Parity-time-symmetry-induced Janus bound states in the continuum and rings of lasing threshold modes
Phys. Rev. A 112, 063517 – Published 8 December, 2025
DOI: https://doi.org/10.1103/37yr-5pfx
Abstract
The far-field polarization of a photonic crystal (PhC) slab is inherently linked to its spatial symmetries. We demonstrate that introducing a parity-time ()-symmetric perturbation—while breaking up-down mirror symmetry—leads to the emergence of a Janus bound state in the continuum (BIC) and a momentum-space ring of lasing threshold modes. Starting from a PhC slab that hosts both symmetry-protected BIC and accidental BICs, we investigate the effects of -symmetric perturbations on these states. The perturbation transforms accidental BICs into pairs of circularly polarized states ( points) and induces a ring of lasing threshold modes in momentum space, while leaving the symmetry-protected BIC unaffected. As the perturbation strength increases, four points on one side merge with the symmetry-protected BIC, whereas those on the opposite side remain separate, giving rise to a Janus BIC characterized by asymmetric topological charges in the upward and downward radiation. Meanwhile, the ring of lasing threshold modes expands, enhancing the quality () factor of nearby resonant modes due to the diverging at the lasing threshold. As a result, points with ultra-high- factors emerge, offering new opportunities for advanced chiral light-matter interactions and vortex beam generation.
