Speckle-Mediated Gain Localization and Random Lasing Threshold in Scattering Media: A Numerical Study

Abstract

We present a self-consistent numerical study of how the pump speckle field influences the threshold and emission dynamics of random lasing in densely packed scattering layers doped with laser dye. The model is built on experimental parameters for Rhodamine 6G-doped TiO₂ (anatase) nanoparticle films pumped by nanosecond laser pulses at 532 nm. Our hybrid computational approach combines (i) Monte Carlo simulation of coherent pump light transport to obtain the three-dimensional speckle-modulated intensity distribution inside the random medium, (ii) a rate-equation model for the local population inversion and gain saturation in dye molecules, and (iii) a Monte Carlo algorithm for fluorescent photon transport with amplification. This framework self-consistently accounts for the spatially non-uniform gain profile induced by speckles and its feedback on the random lasing process. We analyze the lasing threshold as a function of the speckle contrast and correlation length and find that partial spatial confinement of the gain in high-intensity speckle spots reduces the threshold compared with the uniform pumping case. Furthermore, we examine the statistics of optical paths of emitted photons and show that above threshold the path length distribution becomes strongly modified by the presence of gain, leading to a saturation of the effective radiative loss cross-section, in agreement with earlier experimental observations. The developed model establishes a physically transparent link between microscopic speckle statistics and the macroscopic transport parameters of active random media, providing a tool for optimizing random laser performance and interpreting experimental data.

Speaker

Leonid A. Kochkurov
Yuri Gagarin State Technical University of Saratov
Russia

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