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arXiv:0804.2129 · Physical Review Letters

Effect of the Photon’s Brownian Doppler Shift on the Weak-Localization Coherent-Backscattering Cone

M. Lesaffre, Michaël Atlan, M. Groß

Published Jul 17, 2006• 8 citations

Abstract

We report the first observation of the dependence of the coherent-backscattering (CBS) enhanced cone with the frequency of the backscattered photon. The experiment is performed on a diffusing liquid suspension and the Doppler broadening of light is induced by the Brownian motion of the scatterers. Heterodyne detection on a CCD camera is used to measure the complex field (i.e., the hologram) of the light that is backscattered at a given frequency. The analysis of the holograms yield the frequency and the propagation direction of the backscattered photons. We observe that the angular CBS cone becomes more narrow in the tail of the Brownian spectrum. The experimental results are in good agreement with a simple theoretical model.

DOI: 10.1103/physrevlett.97.033901

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We report the first observation of the dependence of the coherent-backscattering (CBS) enhanced cone with the frequency of the backscattered photon. The experiment is performed on a diffusing liquid suspension and the Doppler broadening of light is induced by the Brownian motion of the scatterers. Heterodyne detection on a CCD camera is used to measure the complex field (i.e., the hologram) of the light that is backscattered at a given frequency. The analysis of the holograms yield the frequency and the propagation direction of the backscattered photons. We observe that the angular CBS cone becomes more narrow in the tail of the Brownian spectrum. The experimental results are in good agreement with a simple theoretical model.

When a quasiresonant laser beam illuminates an optically thick cloud of laser-cooled rubidium atoms, the average diffuse intensity reflected off the sample is enhanced in a narrow angular range around the direction of exact backscattering. This phenomenon is known as coherent backscattering (CBS). By detuning the laser from resonance, we are able to modify the light scattering mean-free path inside the sample and we record accordingly the variations of the CBS cone shape. We then compare the experimental data with theoretical calculations and Monte Carlo simulations including the effect of the light polarization and of the internal structure of the atoms. We confirm that the internal structure strongly affects the enhancement factor of the cone and we show that the unusual shape of the atomic medium---approximately a spherically-symmetric, Gaussian density profile---strongly affects the width and shape of the cone.

We present an experimental study of coherent backscattering (CBS) of photon noise from multiple scattering media. We use a pseudothermal light source with a microsecond coherence time to produce a noise spectrum covering a continuous transition, from wave fluctuations to shot noise over several MHz. The angle-dependent Fano factor of backscattered light shows an enhancement due to CBS in the wave fluctuation regime. The CBS line shape and enhancement factor of the noise power is consistent with theory in the weak-scattering limit and for a large number of open reflection channels. These initial experiments on weakly scattering media demonstrate that sensitive noise measurements can be combined with the separation of path lengths present in CBS, opening up new experiments on noise transport in the localization regime.

Weak localization of light or coherent backscattering (CBS) of light is a phenomena in which partial waves traversing momentum-reversed (time-reversed) scattering paths interfere constructively in the backscattering direction leading to an enhancement of scattered light intensity. The enhancement of intensity is proportional to a reduction of light transported in the scattering medium. Thus the CBS signal is a measure of localization of light in the scattering medium. We are building an apparatus to observe weak localization of photons in a high density sample of ultra-cold atoms. First we slow the atoms from room temperature (300 m/s) to less than 10 cm/s in a magneto-optical trap (MOT) where the atom number density is 10 11 atoms/cm 3 . In order to increase the atomic density the atoms in the MOT are transferred to a far detuned microscopic dipole trap created by a laser focused to a waist of approximately 5m. We have successfully loaded the MOT and are now in the process of loading the high density microscopic dipole trap. Once this is achieved we will look at the CBS signal as a function of the atom number density and the scattering cross section. The goal of the project is to reach the Joffe-Regel region where mean free path length is about the wavelength of the scattering radiation.