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Acta Physica Polonica A

Laser Cooling: Beyond One Field and One Dimension

N. P. Bigelow, T. Cas, Han Pu, J. Korn, James P. Shaffer, Philip Hemmer, D. P. Katz, M. S. Shahriar

Published Jul 1, 1994• 1 citation
Publisher

Abstract

We describe recent experimental and theoretical results which show that the light pressure force for an atom with more than two levels, in a field with more than one frequency or which is studied in more than one dimension can display striking new features.The importance of these results lies in the fact that they cannot be explained in terms of a two-level, a monochromatic or a one-dimensional theory.

DOI
10.12693/aphyspola.86.29

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We describe recent experimental and theoretical results which show that the light pressure force for an atom with more than two levels, in a field with more than one frequency or which is studied in more than one dimension can display striking new features.The importance of these results lies in the fact that they cannot be explained in terms of a two-level, a monochromatic or a one-dimensional theory.
A review is presented of some of the principal techniques of laser cooling and trapping that have been developed during the past 20 years. Its approach is primarily experimental, but its quantitative descriptions are consistent in notation with most of the theoretical literature. It begins with a simplified introduction to optical forces on atoms, including both cooling and trapping. Then its three main sections discuss its three selected features, (1) quantization of atomic motion, (2) effects of the multilevel structure of atoms, and (3) the effects of polychromatic light. Each of these features is an expansion in a different direction from the simplest model of a classical, two-level atom moving in a monochromatic laser field.
It is shown that nonlinear interaction between two-level atom and the radiation field can profoundly modify the optical force experienced by the atom The dynamic stark shift now provides a cooling force even at zero detuning.Due to two-photon interaction the 'radiation pressure force' experienced by the atom is twice as large as that for one-photon case, consequently temperatures much lower than the one-photon limit can be achieved Keywords Two-photon interaction, laser cooling, optical force
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.
We adapt an earlier semiclassical theory of laser cooling of an arbitrary multistate atom [J. Javanainen, Phys. Rev. A 44, 5857 (1991)] to the limit of low light intensity. The formal theory is implemented analytically on a computer using mathematica. Expressions of light-pressure force and diffusion are provided for j1{\mathit{j}}_{1}j1​=1/2\ensuremath{\rightarrow}j2{\mathit{j}}_{2}j2​=3/2 and j1{\mathit{j}}_{1}j1​=1\ensuremath{\rightarrow}j2{\mathit{j}}_{2}j2​=2 atoms in a one-dimensional optical confinement area in which the pair of counterpropagating waves has orthogonal polarizations. In our models the cooling temperature decreases as the level degeneracy increases.
The usual form of electromagnetic forces is given by but for neutral atoms q = 0. The next order of force is the dipole term, but this also vanishes because neutral atoms have no inherent dipole moment. However, a dipole moment can be induced by a field, and this is most efficient if the field is alternating near the atomic resonance frequency. Since these frequencies are typically in the optical range, dipole moments are efficiently induced by shining nearly resonant light on the atoms. Laser cooling and trapping rely on the interaction between laser light and atoms to exert a controllable force on the atoms, and many sophisticated schemes have been developed using the special properties of the interaction. This chapter begins with the simplest schemes for exerting optical forces on atoms, namely a single-frequency light field interacting with a two-level atom. Although this is the simplest possible scheme, it is pedagogically valuable because it shows many of the features that will be encountered further on. The ultimate temperature using such a scheme is limited, but lower temperatures can be achieved with multilevel atoms. This topic is addressed later in the chapter. Two kinds of optical forces When an atom absorbs nearly resonant light, it makes a transition to the excited state, and its subsequent return to the ground state can be either by spontaneous or by stimulated emission. The nature of the optical force that arises from these two different processes is quite different, and will be described separately [1]. The spontaneous emission case is different from the familiar quantum mechanical calculations using state vectors to describe the state of the system, because spontaneous emission causes the state of the system to evolve from a pure state into a mixed state (see Exercise 6.9). Spontaneous emission is an essential ingredient for the dissipative nature of the optical forces in monochromatic light, and the density matrix is needed to describe it (see Chap. 6). When an atom absorbs light, its energy excites the atom and its angular momentum changes the electron's orbit. Both of these are internal atomic properties. However, the linear momentum of the light, can change only the external, translational motion of the atom.