High-Q guided-mode resonance of a crossed grating with near-flat dispersion

Guided-mode resonances in diffraction gratings are manifested as peaks (dips) in reflection (transmission) spectra. Resonances with smaller line widths, i.e., with higher Q-factors, ensure stronger light-matter interactions and are beneficial for field-dependent physical processes. However, these high-Q resonances often suffer from strong angular and spectral dispersions. We demonstrate that a class of resonant modes with extraordinarily weak dispersion and Q-factor ∼1000 can be excited in crossed gratings simultaneously with the modes with well-known linear dispersion. Furthermore, the polarization of the incoming light can be adjusted to engineer the dispersion of these modes, and strong to near-flat dispersion or vice versa can be achieved by switching between two mutually orthogonal linear polarization states. We introduce a semi-analytical model to explain the underlying physics behind these observations and perform full-wave numerical simulations and experiments to support our theoretical conjecture. The results presented here will benefit all applications that rely on resonances in free-space-coupled geometries.