Exceptional Point Singularities in Multi-Section DFB Lasers

TL;DR

This paper demonstrates how dual-section DFB quantum cascade lasers can be engineered to observe and control exceptional point singularities, revealing unique lasing and absorption behaviors in non-Hermitian photonic systems.

Contribution

It introduces a tunable dual-section DFB QCL platform for studying exceptional points and their associated spectral phenomena in non-Hermitian laser systems.

Findings

Observation of spectral singularities at exceptional points

Control of coupling strength and gain difference in a single device

Lasing and coherent perfect absorption dynamics demonstrated

Abstract

A laser exhibits both controllable gain and loss and, under proper design conditions, is an ideal non-Hermitian system allowing the direct observation and engineering of spectral singularities such as exceptional points (EPs). A dual section distributed feedback (DFB) quantum cascade laser (QCL) is a prototype of such a system, allowing the controlled coupling of a ladder of cavity Fabry-Perot (FP) modes to a quarter wave shifted DFB mode. Tuning the coupling strength and the gain difference between these two set of modes enables probing the regimes from weak coupling to strong coupling and the robust observation of exceptional point singularities. At these exceptional points, the laser exhibits a sequence of lasing and coherent prefect absorption dynamics1,2 when pumped above transparency. Additionally, the pumping scheme allows the deliberate lifting of the exceptional point degeneracy. These results show that dual section QCL is a perfect platform to study exceptional points because the coupling parameter and system loss can be tuned in a single device.