# CPA-laser effect and exceptional points in PT-symmetric multilayer structures

## TL;DR

This paper investigates the CPA-laser effect in PT-symmetric multilayer structures, clarifying its relation to exceptional points and showing that maximal contrast occurs below the EP, with lasing observed above it.

## Contribution

It provides a detailed analysis of CPA-laser phenomena in PT-symmetric multilayers, distinguishing between CPA amplification and true lasing relative to the exceptional point.

## Key findings

- Maximal absorption-amplification contrast occurs below the EP.
- Lasing occurs above the EP regardless of input wave parameters.
- Maximal contrast is associated with CPA amplifier, not CPA laser.

## Abstract

The simultaneous existence of coherent perfect absorption (CPA) and lasing is one of the most intriguing features of non-Hermitian photonics. However, the link between CPA lasing and PT symmetry breaking at the exceptional point (EP) needs clarification. In this paper, we study the manifestations of the CPA-laser effect in a PT-symmetric multilayer loss-gain structure using both the transfer-matrix method and numerical simulations of the Maxwell-Bloch equations. We show that the maximal contrast between absorption and amplification at different phase relations between the input waves is reached well below the EP and therefore is not connected to true lasing. In this regime, there is a good qualitative agreement between both computational approaches. Above the EP, the system demonstrates lasing regardless of the parameters of the input waves. Thus, the maximal contrast between the absorption and amplification rather corresponds to the CPA amplifier than to the CPA laser.