Experiments and numerical simulations on lasing in random media

We report numerical and experimental studies on multiple scattering media with gain. We describe Monte Carlo simulations that model the behavior of such a system through a three dimensional random walk of photons in a disordered medium with amplification. Two experimentally observed phenomena, viz. temperature tunable random lasing and ultra-narrow lasing modes, are analyzed using the model. We compare the results of our model with previous experimental results on a disordered dielectric of which the scattering strength could be tuned by changing the external temperature. The agreement between the numerical and experimental results enables us to predict the spectral features of the emission from the tunable random laser under various conditions. Results obtained from new experimental data are consistent with the predictions of the simulations. The model also explains the observation of ultra-narrow emission modes in random lasers without requiring optical cavities. The introduction of exponential gain in a multiple light scattering process strongly increases the importance of very long light paths. Such long paths are often neglected in passive disordered materials but we show that they can dominate the emission spectrum from an amplifying disordered system.