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Frontiers in Optics

Coherent flash of light emitted through a cold atomic gas

Romain Pierrat, Maryvonne Chalony, Dominique Delande, David Wilkowski

Published Jan 1, 2012• 1 citation
Publisher

Abstract

We study the time dependence of a laser beam transmitted through a cold atomic gas. We show in particular that the balistic beam exhibits a flash behavior when the incident laser is abruptly switch-off.
DOI
10.1364/ls.2012.ltu1i.3

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An optically thick cold atomic cloud emits a coherent flash of light in the forward direction when the phase of an incident probe field is abruptly changed. Because of cooperativity, the duration of this phenomena can be much shorter than the excited lifetime of a single atom. Repeating periodically the abrupt phase jump, we generate a train of pulses with short repetition time, high intensity contrast, and high efficiency. In this regime, the emission is fully governed by cooperativity even if the cloud is dilute.
We study the coherent transmission of light through cold strontium atomic clouds, using the narrow intercombination line. In the first part of the thesis, the cooperative emission in the coherent transmission is studied experimentally. By applying fast changes to the incident probe field, flashes of light are observed in the forward direction. The peak intensity of the flashes can be much larger than the incident intensity. These flashes are called superflashes. Due to cooperative effect, the time scale of the flashes are shortened with respect to the single scatterer lifetime. By packing these flashes, we generate a pulse train with short repetition time, where the dynamics can almost be fully governed by cooperative effect. The effect of atomic density on the coherent transmission of light is also studied theoretically and numerically. The effective refractive index of the cloud is calculated up to the second order in atomic density. It is eventually validated with a numerical simulation of the coherent light transmission. The second part of the thesis focuses on the effect of magnetic field on the coherent transmission of light. The strontium intercombination line has a sensitivity in the milligauss regime, which is the typical value of stray magnetic field in the lab. An active magnetic field control system is implemented to cancel the low frequency and AC 50~Hz stray magnetic field. Using this control system, we study the nonlinear magneto-optical rotation (NMOR) effect due to a high intensity light field transmitting through a dilute and thin cloud of strontium atom. Interestingly, at sufficiently high cloud temperature, the NMOR effect can have a higher sensitivity to the magnetic field, compared to its linear counterpart at the same cloud temperature.