Recovery of topologically robust merging bound states in the continuum in photonic structures with broken symmetry

Optical bound states in the continuum (BICs) provide a unique mechanism of light confinement that holds great potential for fundamental research and applications. Of particular interest are merging BICs realized in planar periodic structures by merging accidental and symmetry-protected BICs. Topological nature of merging BICs renders their Q factors exceptionally high and robust. However, the existence of accidental BICs with the radiation loss canceled in both the upward and downward directions relies on the up-down mirror symmetry of the structure. If this symmetry is broken, e.g., by a substrate, the Q factor of the mode drops down. Consequently, ultrahigh-Q merging BICs cannot be achieved in substrate-supported structures. Here, by studying the case of a one-dimensional periodic dielectric grating, we discover a simple method to fully compensate for the detrimental effect of breaking the up-down mirror symmetry. The method makes use of a thin layer of a high-refractive-index dielectric material on one side of the structure, allowing one to restore the diverging Q factor of the accidental BIC and fully recover the merged BIC. As an application example, we show that the proposed structures can be used as ultrahigh-performance optical sensors.