Wavelength selective mirror using resonant reflection in a two-dimensional photonic crystal

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We proposed a new structure of guided-mode resonant grating (GMRG) filters with low sideband reflectance. This GMRG filter consists of a high-index thin film on an antireflective structured surface called "moth-eye structure". Since the high-index film undulates along the surface structure, the film acts as a modulated optical waveguide. An incident light wave satisfying a resonant condition is reflected by the GMRG filter, and nonresonant light waves pass through the filter. This 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 a triangular antireflective surface structure whose thickness is 2x greater than its period, the sideband reflectance for nonresonant light waves was lower than 0.5% for TM-polarized light in a wade range of wavelength.We also fabricated a GMRG filter on an antireflective surface structure. A base substrate was a triangular surface structure of fused silica with a period of 333 nm and a height of 〜700 nm. TiO_2 thin film of 100 nm thick was spattered on the moth-eye surface. The refractive indices of the fused silica substrate and the TiO_2 thin film are 1.47 and 2.3. This GMRG filter occurs a resonant reflection at wavelength of 680 nrn with FWHM (full-wide at half maximum) of 0.7 nm. However, the sideband reflectance was lower than 4%.