Tracking exceptional points above laser threshold

TL;DR

This paper experimentally demonstrates the existence and tracking of exceptional points in coupled semiconductor nanolasers above the lasing threshold, revealing their dependence on pump power and cavity detuning, and opening avenues for advanced photonic applications.

Contribution

It shows that exceptional points can be accessed and monitored in nonlinear lasing systems above threshold, challenging previous linear-only assumptions and highlighting the role of cavity detuning.

Findings

EPs can be accessed above laser threshold in coupled nanolasers.

Cavity detuning is necessary to compensate carrier-induced frequency shifts.

EP spectral location varies with pump power and can be continuously tracked.

Abstract

Recent studies on non-Hermitian optical systems having exceptional points (EPs) have revealed a host of unique characteristics associated with these singularities, including unidirectional invisibility, chiral mode switching and laser self-termination, to mention just a few examples. The vast majority of these works focused either on passive systems or active structures where the EPs were accessed below the lasing threshold, i.e. when the system description is inherently linear. In this work, we experimentally demonstrate that EP singularities in coupled semiconductor nanolasers can be accessed and tracked above the lasing threshold, where they become branch points of a nonlinear dynamical system. Contrary to the common belief that unavoidable cavity detuning will impede the formation of an EP, here we demonstrate that this same detuning is necessary for compensating the carrier-induced frequency shift, hence restoring the nonlinear EP in the lasing regime. Furthermore, unlike linear non-Hermitian systems, we find that the spectral location of EPs above laser threshold varies as a function of total pump power and can therefore be continuously tracked. Our work is a first step towards the realization of lasing EPs in more complex laser geometries, and enabling the enhancement of photonic local density of states through non-Hermitian symmetries combined with nonlinear interactions in coupled laser arrays.