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Physical Review Letters

Raman Cooling of Atoms in an Optical Dipole Trap

H. J. Lee, Charles S. Adams, Mark A. Kasevich, Steven Chu

Published Apr 8, 1996• 134 citations
Publisher

Abstract

We have Raman cooled sodium atoms below the photon recoil temperature in a novel type of blue-detuned optical dipole force trap. In this trap $4.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms have been cooled to an effective three dimensional temperature of 1.0 \ensuremath{\mu}K at a final density of $4\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\ensuremath{-}3}$. No atoms were lost during the cooling process. The phase space density increased by a factor of 320 over the uncooled sample. This is the highest phase space density achieved by an all-optical cooling method.

DOI
10.1103/physrevlett.76.2658

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We have Raman cooled sodium atoms below the photon recoil temperature in a novel type of blue-detuned optical dipole force trap. In this trap 4.5\ifmmode×\else\texttimes\fi1054.5\ifmmode\times\else\texttimes\fi{}{10}^{5}4.5\ifmmode×\else\texttimes\fi105 atoms have been cooled to an effective three dimensional temperature of 1.0 \ensuremath{\mu}K at a final density of 4\ifmmode×\else\texttimes\fi1011cm\ensuremath−34\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\ensuremath{-}3}4\ifmmode×\else\texttimes\fi1011cm\ensuremath−3. No atoms were lost during the cooling process. The phase space density increased by a factor of 320 over the uncooled sample. This is the highest phase space density achieved by an all-optical cooling method.
We have simultaneously spin polarized and subrecoil cooled sodium atoms trapped in a blue detuned optical dipole trap. Temperatures of 1.5 \ensuremath{\mu}K and less have been achieved for atoms which have been polarized into any of three F=1F=1F=1 ground-state magnetic sublevels of sodium. The final phase-space density is comparable to the best achieved by an all-optical cooling technique.
Laser cooled sodium atoms are trapped in an optical dipole force trap formed by the intersection of two 1.06 \ensuremath{\mu}m laser beams. Densities as high as $4\ifmmode\times\else\texttimes\fi{}{10}^{12}$ atoms/cm3{\mathrm{m}}^{3}m3 at a temperature of $\ensuremath{\sim}140$ \ensuremath{\mu}K have been obtained in a $\ensuremath{\sim}900$ \ensuremath{\mu}K deep trap. By reducing the trap depth over a 2 s interval, we have evaporatively cooled the atoms to a final temperature of $\ensuremath{\sim}4$ \ensuremath{\mu}K at a density of $6\ifmmode\times\else\texttimes\fi{}{10}^{11}$ atoms/cm3{\mathrm{m}}^{3}m3. This corresponds to a factor of 28 increase in atomic phase-space density.
We present a new and efficient implementation of Raman cooling of trapped atoms. It uses Raman pulses with an appropriate frequency chirp to realize a velocity selective excitation through a rapid adiabatic passage. This method allows to address in a single pulse a large number of non zero atomic velocity classes and it produces a nearly unity transfer efficiency. We demonstrate this cooling method using cesium atoms in a far-detuned crossed dipole trap. Three-dimensional cooling of 1×1051 \times 10^{5}1×105 atoms down to 2μ2 μ2μK is performed in 100 ms. In this preliminary experiment the final atomic density is 1.3×10121.3\times 10^{12}1.3×1012 at/cm3^33 (within a factor of 2) and the phase-space density increase over the uncooled sample is 20. Numerical simulations indicate that temperatures below the single photon recoil temperature should be achievable with this method.
We present a method for producing three-dimensional Bose-Einstein condensates using only laser cooling. The phase transition to condensation is crossed with 2.5×10^{4} ^{87}Rb atoms at a temperature of T_{c}=0.6 μK after 1.4 s of cooling. Atoms are trapped in a crossed optical dipole trap and cooled using Raman cooling with far-off-resonant optical pumping light to reduce atom loss and heating. The achieved temperatures are well below the effective recoil temperature. We find that during the final cooling stage at atomic densities above 10^{14} cm^{-3}, careful tuning of trap depth and optical-pumping rate is necessary to evade heating and loss mechanisms. The method may enable the fast production of quantum degenerate gases in a variety of systems including fermions.
Conditions for trapping sodium atoms in far detuned dipole traps are explored with the goal of maximizing phase space density and coherence times. With red detuned traps, we have been able to achieve densities as high as 2 ×1012 atoms/cm3 at temperatures on the order of (11ħk)2/2M. We also report Raman cooling in a red de‐tuned trap to a one dimensional “temperature” of 0.7(ħk)2/2M. With a blue detuned trap, we have been able to observe Ramsey interference fringes between the ground states of sodium for over 4 seconds. Finally, we report an atom interferometer based on adiabatic transfer of ground state populations.
We develop a simple and practical scheme to apply sideband cooling to a cloud of rubidium atoms. A sample containing 4 × 10 70 87 Rb is trapped in a far red detuned optical lattice. Through optimizing the relevant parameters, i.e., laser detuning, magnetic field, polarization, and duration time, a temperature around 1.5 μK and phase space density close to 1/500 are achieved. Compared with polarization gradient cooling, the temperature decreases by around one order of magnitude. This technique could be used in high precision measurement such as atomic clocks and atom interferometer. It could also serve as a precooling means before evaporation cooling in a dipole trap, and may be a promising method of achieving quantum degeneracy with purely optical means.