Resonant Dual‐Grating Metamembranes Supporting Spectrally Narrow Bound States in the Continuum

Abstract Properties of photonic devices fashioned with dual‐grating metamaterials are reported. Enclosed by dual periodic regions, laterally propagating Bloch modes undergo radiative scattering and leaky‐mode resonance whose properties differ markedly from those with single periodicity. The resonance signatures are sensitively controlled by the relative parameters of the periodic regions. In particular, if they are physically identical and separated by a half‐wavelength, there ensues a bound state in the continuum (BIC) with extremely narrow resonance linewidth. On varying the separation between the periodic layers, the linewidth and corresponding Q can be tuned. This is confirmed experimentally via nanoimprinted dual‐grating membranes. The experimental spectra agree well with rigorously computed spectra as well with an analytical model due to Avrutsky. At grating‐depth and thickness values satisfying this model, three different types of BICs are supported by a single metamembrane. Two BICs appear at normal incidence at the Γ point with one being a quasi‐BIC on one band edge while a true symmetry‐protected BIC resides on the other edge. Moreover, a quasi‐BIC state away from the Γ point in the same device is demonstrated. Whereas these results are based on a simple model with 1D periodicity, the primary properties will carry over to general 2D/3D photonic lattices.