Field and intensity correlations in amplifying random media

TL;DR

This paper investigates how amplification affects light correlations in random media, showing amplification enhances nonlocal correlations and can induce localized-like transport due to narrow resonant modes.

Contribution

It introduces a conditional averaging method to handle divergence in correlation functions and reveals amplification's selective enhancement of long-path contributions.

Findings

Amplification does not alter local spatial correlations.

Nonlocal intensity correlations are significantly magnified by gain.

Increasing gain narrows mode linewidths below mode spacing, mimicking localization.

Abstract

We study local and nonlocal correlations of light transmitted through active random media. The conventional approach results in divergence of ensemble averaged correlation functions due to existence of lasing realizations. We introduce conditional average for correlation functions by omitting the divergent realizations. Our numerical simulation reveals that amplification does not affect local spatial correlation. The nonlocal intensity correlations are strongly magnified due to selective enhancement of the contributions from long propagation paths. We also show that by increasing gain, the average mode linewidth can be made smaller than the average mode spacing. This implies that light transport through a diffusive random system with gain could exhibit some similarities to that through a localized passive system, owing to dominant influence of the resonant modes with narrow width.