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Journal of Physics B Atomic Molecular and Optical Physics

Sisyphus polarization gradient laser cooling of Cr atoms on theJ= 3, , 3 and 4 transitions

Michael Drewsen, U. Drodofsky, Claudia Weber, G Schreiber, J. Mlynek

Published Dec 14, 1996• 16 citations
Publisher

Abstract

Sisyphus polarization gradient laser cooling has been performed in a one-dimensional lin lin laser beam configuration on each of the three fine structure transitions of . Hence for the first time Sisyphus polarization gradient cooling on a transition with has been demonstrated. We have investigated the qualitative and quantitative differences between the cooling process for three transitions with . A major qualitative difference of the laser cooling on the and transitions has been observed as expected from simple theoretical arguments. In the case of sub-Doppler cooling is achieved when the laser detuning is blue. Even though the situation for the and case is qualitatively similar, we find quantitative differences which are in good agreement with Monte Carlo wavefunction simulations. For experiments where narrow velocity distributions are essential, such as atom lithography, these results can be applied.

DOI
10.1088/0953-4075/29/23/006

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Sisyphus polarization gradient laser cooling has been performed in a one-dimensional lin lin laser beam configuration on each of the three fine structure transitions of . Hence for the first time Sisyphus polarization gradient cooling on a transition with has been demonstrated. We have investigated the qualitative and quantitative differences between the cooling process for three transitions with . A major qualitative difference of the laser cooling on the and transitions has been observed as expected from simple theoretical arguments. In the case of sub-Doppler cooling is achieved when the laser detuning is blue. Even though the situation for the and case is qualitatively similar, we find quantitative differences which are in good agreement with Monte Carlo wavefunction simulations. For experiments where narrow velocity distributions are essential, such as atom lithography, these results can be applied.
In sub-Doppler laser cooling of atoms, the lowest temperatures are known to occur at low (but not too low) excitation, and the temperature increases roughly linearly with laser intensity. However, under conditions for which the lowest temperatures are obtained, the small velocity capture range and low optical pumping rate limit the number of atoms that can be collected into the cold sample. In this study, we present measurements for laser cooling of Rb and metastable He atoms for two counterpropagating laser beams with orthogonal linear polarization (lin\ensuremath{\perp}lin) over a wide range of saturation parameters and laser detunings, together with results of semiclassical (Fokker-Planck) and quantum density matrix calculations. We find that at higher laser intensity, a larger number of atoms can be collected into a final velocity distribution that is significantly narrower than that given by a linear extrapolation vs the square root of laser intensity. Two cooling mechanisms are at work: the sub-Doppler Sisyphus mechanism and also Doppler cooling. Under certain conditions that we discuss, Doppler cooling aids Sisyphus cooling by collecting high-velocity atoms beyond the Sisyphus capture range. He∗{\mathrm{He}}^{*}He∗ presents a rather unusual situation in that the minimum average kinetic energy (Eav){(E}_{\mathrm{av}})(Eav​) for purely Sisyphus cooling is comparable to the minimum value of Eav{E}_{\mathrm{av}}Eav​ with Doppler cooling alone.
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.
We present a general theoretical description of laser cooling of three-level atoms in a Λ configuration in two standing waves. Detunings, intensity, and spatial phase difference of the two waves are arbitrary. We demonstrate that, for a zero spatial phase shift, sub-Doppler cooling requires equal detunings. The basic mechanism is then coherent population trapping. For different detunings sub-Doppler cooling occurs only when the spatial phases of the two standing waves are different. This effect can be traced back to a mechanism of the Sisyphus type. These schemes of sub-Doppler cooling require neither polarization gradients nor magnetic fields.
In order to study the 3-D features of Cr atoms in laser cooling field,the interaction process of Cr atom and laser cooling field was analyzed.Based on the semi-classical theory of Doppler and polarization gradient force,the fourth-order Runge-Kutta algorithm was adopted to simulate 3-D trajectory of Cr atoms,at the same time,three-dimension distribution of them was also obtained.After passing through laser cooling field,the divergence angle of Cr atomic beam becomes smaller,and the corresponding velocity also decreases.The results show that the three-dimension features of Cr atoms agree with the two-dimension cases,and can provide more comprehensive,richer cooling information.
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.