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arXiv:physics/9611015 · Physical Review Letters

Ramsey-Type Subrecoil Cooling

F. Sander, T. Devolder, Tilman Esslinger, Theodor W. Hänsch

Published May 26, 1997• 12 citations
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Abstract

We experimentally study the motion of atoms interacting with a periodically pulsed near resonant standing wave with a polarization gradient. The atoms are cooled to a comblike momentum distribution. The peaks have widths of $\ensuremath{\approx}0.3\ensuremath{\Elzxh}k$ and a spacing which is an integer multiple of the recoil momentum $\ensuremath{\Elzxh}k$. The atomic population is accumulated in ground states which periodically evolve to dark states each time the standing wave is switched on. These light pulse synchronously recurring dark states exist for discrete pulse frequencies, even if no stationary dark states are present.

DOI
10.1103/physrevlett.78.4023

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We experimentally study the motion of atoms interacting with a periodically pulsed near resonant standing wave with a polarization gradient. The atoms are cooled to a comblike momentum distribution. The peaks have widths of \ensuremath≈0.3\ensuremath\Elzxhk\ensuremath{\approx}0.3\ensuremath{\Elzxh}k\ensuremath≈0.3\ensuremath\Elzxhk and a spacing which is an integer multiple of the recoil momentum \ensuremath\Elzxhk\ensuremath{\Elzxh}k\ensuremath\Elzxhk. The atomic population is accumulated in ground states which periodically evolve to dark states each time the standing wave is switched on. These light pulse synchronously recurring dark states exist for discrete pulse frequencies, even if no stationary dark states are present.
The \ensuremath{\Lambda} system resulting from the interaction of counterpropagating orthogonally polarized beams of light with a Jg{\mathit{J}}_{\mathit{g}}Jg​=1\ensuremath{\rightarrow}Je{\mathit{J}}_{\mathit{e}}Je​=1 transition is known to exhibit laser cooling without the presence of a damping force, through velocity selective coherent population trapping. This occurs independent of the sign of the laser's detuning from the atomic resonance frequency. We present measurements and calculations that show a detuning-dependent transient peak or dip in the ground-state momentum distribution centered at pg{\mathit{p}}_{\mathit{g}}pg​=0, however, which is reminiscent of laser cooling by a damping force. We explain this in terms of the character of the quantum-mechanical eigenstates of the total ground-state Hamiltonian of the system, using both analytic expressions and numerical calculations.
A quantum analytic treatment of the resonant scattering of atoms with jg equals j yields je equals j (j is an integer) transitions by a pulsed (sigma) + - (sigma) - field is developed. In the approximation of motionless atoms, the atomic density matrix after action of single light pulse is evaluated with exact consideration of the recoil effects. Recurrent equations, which allow to calculate the distribution after arbitrary pulse sequences are found in the analytic form. It is shown that for discrete pulse frequencies the action of N light pulses leads to the formation and narrowing of peaks in discrete points of the momentum space, and also to the broadening of the envelope of the momentum distribution. In the case of the wide (with respect to the photon momentum) initial distribution the explicit formulae for peaks and envelope are obtained and an asymptotic behavior of the solution is analyzed. In the low-intensity limit a dependence of the scattering pattern contrast on the pulse duration is studied.
We report the first observation of subrecoil laser cooling in two dimensions by velocity selective coherent population trapping. We have compressed the momentum distribution of a cloud of trapped metastable helium atoms along two orthogonal axes to $\ensuremath{\delta}p=\frac{\ensuremath{\hbar}k}{4}$, where $\ensuremath{\hbar}k$ is the photon momentum. The corresponding temperature, 16 times below the single-photon recoil temperature, is 250 nK. Atoms are pumped into a nonabsorbing state exhibiting four well-resolved momentum peaks with subrecoil widths. The momentum space density in the peaks is significantly greater than that in the uncooled distribution.
We present a new scheme for ID subrecoil laser cooling by velocity-selective coherent population trapping of even-even isotopes of elements found in the second group of the periodical table, particularily, zaaHg and 40Ca. The scheme envolves the transitions (a) between the ground state with the total angular momentum Pg = 0 and the excited one with Fe = 1, @) between the latter state and the second excited one with Fe* = 2. The second transition belongs to the infrared region and has a width much smaller than the transition from the ground state. The lower transition is driven by two counterpropagating o+ and U- polarized running waves. The laser beam applied in the perpendicular direction is a linearily polarized standing wave exciting the
The effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave has been investigated, both theoretically and experimentally. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous emission lifetimes to several hundred. For a standing wave frequency blue-detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings the rate of diffusion is so slow that the resulting distribution approaches a quasi-steady state. For red detunings the diffusion is balanced with the force and a true steady-state is achieved. A Monte Carlo method based on the density matrix equations in the dressed-state representation is applied to simulate the atomic motion. The dynamics of atom channeling is studied. One of the important applications of this investigation is adiabatic laser cooling of atoms. Lithium atoms channeled in the nodes of an intense standing wave radiation field are cooled to near the recoil limit, a single photon's momentum hbark\\hbar{k}hbark, by adiabatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root mean squared momentum of 2 hbark\\hbar{k}hbark in one-dimension. The data are compared with the results obtained by the Monte Carlo method. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.
We have observed a new type of sub-Doppler laser cooling with neither polarization gradients nor magnetic fields. All known sub-Doppler laser cooling methods require one of these two to resdistribute the atomic population among the energy levels. This allows the irreversible processes of optical pumping vels. This allows the irreversible processes of optical pumping (OP) to manipulate the conservative force of light shifts (e.g. resonant exchange between the two beams comprising a standing wave) into a damping force. Our scheme employs stimulated Raman transitions between ground state hyperfine levels of Rb with a bichromatic standing wave, accompanied by a momentum change of {Delta}p = 2{h_bar}k. OP, with {Delta}p = 1{h_bar}k, creates a population imbalance allowing repetition of the Raman process. When the detuning of the light from Raman resonance {delta} is properly chosen, the net momentum exchange damps the atomic motion. Changing {delta} by only {gamma}/6(1 MHz) replaces cooling by heating. This process separates the velocity selective Raman resonance and OP processes differently from others, and helps clarify the role of these different mechanisms.