Abstract
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 ${{\rm TiO}_2}$ grating layer on a SiN guiding core layer on a ${{\rm SiO}_2}$ substrate. A narrowband flat-top filter is demonstrated with a 7 nm full-width at half-maximum.
© 2020 Optical Society of America
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