Exploring the Nature of Quasi-Bound States in the Continuum Using Metasurface Models

Bound states in the continuum (BICs) are non-radiating modes supported by wave-based systems, although their spectra are compatible with radiation. The most well-known feature of BICs is the formation of resonant behaviors with exceedingly high quality factors (Q-factors). In practice, due to the imperfect nature of the considered systems, the Q-factors are finitely large, and the modes are referred to as quasi-bound states in the continuum (Q-BICs). As these modes are of particular interest in numerous applications, many physical systems supporting Q-BICs and various techniques for their realization have been reported in recent studies. In this work, metasurface models are proposed to investigate the nature of Q-BICs. It is shown that by utilizing metasurface models for BIC-supporting electromagnetic metastructures, which do not necessarily possess deeply subwavelength thickness, one can capture and analyze the salient features of BICs (or Q-BICs) in a far-field manner. This is achieved even though their formation is inherently related to internal information within the considered structures, which is not accessible in a far-field model. Multiple illustrative examples and numerical results are presented to corroborate the proposed approach. The proposed modeling technique provides an elegant method to predict, optimize, and analyze BIC-based phenomena across a wide range of wave-physics branches.