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arXiv:physics/0412119 · Journal of the Optical Society of America B

Extra-heating mechanism in Doppler cooling experiments

T. Chanelière, J.L. Meunier, Robin Kaiser, Christian Miniatura, David Wilkowski

Published Sep 1, 2005• 37 citations
arXivHTMLPDF

Abstract

We experimentally and theoretically investigate laser cooling of strontium-88 atoms in one-dimensional optical molasses. In our case, since the optical cooling dipole transition involves a Jg=0 ground state, no Sisyphus-type mechanisms can occur. We are thus able to test quantitatively the predictions of the Doppler cooling theory. We have found, in agreement with other similar experiments, that the measured temperatures are systematically larger than the theoretical predictions. We quantitatively interpret this discrepancy by taking into consideration the extra-heating mechanism induced by transverse spatial intensity fluctuations of the optical molasses. Experimental data are in good agreement with Monte Carlo simulations of our theoretical model. We thus confirm the important role played by intensity fluctuations in the dynamics of cooling and for the steady-state regime.

DOI
10.1364/josab.22.001819

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We experimentally and theoretically investigate laser cooling of strontium-88 atoms in one-dimensional optical molasses. In our case, since the optical cooling dipole transition involves a Jg=0 ground state, no Sisyphus-type mechanisms can occur. We are thus able to test quantitatively the predictions of the Doppler cooling theory. We have found, in agreement with other similar experiments, that the measured temperatures are systematically larger than the theoretical predictions. We quantitatively interpret this discrepancy by taking into consideration the extra-heating mechanism induced by transverse spatial intensity fluctuations of the optical molasses. Experimental data are in good agreement with Monte Carlo simulations of our theoretical model. We thus confirm the important role played by intensity fluctuations in the dynamics of cooling and for the steady-state regime.
We present two cooling mechanisms that lead to temperatures well below the Doppler limit. These mechanisms are based on laser polarization gradients and work at low laser power when the optical-pumping time between different ground-state sublevels becomes long. There is then a large time lag between the internal atomic response and the atomic motion, which leads to a large cooling force. In the simple case of one-dimensional molasses, we identify two types of polarization gradient that occur when the two counterpropagating waves have either orthogonal linear polarizations or orthogonal circular polarizations. In the first case, the light shifts of the ground-state Zeeman sublevels are spatially modulated, and optical pumping among them leads to dipole forces and to a Sisyphus effect analogous to the one that occurs in stimulated molasses. In the second case (σ+−σ− configuration), the cooling mechanism is radically different. Even at very low velocity, atomic motion produces a population difference among ground-state sublevels, which gives rise to unbalanced radiation pressures. From semiclassical optical Bloch equations, we derive for the two cases quantitative expressions for friction coefficients and velocity capture ranges. The friction coefficients are shown in both cases to be independent of the laser power, which produces an equilibrium temperature proportional to the laser power. The lowest achievable temperatures then approach the one-photon recoil energy. We briefly outline a full quantum treatment of such a limit.
Optical molasses is a three-dimensional (3-D) configuration of laser beams used to laser-cool and to viscously confine neutral atoms. Atoms laser cooled in optical molasses reach temperatures much lower than the limit given by the original theories of laser cooling based on the Doppler effect. This cooling below the Doppler-cooling limit is now seen as being due to new laser cooling mechanisms not considered in the original theories. The dependence of the atomic temperature on parameters such as laser intensity and detuning shows good agreement between calculations performed in 1-D and experiments performed in 3-D. For cooling of Na and Cs atoms, the lowest observed temperatures correspond to rms velocities between three and four times the single photon recoil velocity. For Cs the temperature is 2.5 ± 0.6 μK and is the lowest temperature ever measured for 3-D cooling.
We present a summary of the results of a simple two-level theory of Doppler cooling in optical molasses and contrast it with the recent theories of multilevel, polarization-gradient cooling. The effects of single-photon recoil and of trapping in microscopic optical potential wells are also considered. Experiments are described in which the temperature of sodium atoms released from optical molasses is measured and found to be well below the Doppler-cooling limit. Measurements of the temperature dependence on many experimental parameters are found to be in good qualitative agreement with the new theories of polarization-gradient cooling.
The semiclassical theory of laser cooling is applied for the analysis of cooling of unbound atoms with the values of the ground and exited state angular moments 1/2 in a one-dimensional nondissipative optical lattice. We show that in the low-saturation limit with respect to the pumping field a qualitative interpretation of the cooling mechanisms can be made by the consideration of effective two-level system of the ground-state sublevels. It is clarified that in the limit of weak Raman transitions the cooling mechanism is similar to the Doppler mechanism, which is known in the theory of two-level atom. In the limit of strong Raman transitions the cooling mechanism is similar to the known Sisyphus mechanism. In the slow atom approximation the analytical expressions for the coefficients of friction, spontaneous and induced diffusion are given, and the kinetic temperature is estimated.
Abstract We theoretically and computationally investigate the cooling of antihydrogen, H ¯ , using optical molasses cooling. This updates the results in Donnan et al (2013 J. Phys. B: At. Mol. Opt. Phys. 46 025302) to the current capabilities of the ALPHA experiment. Through Monte Carlo simulation, we show that H ¯ s do not give the standard cooling even in an ideal optical molasses because of their small mass and large transition frequency. For optical molasses cooling in the ALPHA trap, the photons are constrained to travel in one direction only. It is only through the phase space mixing in the trap that cooling in all directions can be achieved. We explore the nontrivial role that laser intensity plays in the cooling. We also investigate the possibility for simultaneously cooling atoms in either of the trapped ground states.