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Chinese Journal of Lasers

The Properties of Light Pressure Force with High Order in Laser Fields

Xianfeng Chen, Fang Jian-Xing, Zhu Shi-Qun

Published Jan 1, 2002• 0 citations
OpenAlex

Abstract

In this paper, the light pressure force in low and high intensity laser fields is derived. The exact numerical results of forces F n ∥(n=0,1,2,3,4,5,6...) through the matrix continued fraction method are presented. At low intensity field (G=1), the spatially averaged force F 0 ∥ gives a cooling effect at the negative detuning. At high intensity (G=64), the effects of the forces with higher order (n≥2) appear and the contributes of the forces with odd or even order are opposite. It is great different from no high order force at low intensity.

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In this paper, the light pressure force in low and high intensity laser fields is derived. The exact numerical results of forces F n ∥(n=0,1,2,3,4,5,6...) through the matrix continued fraction method are presented. At low intensity field (G=1), the spatially averaged force F 0 ∥ gives a cooling effect at the negative detuning. At high intensity (G=64), the effects of the forces with higher order (n≥2) appear and the contributes of the forces with odd or even order are opposite. It is great different from no high order force at low intensity.
Abstract Described is the application of a matrix form of the continued fraction technique to the calculation of the velocity dependent force experienced by a multi-level atom interacting with a complex optical field. Unlike other techniques, this approach provides a solution which is valid for a wide range of optical field configurations and atomoc velocities and which includes the effects of both single and multi-photon processes. As a specific example of this technique the force experienced by a three-level atom (V) in a bichromatic light field is calculated.
We give a general theoretical description of radiative forces on atoms in a monochromatic radiation field. The driven transition has a lower level consisting of several substates, and we consider the limit of low velocities and weak intensities. This situation comprises the schemes where cooling to temperatures below the Doppler limit is possible. After expressing the atomic dipole in terms of the polarizability tensor, we obtain an expression of the force as the sum of the radiation pressure, the dipole force, and a contribution from the gradient of the polarization. This latter contribution contains a part resulting from redistribution of photons between the plane waves that compose the field, and a part resulting from fluorescence. We express the average force and the momentum-diffusion tensor in terms of a closed evolution equation for the lower-state density matrix. We show by a scaling argument that in any case of cooling, the final temperature goes down linearly with the intensity, until the recoil limit is approached. The description is valid for a weak and monochromatic, but otherwise arbitrary field, and it allows for the presence of an external magnetic field, and of hyperfine splitting. It is applicable to cooling in one, two, or three dimensions. We give numerical results for the average force in a number of specific cases of two counterrunning plane waves.
Kinetic phenomena of resonant-particle motion in a standing-light-wave field are theoretically discussed. The light-pressure force and its fluctuations are found in a wide range of the parameters: the atomic velocity, the intensity, and the detuning of the field. There are two characteristic regions of the detunings in a strong field: the adiabatic region and the region of Landau–Zener resonances. The quantum fluctuations of the inner atomic state that are due to the Landau–Zener transitions result, specifically, in the interference effect in the mean light-pressure force. The effects of the spatial grating of the cooled atoms, the particle velocity bunching, and the recoil-effect dependence on the nonlinear absorption are considered for slow particles. The effect of optical pumping by linearly polarized light taking into account the recoil effect is fundamentally new. Because of this effect, the anisotropic resonance medium becomes gyrotropic.
An analytical description of long-range collisions between atoms in a laser cooling field is developed. We begin by considering an N-atom master equation. In the regime of low atomic densities (i.e., where the mean distance between two atoms is much larger than the laser wavelength) it is possible to treat the atom-atom interactions in perturbation theory. Furthermore we assume temperatures which allow a semiclassical treatment of the cooling process. The effect of the presence of other atoms can be separated analytically into two parts; an attenuation force due to the absorption of the laser beams in the atomic cloud similar to the results of Dalibard [Opt. Commun. 68, 203 (1988)], which tends to compress the atomic cloud, and a two-atom force due to photon emission and absorption cycles between different atoms. This force proves to be repulsive for the configurations studied and prevents the cloud from collapsing. The result for the first-order perturbation expansion in collision strength generalizes the model proposed by Walker, Sesko, and Wieman [J. Opt. Soc. B 8, 946 (1991)] by including additional terms, such as those associated with Raman couplings.
The present state of research on the cooling of atomic ensembles by means of laser radiation pressure is discussed. First the basic theory of resonance radiation pressure is reviewed. The effect of recoil on the drift and fluctuations of the momentum of an atom interacting with resonance radiation is examined. There is an analysis of the forces acting on an atom in some simple light fields: a traveling plane wave, a Gaussian beam, and counterpropagating Gaussian beams. The review then focuses on the monochromatization of atoms by resonance radiation pressure and related methods for laser cooling of atoms. The roles played by the radiation pressure force and by momentum diffusion in shaping the narrow velocity distributions of the ensemble of atoms are analyzed. The lowest temperature obtainable in the radiation cooling of atoms is estimated. The basic experimental methods for laser cooling of atomic beams are described. Problems of the three-dimensional cooling of atomic ensembles and problems of localizing cold atoms in light fields, steady-state magnetic fields, and electrostatic fields are discussed. The review concludes with a discussion of some applications of cold atoms in high-precision spectroscopy, frequency standards, and atomic physics.