Mode Switching Through Exceptional Points Induced by Lasing-Inversion Coupling

TL;DR

This paper uncovers a new mode-switching mechanism in laser systems driven by exceptional points caused by lasing-inversion coupling, enabling tunable photonic applications like adaptive communication and spectroscopy.

Contribution

It introduces a first-principles Bloch coupled-mode theory revealing how dynamical couplings create exceptional points that enable mode switching and frequency comb formation.

Findings

Identification of extra exceptional points due to lasing-inversion coupling

Spectral structure governs switching between single-mode lasing and frequency combs

Mechanism enables tunable and adaptive photonic systems

Abstract

The gain-loss coupling in optical cavities induces exceptional points (EPs), where two optical modes coalesce. The large modal overlap near an EP intensifies gain competition, favoring single-mode lasing. Recent studies further revealed self-modulation closer to the EP that transforms the lasing mode into a frequency comb. Such EP-enabled comb formation suggests a previously unaccounted-for mechanism that overcomes the strong gain competition and drives a second mode to threshold. Here, using a Bloch coupled-mode theory derived from first principles, we show that the second threshold arises from dynamical couplings among the population inversion, the lasing field, and a dark cavity mode. The lasing-inversion coupling produces extra EPs, whose spectral structure governs switching among single-mode lasing and frequency combs with different repetition rates. This above-threshold mode-switching mechanism enables new opportunities for tunable photonic systems, including adaptive optical communication links and dual-comb spectroscopy.