Definition of the stimulated emission threshold in high-$\beta$ nanoscale lasers through phase-space reconstruction

TL;DR

This paper introduces a new method to define the lasing threshold in high-$eta$ nanoscale lasers by analyzing phase-space trajectories, providing a clear criterion even when traditional methods fail.

Contribution

It proposes a phase-space reconstruction approach to determine the lasing threshold, applicable to $eta=1$ lasers, validated through theoretical analysis and experimental demonstration.

Findings

Phase-space trajectory shape changes at threshold

Area in phase-space increases at threshold

Method works for $eta=1$ nanoscale lasers

Abstract

Nanoscale lasers sustain few optical modes so that the fraction of spontaneous emission $\beta$ funnelled into the useful (lasing) mode is high (of the order of few 10$^{-1}$) and the threshold, which traditionally corresponds to an abrupt kink in the light in- light out curve, becomes ill-defined. We propose an alternative definition of the threshold, based on the dynamical response of the laser, which is valid even for $\beta=1$ lasers. The laser dynamics is analyzed through a reconstruction of its phase-space trajectory for pulsed excitation. Crossing the threshold brings about a change in the shape of the trajectory and in the area contained in it. An unambiguous definition of the threshold in terms of this change is shown theoretically and illustrated experimentally in a photonic crystal laser.