Accidental BICs and Chiroptical Response of Hollow Channels in Silicon Nanodisk Resonator-Based Metasurfaces for Chiral Sensing

Achieving light confinement at the nanoscale is essential for enhancing light-matter interactions, and bound states in the continuum (BICs) have emerged as promising approaches. This study presents a dielectric metasurface design featuring air channels within a silicon nanodisk resonator, demonstrating symmetry-protected BICs (SPBICs) and accidental BICs (ABICs). Here, we report SPBICs that exhibit an exceptionally high quality (Q) factor at very low asymmetry parameters. An increase in the asymmetry decreases the Q-factor quadratically. To alleviate this, we observe that the SPBIC can be transformed into ABICs by adjusting the radius of one air channel that converts the resonances from quasi-SPBICs to quasi-ABICs. Introducing a third air channel with an optimized radius helps to achieve an ultrahigh Q-factor approximate to 38000. Multiple ABICs with enhanced local fields were observed for both x- and y-polarizations. The triple air channel design also achieves a maximal extrinsic chiral response by breaking the symmetry through an oblique incidence angle, which influences circular dichroism (CD) and transmittance for different circular polarizations. Numerical simulations reveal that the proposed chiral metasurface achieves a near-perfect value of CD = -0.99, a Q-factor of 8846, and a field enhancement by a factor of 200. Moreover, slightly tilting the nanodisk can realize a high intrinsic chirality (CD = -0.88) with a Q-factor approximate to 104. The concept of chiral BICs studied here can be utilized in, but not limited to, applications such as chiral sensing, bioimaging, and chiroptical spectroscopy.