Micro-nano photonic structures hold immense potential in biochemical analysis, environmental monitoring, and medical detection. Chiral metasurfaces, as an emerging micro-nano photonic structure, have attracted wide attention due to their simple fabrication process and ease of integration. In recent years, chiral metasurfaces have been widely used in many practical scenarios, especially for enhancing circular dichroism (CD) and chiral biosensing. Various chiral metallic metasurfaces have been designed to improve chiral responses and CD. However, the inherent ohmic losses in metallic materials result in a low quality factor (Q-factor) for the CD spectrum. The bound state in the continuum (BIC) enables chiral metasurface sensors to achieve CD spectra with high Q-factors. Strong CD spectra with high Q-factors can be achieved by breaking in-plane or out-of-plane geometric symmetry to excite intrinsic chiral quasi-BIC (QBIC), thereby enhancing chiral sensing performance. However, most studies on BIC-based chiral metasurface sensors focus on enhancing the Q-factors of CD spectra, with limited emphasis on the stability of CD peak values. The CD peak value is usually sensitive to variations in structural parameters, which requires precise device manufacturing. In this study, we propose an inverse-S chiral all-dielectric metasurface sensor based on chiral QBIC. The sensor achieves a stabilized and strong CD peak value and exhibits excellent performance in refractive index and chiral sensing.

The proposed chiral metasurface structure consists of periodically arranged inverse-S silicon unit cells on a silica substrate (Fig. 1). The optical properties of the metasurface are analyzed using the finite element method (FEM). Periodic boundary conditions are applied in the x and y directions of the unit cell, with perfect matching layers set in the z direction to ensure result accuracy. Circularly polarized light (CPL) is incident vertically on the metasurface along the -z direction. Firstly, the eigenmodes of the inverse-S structure with in-plane C₂ symmetry are analyzed. Then, the eigenpolarization state of the symmetry-breaking structure is discussed, which demonstrates that the metasurface realizes the chiral response (Fig. 2). The CD response of the structure is calculated using the Jones matrix method. The stability of the CD peak values within a 20 nm structural parameter range is further explored. The far-field scattering power of the structure is analyzed using electromagnetic multipole theory, and the physical mechanism of stabilized strong CD is discussed. Finally, the refractive index sensing performance of the chiral metasurface sensor is analyzed alongside the local enhancement of optical chirality, with the results confirming that the structure significantly enhances chiral light-matter interactions.

By breaking the in-plane C₂ symmetry of the chiral metasurface, the symmetry-protected BIC (SP-BIC) is transformed into an intrinsic chiral QBIC, which achieves a strong CD value of 0.96 with a high Q-factor of 45985. As the asymmetry parameter increases, the CD peak value consistently exceeds 0.85 (Fig. 3). The strong CD of the chiral metasurface is attributed to the main contribution of the magnetic quadrupole (MQ) at left-handed circularly polarized (LCP) incidence (Fig. 4). The metasurface consistently exhibits a strong CD peak value exceeding 0.85 when the structural parameters vary within a range of 20 nm (Fig. 6). The stable strong CD is attributed to the fact that the MQ is significantly enhanced at LCP incidence and strongly suppressed at right-handed circularly polarized (RCP) incidence (Fig. 7). The chiral metasurface sensor demonstrates a refractive index sensing sensitivity of 375.86 nm/RIU and a figure of merit (FOM) of 12453.94 RIU^-1, which indicates high-performance refractive index sensing capabilities. In addition, the CD peak value consistently exceeds 0.85 as the environmental refractive index changes (Fig. 8). The biosensor based on the inverse-S chiral metasurface exhibits excellent stability and reliability for analyte detection. Driven by chiral QBIC, the metasurface achieves a local enhancement of optical chirality by up to four orders of magnitude, which significantly improves chiral light-matter interaction. The chiral enantiomers can be sensitively detected by analyzing distinct CD spectral shifts (Fig. 9).

In this paper, we propose an inverse-S chiral all-dielectric metasurface. By breaking the structural symmetry, the intrinsic chiral QBIC is excited, which results in a stabilized and strong CD. The eigenpolarization state, far-field scattered power, and CD spectrum of the metasurface are analyzed using FEM. Simulation results indicate that the proposed chiral metasurface maintains a stable CD peak value above 0.85 within a 20 nm range of structural parameter variations. The stable CD peak value is attributed to the fact that the MQ is significantly enhanced at LCP incidence and strongly suppressed at RCP incidence. The structure achieves a refractive index sensitivity of 375.86 nm/RIU and a FOM as high as 12453.94 RIU^-1. In addition, the local enhancement of optical chirality in the metasurface is up to four orders of magnitude, which enables sensitive detection of chiral enantiomers. We provide a theoretical reference for the design of high-performance chiral metasurface sensors.