Temporal dynamics of Levy flights of photons in a three-dimensional geometry for hot vapor

Abstract

Multiple scattering of light by resonant vapor is characterized by Lévy-type superdiffusion with a step size distribution $P(x)\propto 1/x^{1+\alpha }$, with $0<\alpha <2$. The Lévy parameter $\alpha$ was measured from $P(x)$, steady fluorescence, frequency-dependent fluorescence and time-resolved transmission, where the latter was observed for quenched disordered media, not yet for annealed disordered media. Here we report first measurements of this quantity from time-resolved fluorescence for photons that are forward diffused from light pulses exciting a hot rubidium vapor, a medium characterized by annealed disorder, where correlations between the photon steps are absent. Also, the vapor is contained in a three-dimensional geometry, a turned cylinder with respect of the incident photon direction. We show experimentally that $\alpha$ can be extracted from this setup, and the results are consistent with those given by the steady frequency-dependent forward fluorescence. Theoretical simulations are consistent with these results. Also, our simulations show the validity of the one-dimensional theories for our actual geometry and other three-dimensional ones, as thin and thick disks, and a measurement of the number of steps that the photon performed before leaving the medium.

8 pages, 6 figures