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Physical Review A

Transformation of optical speckles by atomic vapors: From Rayleigh to Weibull intensity statistics

Inaiara Leite Rodrigues, J. P. Lopez, Thierry Passerat de Silans

Published Aug 26, 2024• 0 citations
Publisher

Abstract

We investigate the transformation of Rayleigh speckle patterns by an atomic cesium vapor. After propagation through the atomic vapor, the incident exponentially decaying intensity distribution of the Rayleigh speckles is transformed with enhancement of high intensities probabilities (extreme events). The measured intensity distributions are fitted by Weibull distributions. The nonlinear interaction between the atomic vapor and the optical field leads to intensity-dependent refraction and absorption, resulting in changes of the speckle spots' intensities with propagation. We measure the inverse of the shape parameter of the Weibull distributions as a function of laser detuning and interpret the results by modeling the speckle propagation.

DOI
10.1103/physreva.110.023525

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We investigate the transformation of Rayleigh speckle patterns by an atomic cesium vapor. After propagation through the atomic vapor, the incident exponentially decaying intensity distribution of the Rayleigh speckles is transformed with enhancement of high intensities probabilities (extreme events). The measured intensity distributions are fitted by Weibull distributions. The nonlinear interaction between the atomic vapor and the optical field leads to intensity-dependent refraction and absorption, resulting in changes of the speckle spots' intensities with propagation. We measure the inverse of the shape parameter of the Weibull distributions as a function of laser detuning and interpret the results by modeling the speckle propagation.
We study the propagation of speckle patterns in a tunable nonlinear medium. Starting from a spatial pattern of photons with a normal distribution (speckle field), we characterize the evolution of the radiation to either subthermal or superthermal distributions and demonstrate the control of this evolution through a single parameter. Our experimental control of the local degree of coherence of the output photons is well described by numerical simulations that consider both dispersive and absorptive nonlinear effects. Particularly, we analyze the role of the nonlinear absorption that either enhances or competes with the dispersive effects in the evolution of speckle fields. In some realizations, an asymptotic power law is observed in the photon spatial distribution.
The irradiation of a dilute cloud of cold atoms with a coherent light field produces a random intensity distribution known as laser speckle. Its statistical fluctuations contain information about the mesoscopic scattering processes at work inside the disordered medium. Following up on earlier work by Assaf and Akkermans [Phys. Rev. Lett. 98, 083601 (2007)], we analyze how static speckle-intensity correlations are affected by an internal Zeeman degeneracy of the scattering atoms. It is proven on general grounds that the speckle correlations cannot exceed the standard Rayleigh law. On the contrary, because which-path information is stored in the internal atomic states, the intensity correlations suffer from strong decoherence and become exponentially small in the diffusive regime applicable to an optically thick cloud.
Investigates the noise due to Rayleigh scattering and its reduction via the spatial redistribution of atoms due to the standing wave pattern of the pump beams. For operation near the resonance, the interaction of the induced atomic dipole with the optical field leads to a periodic potential for the atoms due to the interference standing wave pattern. An assumption of the Boltzmann distribution yields a periodic distribution of the number density of atoms. In the paper, we investigate the effect of detuning and saturation on the noise due to Rayleigh scattering and address the issue of signal to noise ratio in nonlinear optical phase conjugation. We also investigate the minimum incident intensity needed to achieve a signal to noise ratio of one (S/N=1).
The phenomenon of random intensity patterns, for waves propagating in the presence of disorder, is well known in optics and in mesoscopic physics. We study this phenomenon for cold atomic gases expanding, by a diffusion process, in a weak random potential. We show that the density-density correlation function of the expanding gas is strongly affected by disorder and we estimate the typical size of a speckle spot, i.e., a region of enhanced or depleted density. Both a Fermi gas and a Bose-Einstein condensate (in a mean-field approach) are considered.
There are several nonlinear optical processes that can lead to instabilities in a laser beam as it propagates through a nonlinear optical medium. Self focusing effects, for instance, can lead to the growth of new spatial frequency components in a laser beam. A laser beam can also be unstable to the growth of new spectral components. Stimulated Raman scattering is a well-known example of a process that can lead to this sort of instability. In this paper, we present the results of a series of experiments on the somewhat different instabilities that occur for the case of the propagation of a laser beam through an atomic vapor. We find that these instabilities become particularly pronounced when the frequency of the incident laser is near to that of a one- or two-photon resonance of the atom. In either case, we find that the laser beam is unstable to the growth of new frequency components, and that in general these new frequencies are not emitted collinearly with the incident laser. These instabilities are important because they place a limit on the laser intensity that can be used to drive nonlinear optical processes.