Perturbation approach to improve the angular tolerance of high-Q resonances in metasurfaces.

The interest in high-Q resonances in metasurfaces has been rekindled with the rise of the Bound States in the Continuum (BIC) paradigm, which describes resonances with apparently limitlessly high Q-factors. The application of BICs in realistic systems requires the consideration of the angular tolerance of resonances, however, which is an issue that has not yet been addressed. Here, we develop an ab-initio model, based on Temporal Mode Coupled Theory, to describe the angular tolerance of distributed resonances in metasurfaces that support both BICs and Guided Mode Resonances (GMRs). We then discuss the idea of a metasurface with a perturbed unit cell, similar to a supercell, as an alternative approach for achieving high-Q resonances and we use the model to compare the two. We find that, while sharing the high-Q advantage of BIC resonances, perturbed structures feature higher angular tolerance due to band planarization. This observation suggests that such structures offer a route towards high-Q resonances that are more suitable for applications.