Modes of Random Lasers

TL;DR

This paper reviews the nature of lasing modes in random lasers, exploring their relation to passive system resonances and introducing constant-flux states as a comprehensive description.

Contribution

It provides a comprehensive review of numerical and theoretical studies on random laser modes, highlighting the link to passive resonances and introducing constant-flux states for complex cavities.

Findings

In localized regimes, lasing modes closely match passive resonances.

Differences between quasi-bound states and lasing modes increase in diffusive regimes.

Constant-flux states offer a unified description of complex random laser modes.

Abstract

In conventional lasers, the optical cavity that confines the photons also determines essential characteristics of the lasing modes such as wavelength, emission pattern, ... In random lasers, which do not have mirrors or a well-defined cavity, light is confined within the gain medium by means of multiple scattering. The sharp peaks in the emission spectra of semiconductor powders, first observed in 1999, has therefore lead to an intense debate about the nature of the lasing modes in these so-called lasers with resonant feedback. In this paper, we review numerical and theoretical studies aimed at clarifying the nature of the lasing modes in disordered scattering systems with gain. We will discuss in particular the link between random laser modes near threshold (TLM) and the resonances or quasi-bound (QB) states of the passive system without gain. For random lasers in the localized regime, QB states and threshold lasing modes were found to be nearly identical within the scattering medium. These studies were later extended to the case of more lossy systems such as random systems in the diffusive regime where differences between quasi-bound states and lasing modes were measured. Very recently, a theory able to treat lasers with arbitrarily complex and open cavities such as random lasers established that the TLM are better described in terms of the so-called constant-flux states.