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Physical Review A

rf-induced Sisyphus cooling in an optical dipole trap

Kurt W. Miller, Stephan Dürr, Carl Wieman

Published Aug 19, 2002• 25 citations
Publisher

Abstract

A cooling process for optical dipole traps, rf-induced Sisyphus cooling in a circularly polarized far-off-resonance trap, is demonstrated. Cooling rates of up to $\ensuremath{\approx}300 \ensuremath{\mu}\mathrm{K}/\mathrm{s}$ are observed without significant atom loss. The final temperature achieved was (a somewhat disappointing) $17 \ensuremath{\mu}\mathrm{K}.$ Experimental studies and theoretical analysis of this cooling process were carried out, which explain that the temperature limit is due to an equilibrium between the small fraction of total thermal energy that is removed per Sisyphus cycle and the multiple photon recoils that are required to complete each cycle. Light-induced collisional loss made it difficult to operate under conditions where a larger fraction of the energy would be removed. A method for storing atoms in an optical trap with only a single confined spin state is also demonstrated.

DOI
10.1103/physreva.66.023406

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A cooling process for optical dipole traps, rf-induced Sisyphus cooling in a circularly polarized far-off-resonance trap, is demonstrated. Cooling rates of up to \ensuremath≈300\ensuremathμK/s\ensuremath{\approx}300 \ensuremath{\mu}\mathrm{K}/\mathrm{s}\ensuremath≈300\ensuremathμK/s are observed without significant atom loss. The final temperature achieved was (a somewhat disappointing) 17\ensuremathμK.17 \ensuremath{\mu}\mathrm{K}.17\ensuremathμK. Experimental studies and theoretical analysis of this cooling process were carried out, which explain that the temperature limit is due to an equilibrium between the small fraction of total thermal energy that is removed per Sisyphus cycle and the multiple photon recoils that are required to complete each cycle. Light-induced collisional loss made it difficult to operate under conditions where a larger fraction of the energy would be removed. A method for storing atoms in an optical trap with only a single confined spin state is also demonstrated.
We propose a laser-driven Sisyphus-cooling scheme for atoms confined in a far-off resonance optical dipole trap. Utilizing the differential trap-induced ac Stark shift, two electronic levels of the atom are resonantly coupled by a cooling laser preferentially near the trap bottom. After absorption of a cooling photon, the atom loses energy by climbing the steeper potential, and then spontaneously decays preferentially away from the trap bottom. The proposed method is particularly suited to cooling alkaline-earth-metal-like atoms where two-level systems with narrow electronic transitions are present. Numerical simulations for the cases of 88{}^{88}88Sr and 174{}^{174}174Yb demonstrate the expected recoil and Doppler temperature limits. The method requires a relatively small number of scattered photons and can potentially lead to phase-space densities approaching quantum degeneracy in subsecond time scales.
In this thesis, I study optical pumping as a powerful cooling tool for trapped ultra-cold atoms in a highly collisional regime. First application of optical pumping is a continuous loading scheme used to transfer atoms from a guided beam into a hybrid trap. Further, I introduce a Sisyphus cooling scheme based on radio-frequency transitions and optical pumping, operating simultaneously to the accumulation of atoms in the trap. The combined scheme of continuous loading and Sisyphus cooling is demonstrated for a large range of initial conditions of the guided atoms. Thereby, I show that collisional thermalization occurs in a steady-state for almost arbitrary initial conditions, provided that the first dissipative step is able to prevent the atom from leaving the trap during its first passage. On the one hand, this scheme could be applied to a wide range of atomic or molecular beams. On the other hand, phase-space density of 4*10^-4 is reached in a continuous operation mode with chromium atoms. In the second part, I investigate demagnetization cooling based on dipolar relaxation collisions driving the thermalization of the internal (spin) and the external (motional) degrees of freedom. In the case of a gas, one has the advantage that the spin degree of freedom can be cooled very efficiently using optical pumping. It is shown, that demagnetization cooling of a gas is more efficient than evaporation cooling in terms of phase-space density gain versus loss of atoms. This allows reaching a temperature of 6uK at a phase-space density of 0.03. It is observed, that both, continuous Sisyphus cooling and demagnetization cooling are limited by a density dependent loss mechanism. I present circumstantial evidence for excited-state collisions as the dominant limiting process. Finally, I discuss possible extensions to the current experimental procedures, possibly allowing reaching quantum degeneracy by optical means only.
The behavior of atoms in an optical far-off resonance trap is found to depend strongly on the trap polarization. Large loss rates occur unless the polarization is perfectly linear or perfectly circular, in which case exponential lifetimes up to 10 s are observed. Through optimization of trap loading, samples of $4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms are obtained in a circularly polarized trap. Spin polarization of 98(1)98(1)%98(1) is measured. This trap should prove useful for precision measurements such as $\ensuremath{\beta}$-decay asymmetry and should make rf cooling possible in optical traps.
We present a study of Sisyphus cooling of molecules: the scattering of a single-photon remove a substantial amount of the molecular kinetic energy and an optical pumping step allow to repeat the process. A review of the produced cold molecules so far indicates that the method can be implemented for most of them, making it a promising method able to produce a large sample of molecules at sub-mK temperature. Considerations of the required experimental parameters, for instance the laser power and linewidth or the trap anisotropy and dimensionality, are given. Rate equations, as well as scattering and dipolar forces, are solved using Kinetic Monte Carlo methods for several lasers and several levels. For NH molecules, such detailed simulation predicts a 1000-fold temperature reduction and an increase of the phase space density by a factor of 10^7 . Even in the case of molecules with both low Franck-Condon coefficients and a non-closed pumping scheme, 60% of trapped molecules can be cooled from 100 mK to sub-mK temperature in few seconds. Additionally, these methods can be applied to continuously decelerate and cool a molecular beam
We present a study of Sisyphus cooling of molecules: The scattering of a single photon removes a substantial amount of the molecular kinetic energy and an optical pumping step allows one to repeat the process. A review of the produced cold molecules so far indicates that the method can be implemented for most of them, making it a promising method able to produce a large sample of molecules at sub-mK temperatures. Considerations of the required experimental parameters, for instance the laser power and linewidth or the trap anisotropy and dimensionality, are given. Rate equations, as well as scattering and dipolar forces, are solved using kinetic Monte Carlo methods for several lasers and several levels. For NH molecules, such detailed simulation predicts a 1000-fold temperature reduction and an increase of the phase-space density by a factor of 107{}^{7}7. Even in the case of molecules with both low Franck-Condon coefficients and a nonclosed pumping scheme, 60% of trapped molecules can be cooled from 100 mK to sub-mK temperatures in a few seconds. Additionally, these methods can be applied to continuously decelerate and cool a molecular beam.
We confine 85Rb^{85}\mathrm{Rb}85Rb atoms in an optical dipole force trap with a very large detuning from resonance of up to 65 nm. Confinement times of 200 ms, limited only by background-gas collisions, are obtained without additional cooling. A typical trap contains 1300 atoms at a temperature of 0.4 mK and a peak density of 8\ifmmode\times\else\texttimes\fi{}1011{10}^{11}1011 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. We measure spontaneous photon scatter rates of the trapped atoms to be less than 1.6\ifmmode\times\else\texttimes\fi{}103{10}^{3}103 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, corresponding to recoil heating rates below 0.6 mK/s. The far-detuned trap confines atoms with a strong, nearly conservative optical force and negligible atomic excitation.