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Physical Review A

Forces on atoms in a standing-wave laser field

Emilio Fiordilino, Marvin H. Mittleman

Published Jul 1, 1984• 6 citations
Publisher

Abstract

A previous calculation of forces on atoms in external (dc) fields and in homogeneous traveling-wave laser fields is modified for the case of a standing-wave laser field. The case of intense laser fields, where the Rabi frequency is much greater than the natural decay rate of the atom, is described. A general result for any Doppler shift of the laser frequency due to the atomic motion is obtained. It is specialized to the two cases in which this shift is either much smaller or larger than the natural decay rate. In the first case, we get the same result as that of a previous calculation. The second case has not been discussed previously.

DOI
10.1103/physreva.30.177

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A previous calculation of forces on atoms in external (dc) fields and in homogeneous traveling-wave laser fields is modified for the case of a standing-wave laser field. The case of intense laser fields, where the Rabi frequency is much greater than the natural decay rate of the atom, is described. A general result for any Doppler shift of the laser frequency due to the atomic motion is obtained. It is specialized to the two cases in which this shift is either much smaller or larger than the natural decay rate. In the first case, we get the same result as that of a previous calculation. The second case has not been discussed previously.
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It is conventional to describe the motion of an atom in a laser field (and other D.C fields) by a Fokker-Planck equation which is characterized by a force and a diffusion coefficient. Gordon and Ashkin have obtained a result for the diffusion coefficient in a spherically symmetric approximation. We have recently treated it as a second order tensor and have gotten a different result. The tensor has three terms. The leading term arises from the recoil of the atom during fluorescence, the second comes from gradients of the laser intensity and the D.C. fields and the last comes from a combination of the recoil term and gradients. The trace of our leading term agrees with the result of Gordon and Ashkin but there is no agreement at all with the other terms. The calculation was performed for a traveling wave laser and is being extended now to a standing wave laser by a graduate student. The experiment being undertaken here will use a swept laser frequency to compensate for the change in resonance frequency (due to the Doppler effect) of the atom as it slows down.
The optical forces exerted on a slowly moving two-level atom in a standing wave are profoundly modified when the normal decay rates of the atom are altered. The theory presented in this paper shows that the stimulated emission force becomes dominant at a very low laser intensity. Furthermore, it can change sign to provide an additional cooling force instead of a heating force at laser detunings below the atomic resonance. These phenomena can lead to equilibrium temperatures below the Doppler limit.
A theoretical analysis is given of the interaction of a laser field consisting of two orthogonal, linearly polarized, low-power standing waves, with an ensemble of J=1/2 ground-state atoms. The atoms are assumed to move with velocities below the Doppler limit of laser cooling. The two-dimensional light-induced atomic force is calculated for the specific case when the field polarization directions differ by \ensuremath{\pi}/2. The effective optical-pumping time is shown to increase strongly near the nodes of the laser field, leading to a force that can be much larger than that in the one-dimensional case. The force is not isotropic, and can lead to the bunching of the atoms in velocity space.
The effect of field fluctuations on the motion of a two-level atom in resonant laser radiation is investigated theoretically. The internal motion of the atom is treated quantum mechanically (optical Bloch equations); the translational motion, which is coupled to the internal motion, is treated classically (Ehrenfest's theorem); and the fluctuating radiation is treated as a classical electromagnetic wave with stochastic phase (phase-diffusion model). An expression for the radiation force and equations of motion for the ensemble-averaged Bloch vector are derived and used to calculate (i) the radiation force in a plane running wave, (ii) the steady-state radiation force in a general standing wave, and (iii) the dissipative force in a plane standing wave in the limit of strong phase fluctuations. In case (iii) it is found that the atomic velocity is damped when the radiation is tuned above resonance, which is opposite to the case of a weak coherent standing wave, in which damping occurs for radiation tuned below resonance. It is shown that fluctuations have a substantial effect on the radiation force only when the spectral width of the radiation approaches or exceeds the spontaneous emission rate.