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Journal of Experimental and Theoretical Physics

The light pressure force and the friction and diffusion coefficients for atoms in a resonant nonuniformly polarized laser field

A. V. Bezverbnyĭ, Прудников Олег Николаевич, А. В. Тайченачев, Anatolii M Tumaikin, Valeriy I. Yudin

Published Mar 1, 2003• 20 citations
Publisher

Abstract

The motion of slow atoms with degenerate energy levels in a resonant, nonuniformly polarized laser field is described by the Fokker-Planck equation for the atomic distribution function in phase space in terms of the semiclassical approach. Field gradient expansions are used for the spatially nonuniform coefficients of the equation. For closed atomic transitions J g =J→J e =J+1 (J g and J e are the total angular momenta of the ground and excited states, respectively), new analytical results are presented for the light pressure force and the friction and diffusion coefficients in momentum space. These results allow the kinetic effects (laser cooling, localization in optical potential wells, etc.) in a field of arbitrary one-, two-, or three-dimensional configuration to be investigated. In several cases, the new contributions to the friction coefficient are interpreted qualitatively.

DOI
10.1134/1.1567411

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The motion of slow atoms with degenerate energy levels in a resonant, nonuniformly polarized laser field is described by the Fokker-Planck equation for the atomic distribution function in phase space in terms of the semiclassical approach. Field gradient expansions are used for the spatially nonuniform coefficients of the equation. For closed atomic transitions J g =J→J e =J+1 (J g and J e are the total angular momenta of the ground and excited states, respectively), new analytical results are presented for the light pressure force and the friction and diffusion coefficients in momentum space. These results allow the kinetic effects (laser cooling, localization in optical potential wells, etc.) in a field of arbitrary one-, two-, or three-dimensional configuration to be investigated. In several cases, the new contributions to the friction coefficient are interpreted qualitatively.
The kinetics of atoms with degenerate energy levels in the field produced by elliptically polarized waves is considered in the semiclassical approximation. Analytic expressions for the force acting on an atom and for the diffusion coefficient in the momentum space are derived for the optical transition J g =1/2→J e = 1/2 in the slow atom approximation. These expressions are valid for an arbitrary one-dimensional configuration of the light field and for an arbitrary intensity. The peculiarities of the atomic kinetics are investigated in detail; these peculiarities are associated with ellipticity of light waves and are absent in particular configurations formed by circularly or linearly polarized waves, which were considered earlier.
The stochastic dynamics of atoms with degenerate energy levels in a resonant nonuniformly polarized laser field is considered within the framework of the quasi-classical approach. The regular force entering into the Langevin equation and the correlation function of a random force are represented in the form of expansions in spatial gradients of the total light field. Specific features of kinetic distributions that are caused by the multiplicative character of noise, by anisotropy of dissipative processes, and by the presence of a vortex component in the light-induced force are considered for the example of a model system and a two-dimensional field configuration.
In the limit of asymptotically small photon momentum, a Fokker–Planck equation for the center-of-mass Wigner function suffices to describe laser cooling of an atom. We have devised a procedure to compute the light pressure force and the diffusion tensor for an atom with an arbitrary level structure, traveling with an arbitrary velocity in any given three-dimensional light field. The scheme is based on solving a set of differential equations closely related to the density matrix equations of motion encountered in conventional recoilless laser spectroscopy. Computer programs have been written that automatically construct and solve the required equations from broad specifications of the problem, such as energies and angular momenta of the atomic levels. A Langevin equation stochastic simulation has been implemented to solve the Fokker-Planck equation of laser cooling using the computed force and diffusion. Comparisons of optical-molasses experiments and simulations carried out for a model sodium atom with a J = 2→J = 3 transition show qualitative similarities such as sub-Doppler limit temperatures, but the quantitative agreement of theory and experiment is not satisfactory.
Methods are presented that allow numerical calculations of the friction and momentum diffusion coefficients for atomic motion in laser fields with periodicity in one, two, and three dimensions. Polarization gradients in the fields lead to increased friction on atoms with a Zeeman sublevel structure, and new features arising in the cases of motion in two and three dimensions are discussed. There is a marked position dependence of the calculated quantities, and the consequence for the forces on atoms following specified trajectories is studied. Temperatures determined from the position-averaged friction and diffusion coefficients scale with the laser power divided by the frequency detuning, except in some special configurations. Of relevance for experiments on cooling, the relative merits of four different laser configurations in three dimensions are studied.
We have investigated the localization of atoms in two configurations of laser fields: monochromatic and bichromatic standing waves. A quasi resonant monochromatic laser field creates a spatially periodic potential field with period equal to half of the wavelength. If the atomic kinetic energy is less than the height of the potential wells then atoms could be trapped in the wells. The motion of atoms in a standing wave is governed by the gradient force, friction force, and momentum diffusion. We studied one dimensional localization of sodium atoms in such a standing wave, as well as the influence of friction and diffusion on the motion of these atoms. The effect of accumulation of atoms at the bottom of potential wells (nodes of a standing wave) was observed. This leads to a new scheme of laser cooling of atoms with a minimum temperature below the Doppler limit.
The quantum kinetic equation describing slow atomic motion in laser light is derived by an operational method which provides mathematical expressions with a transparent physical structure. The authors prove in a general way that the coefficients appearing in this equation, which is of a Fokker-Planck type, are simply related to the mean value and to the correlation functions of the Heisenberg radiative force of the semiclassical approach, where the atomic position is treated classically. They derive in particular a new theoretical expression for the damping force responsible for radiative cooling and they interpret it in terms of linear response theory. The authors also obtain a new crossed r-p derivative term, which does not appear in semiclassical treatments, but which they find to be very small in most situations. Finally, all the theoretical expressions derived are valid for any J g to J e transitions and are not restricted to two-level atoms.