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Chiral metasurfaces have attracted considerable attention because of their immense potential for diverse applications requiring chiral light-matter interactions. Recently, boosted by a mechanism based on the concept of bound states in the continuum (BICs), high Q factor chiroptical resonances have been exhibited by breaking the inversion symmetries of planar metasurfaces. However, the optical chirality of these chiral metasurfaces is generally intolerable with respect to the structural geometries, especially the geometric asymmetry. Here, we present a novel chiral quasi-BIC with strong optical chirality in an all-dielectric metasurface. By simultaneously breaking the in-plane C 2 rotational and mirror symmetries, the chiral metasurface shows enhanced chiroptical resonances with near-unity CD (∼0.996) and high Q factors (∼2274) at terahertz frequencies. Further analyses based on numerical simulations reveal that the CD of the chiroptical resonance depicts exceptional remarkable tolerableness to the geometry asymmetry δ when δ are present in a broad range, while the corresponding Q factor is modulated accordingly. The results may develop a novel approach to manipulating the advanced optical chirality for potential applications requiring strong CD with enhanced light-matter interactions. [ABSTRACT FROM AUTHOR]

Active control of chirality in plasmonic metamaterials is of great importance due to their potential for diverse applications in imaging, communication and spectroscopy. Recently, inspired by the concept of bound states in the continuum (BIC), strong chiroptical responses are constructed in metamaterials by introducing structural asymmetries. However, most of these chiral metamaterials are static and cannot be modulated. Herein, we theoretically demonstrate a novel approach for manipulating chiroptical responses with enhanced circular dichroism (CD) and large modulation depths in a graphene–metal hybridized metamaterial. By introducing a structured graphene and adjusting the Fermi energy (EF), the conversion between BIC and quasi-BIC states is achieved successfully. The proposed device demonstrates a tuneable CD in the range of 0.693–0.008 when EF is adjusted from 0.01 eV to 1.0 eV, which can be further improved by optimizing its geometry. The proposed graphene–metal hybridized metamaterial paves a new way for manipulating polarization states at terahertz frequencies and is of great potential for practical applications such as dynamic display and optoelectronic modulation. [ABSTRACT FROM AUTHOR]