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Physical Review Letters

Velocity-selective resonances and sub-Doppler laser cooling

S-Q. Shang, B. Sheehy, H. Metcalf, P. van der Straten, Gerard Nienhuis

Published Aug 26, 1991• 57 citations
Publisher

Abstract

We describe several one-dimensional sub-Doppler laser-cooling experiments that cool Rb to finite and zero velocities in a magnetic field and polarization gradients. We then present a physical picture that unifies these measurements in terms of coherent two-photon velocity-selective resonances between atomic ground-state sublevels (including Raman resonances). We propose several sub-Doppler cooling schemes without fields or without polarization gradients, based on VSR between nondegenerate ground states.

DOI
10.1103/physrevlett.67.1094

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We describe several one-dimensional sub-Doppler laser-cooling experiments that cool Rb to finite and zero velocities in a magnetic field and polarization gradients. We then present a physical picture that unifies these measurements in terms of coherent two-photon velocity-selective resonances between atomic ground-state sublevels (including Raman resonances). We propose several sub-Doppler cooling schemes without fields or without polarization gradients, based on VSR between nondegenerate ground states.
Sub-Doppler laser cooling requires optical pumping among differently light-shifted ground-state sublevels. We describe a study of the simplest possible angular-momentum configuration that permits all sub-Doppler cooling phenomena. The Jg = 1 ⇒ Je = 0 angular-momentum configuration shows recoil-limited cooling in the two most well-known types of polarization gradient, magnetically induced laser cooling, velocity-selective resonances, transient cooling, and velocity-selective population trapping.
We describe a new sub-Doppler cooling scheme that cools atoms to nonzero velocity vr{\mathit{v}}_{\mathit{r}}vr​. The atomic motion in a combined optical standing wave and constant magnetic field induces a velocity-selective magnetic resonance that produces strong cooling for negative laser detuning to vr{\mathit{v}}_{\mathit{r}}vr​=${\mathrm{\ensuremath{\omega}}}_{\mathit{Z}}$/2k, with ${\mathrm{\ensuremath{\omega}}}_{\mathit{Z}}$ the Larmor frequency. In a one-dimensinal optical molasses transverse to a thermal Rb beam we observe the bunching of the atoms into two peaks corresponding to \ifmmode\pm\else\textpm\fi{}vr{\mathit{v}}_{\mathit{r}}vr​. We present a quantitative comparison between our theory and experiment and suggest some applications.
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.
We report a 1D study of optical forces on atoms in a two-frequency laser field. The light couples two ground state hyperfine structure levels to a common excited state of 85Rb^{85}\mathrm{Rb}85Rb, thus forming a \ensuremath{\Lambda} system. We observe a new type of sub-Doppler coupling with blue-tuned light that uses neither polarization gradients nor magnetic fields, efficient heating with red tuning, and the spatial phase dependence of these. We observed deflection from a rectified dipole force and determined its velocity dependence and capture range. We report velocity selective resonances associated with Raman transitions. A simplified semiclassical calculation agrees qualitatively with our measurements.
We report theoretical velocity distributions for sub-Doppler laser cooling of metastable He*(2{sup 3}S), calculated with the Density Matrix and Monte Carlo Wavefunction approaches. For low-field (B = 50 mG) magnetic-field induced laser cooling on the 2{sup 3}S {yields} (2{sup 3}P, J = 2) transition ({lambda} = 1083 nm), we get a narrow, sub-Doppler structure, consisting of three, {approximately}1 photon recoil wide peaks, spaced {approximately}1 recoil apart. With increasing field, this three-peak structure develops into two velocity-selective resonance (VSR) peaks, each {approximately}2 recoils wide. For the 2{sup 3}S {yields} (3{sup 3}P, J = 2) transition ({lambda} 389 nm), VSR peaks are predicted to appear at low field without the third, central peak, which only develops at higher field (B = 200 mG). Additional computations deal with polarization-gradient cooling. In general, we find that for one-dimensional cooling calculations, the Density Matrix method is more efficient than the Monte Carlo Wavefunction approach. Experiments are currently under way to test the results.