Monte Carlo calculations of spectral features in random lasing

Raman Research Institute, C V Raman Avenue, Sadashivnagar, Bangalore, 560080, Indiahema@rri.res.inAbstract. Continuing our ongoing investigations of random lasing, we used the Monte Carlomethod to simulate random walks of photons within a multiply scattering medium. By initiallyapplying this technique to calculate pulse-stretching in a passive disordered medium, we elu-cidated its agreement with analytical diffusion theory. Thereafter, we introduced conditions ofopticalamplification,andreproducedtheexperimentallyobservedspectralfeatureslikespectralnarrowing, intensity enhancement, bichromaticity, mode competition, etc., in a random laser.After investigating diffusive and sub-diffusive regimes of scattering, we formulated our resultsin terms of a gain subvolume, the functioning of which depends upon local gain conditions. Wethen used a modified approach of this technique to study ultranarrow random lasing modes, andsuccessfully reproduced these modes observed in a random laser. Based on our simulations, wewere able to explain the origins of ultra-narrow lasing modes as excessively amplified extendedmodes.Keywords: Random lasing, multiple scattering, Monte Carlo, light diffusion, random walks