Tunable high-$Q$ Janus-to-chiral bound states in the continuum in bilayer PhCs

TL;DR

This paper introduces a bilayer dielectric photonic crystal design that enables tunable, high-quality-factor Janus and chiral bound states in the continuum, with active control over optical chirality and dissipation.

Contribution

It demonstrates a novel approach to controlling topological and chiral optical states in bilayer photonic crystals via symmetry perturbations and material conductivity.

Findings

Generation of Janus-chiral BIC with strong handedness selectivity.

Continuous tuning of circular dichroism and resonance wavelength.

Active modulation of chiral response using material conductivity.

Abstract

We propose a bilayer all-dielectric PhC for controlling Janus bound states in the continuum (BIC) and optical chirality through symmetry-selective perturbations. Starting from a symmetry-protected $\Gamma$-point BIC, we use interlayer displacement as one geometric control knob to generate different topological charges in the upward radiation and downward radiation channels. A subsequent diagonal in-plane displacement reconstructs the polarization topology around the BIC and generates a Janus-chiral BIC with strong handedness selectivity. In contrast, other in-plane perturbations generate chiral quasi-BICs with finite radiative coupling, for which the circular dichroism (CD) and resonance wavelength can be continuously tuned. We further show that material conductivity provides an additional dissipative degree of freedom for actively modulating the chiral response, with a switchable CD exceeding 0.89. Near-field optical-chirality distributions and multipole decompositions reveal that the chiral response originates from a symmetry-induced imbalance of local optical handedness and a spin-selective magnetic-dipole resonance. These results reveal the topological relationship between Janus radiation, polarization singularities and intrinsic chirality, thus paving a scalable route toward reconfigurable high-$Q$ chiral photonics.