mask. Since an electric field induced near the substrate was focused on the edges of the metal disks, these disks gradually shrank. Consequently, a conical shape was formed. The fabricated cone array has a period of 250 nm and a height of 750 nm. Measured reflectivity of the antireflection structured surface is less than 0.5% in the wavelength range of 400–800 nm for normal incidence.

Polarization-multiplexed diffractive optical elements fabricated by subwavelength structures.

Polarization-multiplexed phase-only diffractive optical elements with subwavelength structures are proposed and fabricated. The differences among the phase modulations result from the differences among the effective indices exhibited in the subwavelength structures with various filling factors and surface profiles, and the phase retardations are obtained by the relief depth of the structures. The polarization-selective property is achieved by the polarization dependence of the effective indices exhibited in the one-dimensional subwavelength structures and the polarization independence exhibited in the two-dimensional structures. Additionally, the polarization contrast of our polarization-multiplexed elements, defined as the cross talk between the two polarization incidences, is independent of the relief depth. The principle of the polarization multiplexing by use of the subwavelength structures is described, and the fabrication results for the polarization-multiplexed computer-generated holograms are demonstrated.

Low-Temperature Formation of High-Quality

We have fabricated an Al2O3/GeO2 gate-dielectric stack on p-type Ge by electron-cyclotron-resonance plasma oxidation and sputtering without external substrate heating. We show that the midgap interface state density at the GeO2/Ge interface is 4.5 × 1010 cm-2 · eV-1. The hysteresis observed in capacitance-voltage measurements is reduced to 50 mV when the gate bias is swept from accumulation to inversion and back to accumulation or after a single dummy sweep from inversion to accumulation, indicating the possibility that the bulk oxide traps causing the hysteresis are deactivated by the injected holes. The band gap of GeO2 was determined by internal photoemission measurements to be 4.7 eV. The conduction- and valence-band offsets at the GeO2/Ge interface are moderately symmetric and large with values of 1.8 and 2.2 eV, respectively. These promising results suggest that low-temperature plasma-grown GeO2 is a suitable interlayer between high-dielectric-constant dielectrics and Ge.

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This paper describes a novel refractive index sensor with a guided-mode resonant grating (GMRG) filter. The GMRG filter is a narrowband wavelength reflection filter. The incident light is reflected at a resonance condition of incident angle and wavelength. When the grating filter is covered by a liquid to be tested, the resonant condition depends upon its refractive index. The refractive index of the liquid can be determined from the resonance angle for a known wavelength. Since a full-half width of incident angle for the resonance is very narrow (less than 0.1 degree(s)), a high resolution is expected. We designed the GMRG filters for the refractive index sensor. And the resonance angle of incidence was investigated experimentally for the mixture of water and ethyl alcohol.

Interlayer for High-

The electrical properties of tantalum pentaoxide (Ta2O5) films deposited by an electron cyclotron resonance (ECR) plasma sputtering have been investigated using metal–insulator–metal (MIM) capacitor structures. The deposition conditions of Ta2O5 films and the bottom electrode materials of the MIM have been examined from the point of view of both the low leakage current and the high capacitance. On the TiN bottom electrode, additional postdeposition plasma irradiation on the first thin Ta2O5 layer was effective in ensuring a high breakdown strength of the MIM, because of the stabilization of the interlayer between the Ta2O5 film and the TiN bottom electrode film. However, the MIM using a Ru bottom electrode, which shows the characteristics of conductive films even for oxides, showed the high breakdown characteristics of approximately 4.5 MV/cm for as-deposited Ta2O5 in metal-mode deposition in ECR sputtering. In both cases, the dielectric constant of Ta2O5 was approximately 25, so that a high capacitance of approximately 3 fF/µm2 was for frequencies of up to 1 MHz for a MIM with a Ru bottom electrode and 20-nm-thick Ta2O5 film.

