Spin-Orbit-Locking Chiral Bound States in the Continuum

TL;DR

This paper demonstrates that applying a magnetic field to a magneto-optical photonic crystal creates chiral bound states in the continuum with spin-orbit locking, high quality factors, and potential for spin-selective nanophotonics.

Contribution

It introduces a new form of chiral BICs enabled by magnetic fields, breaking time-reversal symmetry and advancing topological photonics with unique spin-orbit locking properties.

Findings

Magnetic field lifts degeneracy of BICs into chiral pairs.

Chiral BICs exhibit high quality factors and circular polarization.

Multipole analysis confirms non-zero angular momenta and spin-orbit locking.

Abstract

Bound states in the continuum (BICs), which are confined optical modes exhibiting infinite quality factors and carrying topological polarization configurations in momentum space, have recently sparked significant interest across both fundamental and applied physics.} Here we show that breaking time-reversal symmetry by external magnetic field enables a new form of chiral BICs with spin-orbit locking. Applying a magnetic field to a magneto-optical photonic crystal slab lifts doubly degenerate BICs into a pair of chiral BICs carrying opposite pseudo-spins and orbital angular momenta. Multipole analysis verifies the non-zero angular momenta and reveals the spin-orbital-locking behaviors. In momentum space, we observe ultrahigh quality factors and near-circular polarization surrounding chiral BICs, enabling potential applications in spin-selective nanophotonics. Compared to conventional BICs, the magnetically-induced chiral BICs revealed here exhibit distinct properties and origins, significantly advancing the topological photonics of BICs by incorporating broken time-reversal symmetry.