Janus bound states in the continuum in structurally symmetric photonic crystals

Abstract: We propose a sigma_z-symmetry-preserving approach to achieve Janus bound states in the continuum (Janus BICs) exhibiting asymmetric topological charges in the upward and downward radiation channels. While prior approaches typically involve explicit structural perturbations to break vertical symmetry, our design leverages a bilayer photonic crystal slab (PCS) system with independently tunable refractive indices, introducing an optical asymmetry without altering the geometric symmetry. In the optical symmetry case, the system supports symmetry-protected BICs at Gamma point with topological charge q = -1, and Friedrich-Wintgen BICs (FW-BICs) at off-Gamma point with q = +1. Upon introducing refractive index detuning, the polarization vortex splits into two circularly polarized states (C points) with half-integer topological charge (q = 1/2), shifting oppositely in momentum space for upward and downward radiation, while the symmetry-protected BICs remain unaffected. Janus BICs are established through the shift of upward-radiating C points shift towards the Gamma point, accumulating a total topological charge of q = -1, while the downward-radiating counterpart contributes q = +1, leading to a net topological charge reversal between the two radiation channels. This purely optical mechanism allows for the realization of Janus BICs without any structural deformation. Their asymmetric topological nature makes them ideally suited for applications in unidirectional light sources, chiral photonic interfaces, and topological photonic circuits, offering a promising platform for on-chip optical communication, sensing, and quantum information processing.