How many parameters do we need to obtain a BIC in symmetric and asymmetric structures?

Abstract

Photonic bound states in the continuum (BICs) are nonradiating eigenmodes of structures with open scattering channels. Most often, BICs are studied in highly symmetric structures with one open scattering channel. In this simplest case, the so-called symmetry-protected BICs can be found by tuning a single parameter, which is the light frequency. Another kind of BICs -- accidental BICs -- can be obtained by tuning two parameters. For more complex structures lacking certain symmetries or having several open scattering channels, more than two parameters might be required. In the present work, we propose an algebraic approach for computing the number of parameters required to obtain a BIC by expressing it through the dimension of the solution set of certain algebraic equations. Computing this dimension allows us to relate the required number of parameters with the number of open scattering channels without solving Maxwell's equations. We show that different relations take place when the scattering matrices describing the system are symmetric or asymmetric. The obtained theoretical results are confirmed by the results of rigorous electromagnetic simulations.