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

TL;DR

This paper demonstrates high-Q guided-mode resonances with near-flat dispersion in crossed gratings, enabling controlled polarization-dependent dispersion engineering, supported by semi-analytical models, simulations, and experiments.

Contribution

The study introduces a method to achieve high-Q resonances with tunable dispersion in crossed gratings, including near-flat dispersion, through polarization control.

Findings

High-Q modes with Q > 1000 and weak dispersion are demonstrated.

Polarization switching enables transition between flat and linear dispersion.

Theoretical and experimental results confirm the dispersion engineering capability.

Abstract

Guided-mode resonances in diffraction gratings are manifested as peaks (dips) in reflection (transmission) spectra. Smaller resonance line widths (higher Q-factors) ensure stronger light-matter interactions and are beneficial for field-dependent physical processes. However, strong angular and spectral dispersion are inherent to such high-Q resonances. We demonstrate that a class of high-Q resonant modes (Q-factor >1000) exhibiting extraordinarily weak dispersion can be excited in crossed gratings simultaneously with the modes with well-known nearly linear dispersion. Furthermore, we show that 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.