High resolution refractive index sensor with a crossed guided-mode resonant grating

Abstract

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.