Guided-mode resonance filter with an antireflective surface consisting of a buffer layer with refractive index equal to that of the grating

A type of guided-mode resonance filter (GMRF) with an antireflective surface consisting of a buffer layer with refractive index equal to that of the grating is proposed, and the approximate design approach is presented. The relation between the filter linewidth and the coupling loss is used to analyze the filter properties by using different derivation methods. It is shown that the dispersion equation of the slab waveguide may provide a reliable approximation in estimating the resonance locations of the GMRF with an antireflective surface. The buffer layer functions as an intermediate layer between the grating and waveguide layers. This reduces the coupling and out-coupling of a mode of the waveguide, which results in significant reduction of the coupling loss and the filter linewidth with the antireflection condition nearly preserved. By changing the thickness of the buffer layer, different linewidths can be obtained with spectral symmetry and sideband suppression almost kept the same. The slight shift of resonance wavelengths due to the variety of the buffer layer thickness and the etching effects can be adjusted to the design value by changing the grating period. Accurate etch depth control to avoid underetching is necessary. The electric field distributions under resonance conditions shows that the buffer layer increases the mode confinement, thus narrowing the filter linewidth.