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Quantum Electronics and Laser Science Conference

Channeling and velocity bunching of atoms in a standing wave laser field

Yuzhu Wang, W. Q. Cai, Y. D. Cheng, L. Liu, Yi Luo

Published Apr 24, 1989• 0 citations
OpenAlex

Abstract

The kinetic behavior of atomic motion in a standing wave laser field has been extensively studied because it is important to trapping, cooling, and manipulating atoms. The radiative force acting on atoms in the standing wave is related to the processes of induced transitions and spontaneous emission. The probabilities of these processes depend on the detuning between laser and atomic frequencies, the intensity and structure of the field, as well as the velocity of the atom.1-3

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•Jan 1, 1994•1 citation
The kinetic behavior of atomic motion in a standing wave laser field has been extensively studied because it is important to trapping, cooling, and manipulating atoms. The radiative force acting on atoms in the standing wave is related to the processes of induced transitions and spontaneous emission. The probabilities of these processes depend on the detuning between laser and atomic frequencies, the intensity and structure of the field, as well as the velocity of the atom.1-3
The effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave has been investigated, both theoretically and experimentally. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous emission lifetimes to several hundred. For a standing wave frequency blue-detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings the rate of diffusion is so slow that the resulting distribution approaches a quasi-steady state. For red detunings the diffusion is balanced with the force and a true steady-state is achieved. A Monte Carlo method based on the density matrix equations in the dressed-state representation is applied to simulate the atomic motion. The dynamics of atom channeling is studied. One of the important applications of this investigation is adiabatic laser cooling of atoms. Lithium atoms channeled in the nodes of an intense standing wave radiation field are cooled to near the recoil limit, a single photon's momentum hbark\\hbar{k}hbark, by adiabatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root mean squared momentum of 2 hbark\\hbar{k}hbark in one-dimension. The data are compared with the results obtained by the Monte Carlo method. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.
This work discusses the effect of a high-intensity standing wave laser field on the motion of neutral atoms moving with a relatively high velocity. The analysis involves a detailed calculation of the force acting on the atoms and the calculation of the diffusion tensor associated with the fluctuations of the quantum force operator. The high-intensity laser field limit corresponds to a Rabi frequency much greater than the natural rate of the atom. The general results are valid for any atomic velocity. Results are then specialized to the case of slow and fast atoms where the Doppler shift of the laser frequency due to the atomic motion is either smaller or larger than the natural decay rate of the atom. The results obtained for the force and diffusion tensor are applied to a particular ideal experiment that studies the evolution of a fast atomic beam crossing a high-intensity laser beam. The theories developed previously, for a similar laser configuration, discuss only the low atomic velocities case and not the more realistic case of fast atoms. Here, an approximate solution of the equation for the distribution is obtained. Starting from the approximate distribution function, the deflection angle and dispersion angle for themore » atomic beam with respect to the free motion are calculated.« less
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.
The dressed atom approach is used to investigate the evolution of atoms moving in an intense standing wave laser field. When coherences between the dressed states can be neglected, a simple expression is obtained for the radiative force acting on the atoms. The Landau-Zener model, well-known in the theory of slow atomic collisions, is here used to include coherences in an analytic treatment. At high velocities, where the simplest approach breaks down, the results are in good agreement with exact numerical calculations. Applications outside the field of laser cooling are discussed.
The interaction of a beam of two level atoms with a standing wave laser field has been the subject of extensive investigation in recent times. A standing wave perpendicular to an atomic beam allows the manipulation of the transverse velocity distribution. The interaction of an atomic beam with a strong standing wave laser field has been systematically investigated by both theoretical simulation and experiment in this thesis. We have developed a numerical model based on a continued fraction solution for the light pressure forces to simulate the near perpendicular interaction of an atomic beam with a strong standing wave laser field. The reshaping of the transverse spatial profile of an atomic beam by a perpendicular, near resonant, standing wave laser field is investigated for red detunings using a longitudinal velocity selective detection system. The experimental dependence of the reshaped profile on the longitudinal and transverse atomic velocities agrees well with a continued fraction solution for the light force. In particular, a regime is found where channelling of the atoms near the antinodes of the standing wave produces a central peak in the spatial distribution of the atomic beam in the far field which have also been observed elsewhere.