Circular-polarization-selective perfect reflection from chiral superconductors

TL;DR

This paper proposes a novel type of mirror based on chiral superconductors that can perfectly reflect circularly polarized light, enabling new chiral optical cavities without additional magnetic components.

Contribution

Introduction of single-crystal circular-polarization-selective mirrors using chiral superconductors that inherently break time-reversal symmetry, facilitating tunable chiral optical cavities.

Findings

CSPR occurs in strong-coupling superconductors in the BCS-BEC crossover regime.

Significant optical Hall conductivity can be achieved in doped quantum Hall insulators and chiral pairing.

Potential application to terahertz chiral cavities with high quality factors.

Abstract

Integrating mirrors with magnetic components is crucial for constructing chiral optical cavities, which provide tunable platforms for time-reversal-asymmetric light-matter interactions. Here, we introduce single-crystal circular-polarization-selective mirrors based on chiral superconductors, which break time-reversal symmetry themselves, eliminating the need for additional components. We show that a circular-polarization-selective perfect reflection (CSPR) occurs for strong-coupling superconductors in the BCS-BEC crossover regime or beyond if the optical Hall conductivity is significant in the unit of conductivity quantum per unit layer, $e^2/ha_z$, where $a_z$ is the lattice constant along the surface normal. While the optical Hall conductivity in chiral superconductors is typically tiny, we classify three routes to obtain a large value. We demonstrate the significant optical Hall conductivity and the resulting CSPR with two examples: (1) superconductivity in doped quantum Hall insulators and (2) chiral pairing that preserves the Bogoliubov Fermi surfaces in the weak-pairing limit. We also discuss the application of our theory to the recently discovered chiral superconducting phase in rhombohedral graphene. Our theory reveals the potential of these classes of chiral superconductors as promising elements for building high-quality-factor terahertz chiral cavities.