High‐ Q Chiroptical Resonances by Quasi‐Bound States in the Continuum in Dielectric Metasurfaces with Simultaneously Broken In‐Plane Inversion and Mirror Symmetries

Abstract Bound states in the continuum (BIC) have recently attracted great attention in photonics. Metasurfaces with broken inversion symmetry exhibit high‐ Q resonances via quasi‐BIC, enabling low threshold lasing, sensing, and efficient nonlinear generation. Recently, chiral BICs have been investigated by using double layer structure or out‐of‐plane perturbation, requiring complicated fabrication processes. This work presents chiral quasi‐BIC by simultaneously breaking the in‐plane inversion and mirror symmetries in dielectric metasurfaces. For such a simultaneous symmetry breaking, the metasurfaces exhibit quasi‐BIC with strong chirality, resulting in near‐unity circular dichroism with high quality factor of several orders‐of‐magnitude. As the chiral quasi‐BIC appears only by in‐plane structural perturbation, it is much more feasible for practical fabrications compared with preceding results. Due to the symmetry‐protected nature of its original BIC, the proposed approach does not require fine tuning of structural parameters for observing the desired effects unlike the accidental BIC. The results presented in this work pave the way for chiral sensing with high spectral resolution, nonlinear chiroptics with high efficiency, low‐threshold circular polarized lasing, and security applications.