Continuing our ongoing investigations of random lasing, we used the Monte Carlo method to simulate random walks of photons within a multiply scattering medium. By initially applying this technique to calculate pulse-stretching in a passive disordered medium, we elucidated its agreement with analytical diffusion theory. Thereafter, we introduced conditions of optical amplification, and reproduced the experimentally observed spectral features like spectral narrowing, intensity enhancement, bichromaticity, mode competition, etc., in a random laser. After investigating diffusive and sub-diffusive regimes of scattering, we formulated our results in terms of a gain subvolume, the functioning of which depends upon local gain conditions. We then used a modified approach of this technique to study ultranarrow random lasing modes, and successfully reproduced these modes observed in a random laser. Based on our simulations, we were able to explain the origins of ultra-narrow lasing modes as excessively amplified extended modes.
We report on experimental and numerical studies of free space terahertz (THz) propagation in strongly scattering random dielectric media. The on-axis ballistic and small angle scattered transmission is measured through media of varying thickness. The experimental variations of the terahertz pulse group delay and scattering-induced effects such as temporal pulse distortion, spectral decay, and power attenuation as a function of sample thickness are well described by a Monte Carlo photon migration model. The transmitted pulses are analysed using the classical Bruggemann effective medium approximation (EMA). It is found that the effective medium approximation underestimates the accumulated pulse phase acquired by the high frequencies during pulse propagation. An empirically modified EMA provides accurate description of the random dielectric medium.
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