Infinite-Q Accidental and Merging BICs with Broken Up-down Symmetry

Due to the exceptional light confinement capabilities, optical bound states in the continuum (BICs) have attracted a lot of interest in both fundamental and applied research. It has recently been shown that planar periodic structures can support different types of BICs - accidental and symmetry-protected BICs - which can be aggregated at a single point in the momentum space, generating the so-called merging BIC. The Q factors of merging BICs can reach ultrahigh values and exhibit exceptional topological robustness. However, even a slight violation of the up-down mirror symmetry transforms accidental BICs into quasi-BICs with finite Q factors and destroys the topological advantage of the merging BICs. Here, we introduce a method to effectively compensate for the deteriorating effect of the symmetry breaking. We demonstrate the method using the example of a one-dimensional dielectric grating on a semi-infinite substrate, in which the symmetry is broken by an interface with air. We show that the dielectric environment on both sides of the grating can be balanced by an additional layer of a high-index dielectric placed on the interface, restoring the diverging Q factors of the accidental BICs and the topological properties of the merging BICs. Our findings make it possible to achieve ultrahigh-Q accidental and merging BICs in the absence of up-down mirror symmetry, removing one of the major obstacles in practical realization of topologically robust BICs in substrate-supported photonic crystals and metasurfaces.