Recovery of topologically robust merging bound states in the continuum in photonic structures with broken symmetry
Huayu Bai, Andriy Shevchenko, Radosław Kołkowski
Abstract
Abstract 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.