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Acta Physica Sinica

High-gain Raman scattering spectrum of trapped atomic gas

Jia Zhen-Qiang, Zhang Lin

Published Jan 1, 2012• 0 citations
Publisher

Abstract

In this paper, the Raman scattering spectra of cold atomic gas in different environments are closely calculated. Comparing the spectra from the atoms in free and trapped environments, the calculation result gives a right spectrum structure for the free atoms and reveals a distinct high-gain spectrum exhibiting a comb-like structure for the harmonic trapped atoms. The spectral peaks of the trapped spectrum are uniformly distributed and the distance between adjacent spectral peaks exactly equals the frequency of the trap. The nonlinear gain scheme and the gain conditions of the Raman scattering process are clearly given in this paper.

DOI
10.7498/aps.61.043206

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In this paper, the Raman scattering spectra of cold atomic gas in different environments are closely calculated. Comparing the spectra from the atoms in free and trapped environments, the calculation result gives a right spectrum structure for the free atoms and reveals a distinct high-gain spectrum exhibiting a comb-like structure for the harmonic trapped atoms. The spectral peaks of the trapped spectrum are uniformly distributed and the distance between adjacent spectral peaks exactly equals the frequency of the trap. The nonlinear gain scheme and the gain conditions of the Raman scattering process are clearly given in this paper.
High-gain resonant nonlinear Raman scattering on trapped cold atoms within a high-fineness ring optical cavity is simply explained under a nonlinear opto-mechanical mechanism, and a proposal using it to detect frequency of micro-trap on atom chip is presented. The enhancement of scattering spectrum is due to a coherent Raman conversion between two different cavity modes mediated by collective vibrations of atoms through nonlinear opto-mechanical couplings. The physical conditions of this technique are roughly estimated on Rubidium atoms, and a simple quantum analysis as well as a multi-body semiclassical simulation on this nonlinear Raman process is conducted.
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Investigations of optical processes for laser cooled and trapped atoms are described. Fluorescence from Cs atoms in a magneto-optical trap is detected under conditions of very low atomic density. Discrete steps are observed in the fluorescent signal versus time and are associated with the arrival and departure of individual trapped atoms. Histograms of the frequency of occurrence of a given level of fluorescence exhibit a series of uniformly spaced peaks that are attributed to the presence of N = 0, 1, 2 atoms in the trap. In addition, numerical absorption and emission spectra for three-level [...], [...], and V systems under intense radiations are calculated. Absorption spectra for a [...] system is used to explain the probe-wave amplification and absorption spectra recorded for Cs atoms cooled and confined in a magneto-optical trap, in which novel spectral features of narrow frequency widths with single-pass gain exceeding 20% are observed. The consequence of the optical gain is demonstrated to lead to negative radiation pressure, which is investigated together with other mechanical forces in the trap. Various alternative trapping schemes in three-level and two-level atoms are proposed as possible means to compress an atomic sample and demonstrated for a two-level magneto-optical trap.
We propose different spectroscopic methods to explore the nature of the thermal excitations of a trapped Bose condensed gas: 1) a four photon process to probe the uniform region in the trap center: 2) a stimulated Raman process in order to analyze the influence of a momentum transfer in the resulting scattered atom momentum distribution. We apply these methods to address specifically the energy spectrum and the scattering amplitude of these excitations in a transition between two hyperfine levels of the gas atoms. In particular, we exemplify the potential offered by these proposed techniques by contrasting the spectrum expected from the non-conserving Bogoliubov approximation, valid for weak depletion, to the spectrum of the finite-temperature extensions like the conserving generalized random phase approximation (GRPA). Both predict the existence of the Bogoliubov collective excitations but the GRPA approximation distinguishes them from the single-atom excitations with a gapped and parabolic dispersion relation and accounts for the dynamical screening of any external perturbation applied to the gas. We present two feasible experiments, one concerns the observation of the gap associated to this second branch of excitations and the other deals with this screening effect.
We study optical gain in a gas of cold 39K^{39}\mathrm{K}39K atoms. The gain is observed during operation of a conventional magneto-optical trap without the need for additional fields. Measurements of transmission spectra from a weak probe show that the gain is due to stimulated Raman scattering between hyperfine ground states. The experimental results are reproduced by a simplified six-level model, which also helps explain why such gain is not observed in similar experiments with rubidium or cesium.
We study superradiant Raman scattering from an ultracold, but finite, temperature Bose gas in a harmonic trap. Numerical simulations indicate the existence of distinct time scales associated with the decoherence of the condensed versus thermal fractions, and the concomitant preferred scattering from atoms in low-lying trap states in the regime where superradiance takes place on a time scale comparable to an inverse trap frequency. As a consequence the scattered atoms experience a modest reduction in temperature as compared to the unscattered atoms.
We present a semi-classical theoretical framework on light-wave mixing and scattering in single-component quantum gases. We show that these optical processes originating from elementary excitations with dominant collective atomic recoil motion are stimulated Raman or hyper-Raman in nature. In the forward direction, the wave-mixing process, which is the most efficient process in normal gas, is strongly reduced by the condensate structure factor even though the Bogoliubov dispersion relation perfectly compensates the optical-wave phase mismatch. In the backward direction, however, the free-particle-like condensate structure factor and Bogoliubov dispersion result in highly efficient light-wave mixing and collective atomic recoil motion that are enhanced by a stimulated hyper-Raman gain and a very narrow two-photon motional state resonance.