This research aims to design and investigate a high-performance chiral metasurface based on bound states in the continuum (BICs), achieving both an ultra-high quality factor (Q) and near-unity circular dichroism (CD=0.99). The study focuses on precise control of chiral responses by breaking the in-plane C₂ rotational symmetry or adjusting the angle of light incidence, while maintaining out-of-plane structural symmetry. In addition, we aim to expand the application of such chiral metasurfaces to optical sensing, specifically refractive index sensing, leveraging BIC properties to enhance sensitivity and figure of merit (FOM=233 nm/RIU).

To achieve these objectives, we design a periodic array of dielectric metasurfaces consisting of square nanodisks with double notches. As shown in Fig. 1(b), the unit cell structure features a period P=850 nm, side length W₀=520 nm, notch width W₁=185 nm, and length L=234 nm. The nanodisks are made of high-refractive-index silicon (n=3.48) with a thickness of 350 nm, and the substrate is SiO₂ (n=1.46). Circularly polarized light propagates along the -z direction, perpendicular to the metasurface. Numerical simulations are conducted using COMSOL MULTIPHYSICS, with periodic boundary conditions in the x and y directions and perfectly matched layers (PMLs) in the z direction to ensure accuracy and reliability. We initially analyze the eigenmodes and Q factors of the metasurface in momentum space, as shown in Fig. 1(c) and (d). The BICs are identified at the Γ point, where the Q factor theoretically tends to infinity, and far-field polarization states are characterized, as shown in Fig. 1(e). To study the chiral response, we introduce an in-plane asymmetry parameter δ and investigate the CD spectra and transmittance components, as illustrated in Fig. 2. We also explore the effects of oblique incidence on the chiral response (Fig. 3) and analyze how the azimuthal angle φ affects CD (Fig. 4). For the refractive index sensing application, we systematically analyze the influence of environmental refractive index changes on transmission components and CD response. Specifically, we examine shifts in cross-polarized transmission components TRL and TLR and the CD spectrum as the refractive index of the surrounding medium varies from 1.00 to 1.18, with a step size of 0.03 [Figs. 5(a) and (b)]. The linear redshift in the CD spectrum and the correlation between the CD peak shift and refractive index are also studied [Figs. 5(c) and (d)].

1) High-Q factor and near-unity CD. By introducing an in-plane asymmetry parameter δ, we achieve a near-unity CD (CD=0.99) alongside an ultra-high Q factor. While the Q factor decreases with increasing δ, CD remains close to unity, confirming that fine-tuning δ optimizes chiral q-BIC performance (Fig. 2). 2) Tunable chirality via oblique incidence. We find that chiral response can be precisely modulated by adjusting the incident angle θ and azimuthal angle φ. At θ=6° and φ=90°, the CD reaches a maximum value of 0.98, and the Q factor follows an inverse quadratic relationship with sin θ (Fig. 3). The magnetic dipole (MD) plays a dominant role in the chiral q-BIC mode, as evidenced by the electromagnetic field distribution [Fig. 3(e)]. 3) Refractive index sensing performance. The chiral metasurface demonstrates high sensitivity (233 nm/RIU) and excellent FOM in refractive index sensing. As the refractive index increases, resonance wavelengths of cross-polarized transmission components TRL and TLR shift linearly to longer wavelengths, and the CD spectrum exhibits a corresponding linear redshift [Fig. 5(c)]. The linear relationship between CD peak shift and refractive index change [Fig. 5(d)] confirms the sensor’s high sensitivity and reliability. 4) Physical mechanisms of CD flipping. We observe that the CD sign flips with changes in azimuthal angle φ (Fig. 4). At specific angles, CD exhibits a sign reversal, corresponding with the helicity flip in momentum space [Fig. 1(f)]. This behavior provides insights into the underlying physical mechanisms and offers new possibilities for designing chiral metasurfaces with custom CD properties.

In this study, we successfully design a high-performance chiral metasurface based on BICs, achieving both ultra-high Q factors and near-unity CD. We demonstrate precise control over the chiral response by breaking in-plane C₂ rotational symmetry or adjusting light incidence angles. Furthermore, we extend the application of this chiral metasurface to refractive index sensing, achieving a sensitivity of 233 nm/RIU and a high FOM. Our results enhance the design principles and application scenarios for chiral metasurfaces, offering new directions for the development of optical sensors and photonic devices. The simultaneous achievement of strong CD, high Q factor, and tunable response opens up new possibilities for advanced optical and sensing applications.