Asymptotically circularly polarized bound states in the continuum

TL;DR

This paper introduces asymptotically circularly polarized BICs in dielectric structures, revealing their properties, types, and potential for chiral optical applications, supported by bifurcation theory and polarization analysis.

Contribution

It identifies and characterizes acp-BICs, including their types, underlying physics, and bifurcation behavior, advancing understanding of chiral and singular optical responses in dielectric systems.

Findings

acp-BICs approach ideal circular polarization near BICs

all-angle acp-BICs exhibit uniform handedness across momentum space

acp-BICs serve as super-BICs with intrinsic polarization

Abstract

We study a class of bound states in the continuum (BICs) in all-dielectric periodic structures, near which resonant states approach ideal circularly polarized states (CPSs). We term these BICs {\em asymptotically circularly polarized BICs} ({\em acp}-BICs) and identify two types: single-angle and all-angle. Single-angle {\em acp}-BICs permit convergence to left- or right-handed CPSs only along a single momentum-space direction, whereas all-angle {\em acp}-BICs exhibit convergence to CPSs of a single handedness throughout the entire momentum space, rendering them exceptionally promising for chiral optical applications. We reveal that the existence of {\em acp}-BICs is underpinned by total reflection of circularly polarized waves. Moreover, all-angle {\em acp}-BICs qualify as super-BICs, with uniform nearby polarization being an intrinsic property. In addition, a bifurcation theory is developed to analyze the emergence of genuine CPSs from {\em acp}-BICs under $C_{2}$-symmetric structural perturbations. Our results suggest {\em acp}-BICs as a platform for singular and chiral optical responses in all-dielectric systems.