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Acta Physica Sinica (Overseas Edition)

Laser cooling in phase fluctuating laser field

LI Yi-min, Donghai Yang, Yiqiu Wang

Published Jun 1, 1998• 1 citation
Publisher

Abstract

The influence of laser linewidth Γ 1 on laser cooling of atoms was investigated theoretically by using a diffusing phase model. The final temperature which can be achieved by Doppler cooling or sub-Doppler cooling in phase fluctuating laser field is presented. It was found that the natural linewidth Γ n of the transition used for cooling the atoms is an important quantity in evaluating the influence of Γ 1 under Doppler cooling mechanism, and the laser linewidth has little effect on the final temperature under sub-Doppler cooling mechanism because the laser detuning is very large in this case.

DOI
10.1088/1004-423x/7/6/002

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The influence of laser linewidth Γ 1 on laser cooling of atoms was investigated theoretically by using a diffusing phase model. The final temperature which can be achieved by Doppler cooling or sub-Doppler cooling in phase fluctuating laser field is presented. It was found that the natural linewidth Γ n of the transition used for cooling the atoms is an important quantity in evaluating the influence of Γ 1 under Doppler cooling mechanism, and the laser linewidth has little effect on the final temperature under sub-Doppler cooling mechanism because the laser detuning is very large in this case.
Abstract Doppler cooling of 88Sr atoms is studied in the presence of off-resonant red-detuned fluctuating laser fields. Using a semi-classical approach, we show that the relevant physical quantities in the cooling process, such as optical forces, the damping coefficient, Doppler temperature, and atom number in the trap, are strongly affected by the laser amplitude and phase fluctuations. We find that the Doppler cooling limit is higher than the predicted Doppler theory for non-fluctuating lasers. This implies an additional heating mechanism exists due to the laser fluctuations. Furthermore, our numerical analysis shows that the effect of laser power stability on reducing the number of trapped atoms in a magneto-optical trap is more substantial than the effect of laser linewidth.
Based on the numerical solution of the quantum kinetic equation for the atomic density matrix, which makes it possible to accurately take into account the recoil effects in the interaction of atoms with field photons, we have studied the limits of laser cooling of atoms using closed optical transitions characterised by different recoil parameters (the ratio of the recoil energy to the natural linewidth). It is shown that for optical transitions with an insufficiently small recoil parameter, the polarisation effects, which lead to the possibility of sub-Doppler laser cooling, lose their efficiency and do not ensure an attainment of the temperature below the Doppler limit. The analysis performed allows one to outline the boundaries of the sub-Doppler theory of laser cooling of atoms.
We examine the laser cooling of three-level (Lambda) -atoms in the field of two standing waves when frequency detunings are considerably greater than natural width of the excited state as well as the Rabi frequencies in the system. It is shown that at the difference of frequency detunings is small when compared to the Rabi frequencies sub-Doppler cooling of the (Lambda) -atom is possible. We present the dependence of the light pressure force on the atoms velocity and dependence of cooled atoms temperature on the parameters of the exciting waves.
Laser cooling dynamics of neutral atoms with a closed optical transition Jg → Je in light fields with nonuniform polarization formed by counterpropagating waves with linear, circular or elliptical polarization was considered. For the case of finite interaction time of atoms with light field both mechanisms of Doppler and Sub-Doppler laser cooling affected on atoms in ensemble having different velocities were taken into account. We get qualitative relations on a base of numerical analysis in a frame of quasiclassical treatment of laser cooling. These relations allow to define optimal parameters of cooling waves, i.e. polarization, detuning from atomic resonance and intensity of light waves for the most rapid and dip laser cooling from initially wide momentum distribution for finite interaction time of atoms with laser field
We analyze the influence of an arbitrarily directed uniform magnetic field on the laser sub-Doppler cooling of atoms. The analysis is done for a (3+5)-level atom excited by a ${\ensuremath{\sigma}}^{+}\ensuremath{-}{\ensuremath{\sigma}}^{\ensuremath{-}}$ laser field configuration. Our analysis shows that the effects of the magnetic field depend strongly on the direction of the magnetic field. In an arbitrarily directed magnetic field the laser cooling configuration produces both the main resonance existing already at zero magnetic field and additional sub-Doppler resonances caused by two-photon and higher-order multiphoton processes. These sub-Doppler resonances are, however, well separated on the velocity scale if the Zeeman shift exceeds the widths of the resonances. This allows one to use the main sub-Doppler resonance for an effective laser cooling of atoms even in the presence of the magnetic field. The effective temperature of the atomic ensemble at the velocity of the main resonance is found to be almost the same as in the absence of the magnetic field. The defined structure of the multiphoton resonances may be of importance for the sub-Doppler laser cooling of atoms, atomic extraction from magneto-optical traps, and applications related to the control of atomic motion.