# Circularly polarized states spawning from bound states in the continuum

## TL;DR

This paper demonstrates how breaking in-plane inversion symmetry in photonic crystal slabs can generate pairs of circularly polarized states from bound states in the continuum, enabling full polarization control.

## Contribution

It reveals a novel mechanism for creating circularly polarized states from BICs through symmetry breaking, expanding topological and polarization manipulation applications.

## Key findings

- Circularly polarized states spawn from BICs with symmetry breaking.
- Full coverage of the Poincaré sphere achieved in photonic systems.
- Ellipticity modulation demonstrated in visible frequencies.

## Abstract

Bound states in the continuum in periodic photonic systems like photonic crystal slabs are proved to be accompanied by vortex polarization singularities on the photonic bands in the momentum space. The winding structures of polarization states not only widen the field of topological physics but also show great potential that such systems could be applied in polarization manipulating. In this work, we report the phenomenon that by in-plane inversion ($C_2$) symmetry breaking, pairs of circularly polarized states could spawn from the eliminated Bound states in the continuum. Along with the appearance of the circularly polarized states as the two poles of the Poincar\'e sphere together with linearly polarized states covering the equator, full coverage on the Poincar\'e sphere could be realized. As an application, ellipticity modulation of linear polarization is demonstrated in the visible frequency range. This phenomenon provides new degree of freedom in modulating polarization.