Exceptional Points and Lasing Thresholds: When Lower-Q Modes Win

TL;DR

This paper reveals that in laser cavities with exceptional points, the usual hierarchy of modes reaching lasing threshold first can be reversed, leading to counterintuitive mode switching driven by non-Hermitian degeneracies.

Contribution

It demonstrates how exceptional points can alter the expected lasing mode hierarchy, showing that lower-Q modes can lase before higher-Q modes due to non-Hermitian effects.

Findings

Exceptional points can cause lower-Q modes to lase first.

Counterintuitive mode switching occurs even with uniform pumping.

Non-Hermitian physics significantly influences lasing dynamics.

Abstract

One of the most fundamental questions in laser physics is the following: Which mode of an optical cavity will reach the lasing threshold first when gain is applied? Intuitively, the answer appears straightforward: When a particular mode is both temporally well confined (i.e., exhibits the highest quality factor) and experiences initially the largest increase of the modal gain, it is naturally expected to lase first. However, in this work, we demonstrate that this intuition can fail in surprising ways. Specifically, we show that in the presence of non-Hermitian degeneracies, known as exceptional points, the expected mode hierarchy can be dramatically altered. These spectral singularities can give rise to counterintuitive mode switching, where a mode with a lower quality factor and initially smaller increase of modal gain reaches the lasing threshold ahead of a more favorable competitor. Remarkably, this effect can occur even under spatially uniform pumping, underscoring the subtle and profound influence of non-Hermitian physics on lasing dynamics.