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

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">T</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> grating layer on a SiN guiding core layer on a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">S</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> substrate. A narrowband flat-top filter is demonstrated with a 7 nm full-width at half-maximum.