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We have proposed a new structure of guided-mode resonant grating (GMRG) filter with low sideband reflectance. This GMRG filter consists of a high refractive index thin-film on an antireflection structured (ARS) surface called “moth-eye structure”. This antireflective GMRG filter is valid for reducing reflection of nonresonant light waves in a wide spectral range. This antireflective GMRG filter is valid for reducing reflection of nonresonant light waves in a wide spectral range. The resonant reflection of this new filter was investigated by numerical calculation based on an electromagnetic grating analysis. In the case of an antireflective GMRG filter with aspect ratio 2, the sideband reflectance for nonresonant light waves was lower than 0.5% for TM polarized light in the wide-wavelength range. We have fabricated an antireflective GMRG filter. The triangular grating of fused silica for ARS surface was fabricated by reactive ion etching due to high-density fluorocarbon plasma with resist line patterns and chromium thin-film line patterns as etching masks. The fabricated antireflective GMRG filter was a period of 333 nm and a height of about 666 nm. The thickness of a TiO2 thin-film deposited on the triangular grating was about 100 nm. Resonant peak was detected at wavelength of 680 nm, and peak intensity was 45%. Moreover, it was found that sideband reflectance was less than 4%.

Gate Dielectrics/Ge by Electron-Cyclotron-Resonance Plasma Techniques

We made a form birefringent micro-retarder array for the real-time imaging polarimetry. The polarimetry system is composed of an imaging lens, the retarder array, a polarization film, and an image sensor. We fabricated a 92x70 retarder array by the electron-beam lithography and the reactive ion etching. Each retarder was a TiO2 subwavelength grating of 0.15(mu) nm in a period on a silica substrate. Some polarization images of four Stokes parameters were obtained with the polarimetry system.

Refractive index sensor with a guided-mode resonant grating filter

Several optical elements with subwavelength structured (SWS) surfaces have been developed. The SWS has optical features of artificial refractive index, form birefringence, resonance and band-gap effects. This paper describes some applications of form birefringent optical elements and a resonant reflection element. A form-birefringent quarter-wave plate was realized by sputtering the high refractive-index thin film on a SWS substrate. The wavelength dispersion of form birefringence restrains the phase retardance from depending on the wavelength of light. An array of form-birefringent wave-plates is useful for the real-time imaging polarimetry. We developed a real-time polarization imaging system for the visible light. A guided-mode resonant grating with a PLZT wave-guide was designed for optical switches. The PLZT is ferroelectric material with an electro-optic (EO) effect. We made a feasibility study on the optical switching by numerical simulations.

Characterization of Metal Insulator Metal Electrical Properties of Electron Cyclotron Resonance Plasma Deposited Ta2O5

The deep gratings with ultrahigh spatial frequencies have been developed for wavelength dispersion applications. The diffraction performances, including diffraction efficiencies, wavelength dispersive abilities and wavelength band, have been theoretically investigated. The high diffraction efficiencies, wide wavelength dispersive abilities and broad wavelength range proved the significant advantages of the developed gratings for our applications. For the fabrication process, a method for synthesizing the grating profiles to produce optimal optical performances for practical applications and for easily achieving high fidelity by microfabrication technologies has been proposed. The experimental results show that the fabricated gratings using the synthesis method have diffraction efficiencies higher than 95%, while the wide wavelength dispersions are reserved, and low diffraction fluctuations are also reduced.

Design and fabrication of guided-mode resonant grating filter with antireflection structured surface

We present the real time polarimeter with a micro retarder array consisting of subwavelength gratings with different directions. A 2x2 form-birefringent micro retarder array was fabricated with an electron-beam lithography method and a reactive ion etching technique. Each retarder was a TiO2 grating with 300-nm period on a silica substrate, and the phase retardance was 0.31 (pi) for 633-nm wavelength light. Using the fabricated retarder array, the real time polarimeter was made. The micro retarder array and a linear polarizer film were stacked up on a 2x2 photo-detector array. Four Stokes parameters were computed at a real time rate from outputs of the detector array. We could measure temporal change in polarization states of light by simple experiments.