Interference in Random Lasers

Abstract

A random laser is a disordered medium that both scatters and amplifies light. Important parameters in this system are the transport mean free path l, the size of the amplifying volume and the amplification length l _amp. Properties of light that is propagating through such a medium are modified by the presence of gain, giving rise to a range of surprising phenomena. One of these is the occurrence of a lasing threshold in the output power, pulse duration and spectral width of the emission [1]. These experiments have raised a large number of questions regarding the mechanism that gives rise to these phenomena. One of the most intriguing is how to connect the multiple scattering picture with the laser picture.

Recently, we have performed measurements and simulations investigating the dependence of the laser threshold on the size of the gain volume [3]. It was found that for a small amplifying volume the threshold is at a much higher pump intensity than for a large volume. This effect was reproduced very accurately in a simulation of diffusion in a two- component (amplifying/passive) medium.

Enhanced backscattering is affected by amplification in the multiple scattering medium [3]. As the amount of gain in the medium increases, the backscattercone gets narrower. This is due to the fact that longer light paths, the ones that make up the top of the cone, are amplified more strongly than shorter paths, contributing to the wings. In ref. 2, however, the laser threshold of the medium could not be reached. We present backscattering experiments in a material which allows us to cross the laser threshold. In this way we want to shed more light on how propagation in a multiply scattering medium is affected by gain.