# Chiral Optical Tamm States: Temporal Coupled-Mode Theory

## TL;DR

This paper models the spectral behavior of chiral optical Tamm states at interfaces using temporal coupled-mode theory, revealing polarization-dependent mode coupling and conditions for optimal excitation.

## Contribution

It introduces a theoretical framework for describing COTS spectral behavior and mode coupling, highlighting polarization effects and conditions for polarization crossover.

## Key findings

- Mode coupling differs for circular polarizations due to helix structure.
- COTS frequency falls into the stop band, causing exponential attenuation for co-handed polarization.
- Cross-handed polarization can excite COTS when reflected from the mirror.

## Abstract

The chiral optical Tamm state (COTS) is a special localized state at the interface of a handedness-preserving mirror and a structurally chiral medium such as a cholesteric liquid crystal or a chiral sculptured thin film. The spectral behavior of COTS, observed as reflection resonances, is described by the temporal coupled-mode theory. Mode coupling is different for two circular light polarizations because COTS has a helix structure replicating that of the cholesteric. The mode coupling for co-handed circularly polarized light exponentially attenuates with the cholesteric layer thickness since the COTS frequency falls into the stop band. Cross-handed circularly polarized light freely goes through the cholesteric layer and can excite COTS when reflected from the handedness-preserving mirror. The coupling in this case is proportional to anisotropy of the cholesteric and theoretically it is only anisotropy of magnetic permittivity that can ultimately cancel this coupling. These two couplings being equal results in a polarization crossover (the Kopp--Genack effect) for which a linear polarization is optimal to excite COTS. The corresponding cholesteric thickness and scattering matrix for COTS are generally described by simple expressions.