Chiral Coherent Perfect Absorption on Exceptional Surfaces

TL;DR

This paper introduces a photonic system operating on an exceptional surface that achieves chiral, degenerate coherent perfect absorption at an exceptional point, enabling unidirectional perfect absorption with potential applications in radiation control.

Contribution

The authors design a waveguide-coupled resonator with feedback to operate on an exceptional surface, demonstrating chiral CPA at an exceptional point, which was previously challenging to stabilize and tune.

Findings

First demonstration of chiral CPA at an exceptional surface.

Observation of squared Lorentzian absorption lineshapes.

System operates at critical coupling on the exceptional surface.

Abstract

Engineering the transport of radiation and its interaction with matter using non-Hermiticity, particularly through spectral degeneracies known as exceptional points(EPs), is an emerging field that has both fundamental and practical implications. Chiral behavior in the vicinity of EPs opens new opportunities in radiation control, such as unidirectional reflection or lasing with potential applications in areas ranging from cavity quantum electrodynamics and spectral filtering to sensing and thermal imaging. However, tuning and stabilizing a system to a discrete EP in parameter space is a challenging task: either the system is operated close to an EP rather than directly at the EP or the true power of EP is obscured by stability issues. Here, we circumvent this challenge by designing a photonic system that operates on a surface of exceptional points, known as an exceptional surface (ES). We achieve this by using a waveguide-coupled optical resonator with an external feedback loop that induces a nonreciprocal coupling between its frequency degenerate clockwise (CW) and counterclockwise (CCW) traveling modes. By operating the system at critical coupling on the ES, we demonstrate, for the first time, the effect of chiral and degenerate coherent perfect absorption (CPA) where input waves in only one direction are perfectly absorbed. This novel type of CPA-EP is revealed through the observation of the predicted and long-sought squared Lorentzian absorption lineshapes. We expect our results and approach to serve as a bridge between non-Hermitian physics and other fields that rely on radiation engineering.