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Physical Review Letters

Spontaneous Generation of a Longitudinal Atomic Density Grating in Sodium Vapor

Gian Luca Lippi, G. P. Barozzi, Sylvain Barbay, Jorge R. Tredicce

Published Apr 1, 1996• 49 citations
Publisher

Abstract

We present measurements of probe beam amplification in Na vapor at intermediate density values, in both the counterpropagating and copropagating directions (with respect to the pump beam) in the case of degenerate pump and probe frequencies. Strong gain is observed only in the counterpropagating direction for parameters that preclude the possibility of Raman or Rayleigh gain. We show that the experimental results can be interpreted in terms of the spontaneous formation of a density grating due to the nonresonant interaction between the electromagnetic field and the atoms.

DOI
10.1103/physrevlett.76.2452

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The results of an experimental and theoretical study of the gain (or absorption) experienced by a weak probe beam propagating through a sodium vapor in the presence of an intense pump field that is nearly resonant with the 3s→3p atomic transition are presented. In general, the probe transmission spectrum is found to be comprised of three distinct features, two of which can lead to amplification of the probe wave. The origin of these features can be traced to the modification of the atomic level structure by the ac Stark effect. The experimentally measured spectra are in good agreement with the predictions of a theoretical model based on the solution of the density-matrix equations of motion for a two-level atom and including the effects of Doppler broadening. The dependence of the induced gain on the pressure of a helium buffer gas has been measured and is found to be in good agreement with the predictions of theory. The maximum gain observed in these experiments occurs at one of the Rabi sidebands and leads to a 38-fold increase in the intensity of the probe wave.
We describe a novel optical gain mechanism observed in a strongly driven Doppler-broadened atomic vapor. Gain and self-stabilizing laser oscillations are observed in a direction which is counterpropagating with respect to the traveling wave pump. Using Bragg diffraction, we show that the underlying gain-lasing mechanism is directly tied to a spontaneously formed optical wavelength grating. We suggest that this grating arises from a self-organization or self-bunching of the atoms, an effect which is mediated by recoil just as in the free electron laser.
This paper investigates stimulated emission and absorption near resonance for a driven system of interacting two-level atoms. Microscopic kinetic equations for the density matrix elements of N-atom states including atomic motion are built, taking into account atom-field and atom-atom interactions. Analytical solutions are given for the resulting macroscopic equations in different limits, for a system composed of a strong coherent “pump” field and a weak counter-propagating “probe” field. It was shown that the existence of a dipole-dipole (long-range) interaction between atoms separated by distance less than the pump wave-length can cause the formation of periodic polarization and population structures (gratings in time and space) in the pumped medium without a probe field. The magnitude of pump induced population grating can have a strong dependence on the relation between the pump field detuning and the polarization decay rate. The “interaction” between pump and probe induced polarization/population gratings through a dipole-dipole interaction mechanism causes the absorption line shape asymmetry. Under certain conditions, this asymmetry is revealed in increasing probe gain for the “red”-shifted (relative to pump) probe and suppressing the gain for the “blue”-shifted probe field when pump is “red”-shifted relative to the ensemble averaged resonant frequency. The theoretical results are consistent with experimental data for the probe gain or absorption as the function of frequency and the dependance of the gain on atomic density for sodium vapor when the pump laser is tuned near the D 2 line. Here the dependance of gain on particle density was explained in the terms of the long-range interaction between the atoms.