Bilayer Metasurface with High-Q Terahertz Chiroptical Response Empowered by Intrinsic Quasi-Multi-Bound States in the Continuum

Bound states in the continuum (BIC) with high quality factor (Q-factor) and narrow bandwidth can manipulate electromagnetic waves and enhance light-matter interactions. This phenomenon has garnered significant attention in the field of photonics, particularly in the context of all-dielectric metamaterials and metasurfaces. Recently, the introduction of BIC into chiral structures has shown that upon circularly polarized light irradiation of metasurfaces with chiral asymmetry, BIC leaks into chiral quasi-BIC (Q-BIC) and induces near-unity circular dichroism (CD). This has broad applications in fields including efficient nonlinear spin optics and chiral biosensing. Here, we numerically demonstrate in the terahertz band that simultaneous breaking of in-plane inversion symmetry and mirror symmetry in monolayer and bilayer cross-shaped structures can lead to Q-BICs with strong chirality, and achieve near-unity CD with high Q-factor. Especially, the bilayer metasurface can realize two Q-BICs in the considered frequency band without introducing complexity of the pattern in each layer. It has been demonstrated that there is a positive correlation between the number of chiral Q-BICs and the layers, which contributes to enhancing the modulation depth of the chiral optical response. Besides, the CD of the two Q-BIC modes in the bilayer metasurface can be flexibly regulated with more freedoms by varying the chiral asymmetry parameters in each layer and the distance between the layers. Additionally, the chiral response induced by Q-BIC in the bilayer structure exhibits a more significant CD and higher Q-factor in both operational modes, indicating higher sensitivity. For example, the figure of merit (FOM) reaches of 357 RIU-1 and 914 RIU-1 for Mode-I and Mode-II in bilayer metasurface, compared to only 85 RIU-1 for the Q-BIC mode in the monolayer metasurface. This work demonstrates a viable technique for controlling terahertz waves on chiral bilayer metasurface, which is expected to enable the practical application of terahertz chiral sensors with high spectral resolution.