Cross-stacking of guided-mode resonance gratings for polarization-independent flat-top filtering

A guided-mode resonance filter (GMRF) consists of a subwavelength grating on a thin-film waveguide and can provide narrowband filtering with fewer layers in contrast to a multilayer dielectric mirror. A GMRF normally shows a peaky reflection spectrum and polarization dependency. In this Letter, two GMRFs are orthogonally stacked to give polarization-independent narrowband flat-top filtering. The GMRFs are designed by numerical methods and then fabricated and stacked. Each GMRF is formed by corrugation of a T i O 2 grating layer on a SiN guiding core layer on a S i O 2 substrate. A narrowband flat-top filter is demonstrated with a 7 nm full-width at half-maximum.