Coherent perfect absorption in a homogeneously-broadened two-level medium

TL;DR

This paper investigates coherent perfect absorption (CPA) in a homogeneously-broadened two-level medium, revealing how dispersive properties affect the CPA conditions and the influence of bistability on CPA observation.

Contribution

It provides detailed analysis of CPA conditions in a two-level medium, highlighting the impact of dispersion and bistability on the process.

Findings

CPA frequency differs from lasing mode due to dispersion

Exact time-reversal symmetry is broken in the medium

CPA in bistable regimes requires operation on the upper hysteresis branch

Abstract

In recent works, it has been shown rather generally that the time-reversed process of lasing at threshold realizes a coherent perfect absorber (CPA). In a CPA, a lossy medium in an optical cavity with a specific degree of dissipation, equal in modulus to the gain of the lasing medium, can perfectly absorb coherent optical waves that are the time-reversed counterpart of the lasing field. Here the time-reversed process of lasing is considered in details for a homogeneously-broadened two-level medium in an optical cavity, and the conditions for CPA are derived. It is shown that, owing to the dispersive properties of the two-level medium, exact time-reversal symmetry is broken and the frequency of the field at which CPA occurs is generally different than the one of the lasing mode. Moreover, at a large cooperation parameter the observation of CPA in the presence of bistability requires to operate in the upper branch of the hysteresis cycle.