A high resolution refractive-index sensor with a guided-mode resonant grating has been proposed. The gratin has a two-dimensionally periodic structured surface, which is covered with liquid to be measured. The resonant wavelength depends on the polarization states of light for oblique incidence. The change in refractive index of the liquid is determined from the difference of reflectance (or transmittance) between the P and S polarized light waves. The lattice structured silica substrate with a period of 380 nm was made. And a hafnium-dioxide thin film was deposited on the substrate. When the grating surface was covered with water, the measured reflectance had resonant peaks at a wavelength of 615 nm for S polarization and 617 nm for P polarization at an incident angle of 0.5°. For a wavelength of 616 nm, the difference of transmittance of P and S polarization was in linear relation to the change in refractive index. The refractive index was detected with a resolution of 4x10-4 in a measurement range of 0.064.
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.
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%.
This research concerns a thin illumination device to be used by mounting between a Reflection-type liquid crystal display device (LCD) and a viewer. Because, the brightness of display is highly dependent on the surrounding environment, an auxiliary illumination is needed to provide against cases when surrounding light is insufficient. We investigated theoretically and experimentally into a grating for monochromatic illumination. First, the requirements of illumination were calculated and a grating shape for illumination was developed. The antireflection effect of the grating was confirmed. In this paper, the diffraction efficiency of the grating was calculated by a rigorous analysis method. It is structured the right triangular profile with the subwavelength period to project light onto the display object by using diffraction effect. Therefore, the light guide of length/thickness equals 50, with the grating, gets an illumination efficiency of 90%. Not occurring reflected light from the illumination device by an antireflection effect prevents that the contrast of display becomes low. Next, the fabrication method by using a reactive ion etching (RIE) that makes the grating of high precision was developed.
Fabrication of a fine diffractive optical element on a Si chip is demonstrated using imprint lithography. A chirped diffraction grating, which has modulated pitched pattern with curved cross section is fabricated by an electron beam lithography, where the exposure dose profile is automatically optimized by computer aided system. Using the resist pattern as an etching mask, anisotropic dry etching is performed to transfer the resist pattern profile to the Si chip. The etched Si substrate is used as a mold in the imprint lithography. The Si mold is pressed to a thin polymer (poly methyl methacrylate) on a Si chip. After releasing the mold, a fine diffractive optical pattern is successfully transferred to the thin polymer. This method is exceedingly useful for fabrication of integrated diffractive optical elements with electric circuits on a Si chip.
We have developed several kinds of diffractive optical elements with subwavelength structures for optical information and transmission system. This paper describes the optical elements and fabrication technique, such as antireflection structured surface, a high efficiency computer generated hologram, a non-polarizing resonant grating filter, and a micro retarder array for the real-time polarimeter.
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.
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.
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.
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