Brillouin Zone Folding Induced Spin–Orbit-Locking Chiral BIC and Quasi-BIC

Chiral phenomena in photonics have been extensively explored in various optical platforms such as chiral nanoparticles and photonic crystal slabs due to their potential in diverse applications such as sensing and light manipulation. Recently, photonic bound states in the continuum (BICs), which inherently exhibit spin and chiral nature, have drawn substantial interest owing to their ultrahigh quality factor and exceptional light confinement. However, achieving spin–orbit-locking chiral BICs or quasi-BICs remains a challenge, limiting their broader application in advanced optical devices. Here, we propose an approach to simultaneously realize spin–orbit-locking chiral BICs and quasi-BICs in magneto-optical photonic crystal slabs. When the period tripling, in-plane symmetry breaking, and time-reversal symmetry breaking operations are applied on a general and widely used honeycomb lattice, the Brillouin zone folding enabled degenerate bulk modes are transformed into a pair of BICs and a pair of quasi-BICs. The chirality of each mode is thoroughly investigated by inspecting the phase profile of the out-of-plane electric field, the radiation profile of the in-plane magnetic field, and the multipolar decomposition of near fields, all of which confirm the spin–orbit-locking feature. Our work introduces a design framework for creating chiral BICs and quasi-BICs, offering significant potential for future developments in chiral photonics and advanced optical engineering.