Redistribution of light frequency by multiple scattering in a resonant atomic vapor

TL;DR

This study uses Monte-Carlo simulations to analyze how multiple scattering in a resonant atomic vapor redistributes light frequency, revealing regimes of superdiffusion and normal diffusion depending on excitation conditions.

Contribution

It demonstrates the evolution towards complete frequency redistribution and identifies different diffusion regimes in resonant atomic vapors through detailed spectral and transport analysis.

Findings

Spectral evolution towards complete frequency redistribution with increasing scattering events.

Superdiffusive light transport near the resonance line center.

Normal diffusion observed far from resonance, with incomplete redistribution due to frequency correlations.

Abstract

The propagation of light in a resonant atomic vapor can \textit{a priori} be thought of as a multiple scattering process, in which each scattering event redistributes both the direction and the frequency of the photons. Particularly, the frequency redistribution may result in L\'evy flights of photons, directly affecting the transport properties of light in a resonant atomic vapor and turning this propagation into a superdifusion process. Here, we report on a Monte-Carlo simulation developed to study the evolution of the spectrum of the light in a resonant thermal vapor. We observe the gradual change of the spectrum and its convergence towards a regime of Complete Frequency Redistribution as the number of scattering events increases. We also analyse the probability density function of the step length of photons between emissions and reabsorptions in the vapor, which governs the statistics of the light diffusion. We observe two different regime in the light transport: superdiffusive when the vapor is excited near the line center and normal diffusion for excitation far from the line center. The regime of Complete Frequency Redistribution is not reached for excitation far from resonance even after many absorption/reemission cycles due to correlations between emitted and absorbed frequencies.