Cross-stacking of Guided-mode Resonance Gratings for Polarization-independent Flat-top Filtering

A stack of two thin film waveguide gratings was designed to provide polarization-independent flat-top filtering for a vertically incident free-space wave. The grating was fabricated by a relief of TiO₂ on a Si-N guiding core layer on a SiO₂ substrate. Predicted filtering performance was experimentally demonstrated with 7-nm bandwidth.