Independent Dual‐Band Bound States in the Continuum Supported by Double Asymmetric Periodic Gratings in Germanium‐Based Structure

Abstract Bound states in the continuum (BIC) are localized states coexisting with extended waves in the continuum spectrum and have infinite lifetimes without emitting radiation. In practice, symmetry‐protected BIC are usually extended to quasi‐BIC by breaking symmetry is commonly employed. A new strategy to realize the dual‐band BIC on an all‐dielectric metasurface is proposed and experimentally proved. This strategy is based on two inverse‐guided modes supported by the double asymmetric periodic gratings (dual‐APGs). In the normal incidence condition, the guided modes are bounded as radiation‐free dark states due to the symmetric incident field and the symmetric structural geometry. By importing symmetry breaking in adjacent gratings or gaps, the guided modes are coupled with radiation mode and converted into resonance states with a high Q‐factor. These two BICs modes supported by air gaps and gratings can be controlled independently. In addition, experimental observation and verification based on the non‐etching fabrication technology in the mid‐infrared band are also implemented. This work provides a new method for the realization and tuning of dual‐frequency BICs, the obtained high Q‐factor independent bi‐quasi BICs have potential applications in nonlinear optics, multi‐mode laser, and optical sensing.