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 propose an optical switch of a guided-mode resonant grating (GMRG) filter with a Kerr medium and simulate optical switching effects by using the nonlinear finite differential time domain (FDTD) method. It is shown that the nonlinear FDTD method is needed for simulating the optical switch effect by analyzing the bistable feature. The doubly periodic structure was used in order to produce the optical Kerr effect efficiently. Because a doubly periodic GMRG filter operates for small beam diameter and grating area, the electric field can be accumulated to the small area. The doubly periodic grating consisted of materials with refractive indices of 1.88 and 1.0, and the material of index 1.88 had a third-order susceptibility of 8.5×10-10esu. The TE polarized plane waves were normally incident on the grating structure as “pump light” and “probe light.” When the intensity of “pump light” increases, the refractive index changes due to the optical Kerr effect, so that the resonant condition of the GMRG filter for the “probe light” also changes. Therefore the transmittance of “probe light” can be controlled by the “pump light.” By changing “pump light” from 0 to 100kW/mm2, the transmittance of “probe light” was controllable from 0 to 0.6.
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 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.