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Journal of the Optical Society of America B

Sub-Doppler laser cooling on combined optical transitions

Прудников Олег Николаевич, E. Arimondo

Published May 1, 2003• 15 citations
Publisher

Abstract

We study the laser cooling process in a complex lambda system in the presence of force-assisted bichromatic velocity-selective coherent population trapping. Our theoretical model includes atomic transitions based on two ground states, Fg=1 and Fg′=3, and one excited state, Fe=2, excited by a bichromatic laser field tuned in resonance with the Fg=1→Fe=2 and Fg′=3→Fe=2 transitions. The analyzed optical scheme may be useful for alternative transverse cooling in experiments with group III atoms (Al, Ga, In).

DOI
10.1364/josab.20.000909

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We study the laser cooling process in a complex lambda system in the presence of force-assisted bichromatic velocity-selective coherent population trapping. Our theoretical model includes atomic transitions based on two ground states, Fg=1 and Fg′=3, and one excited state, Fe=2, excited by a bichromatic laser field tuned in resonance with the Fg=1→Fe=2 and Fg′=3→Fe=2 transitions. The analyzed optical scheme may be useful for alternative transverse cooling in experiments with group III atoms (Al, Ga, In).
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 present a calculation of cooling of Rydberg atoms in bichromatic standing light waves. The atoms in our model are assumed to have a cascade three-level transition scheme, with the second excited level being the Rydberg state. The cooling fields, which also excite the atoms to the Rydberg state, consist of two standing waves whose frequencies are tuned near the two transition frequencies of the cascade atomic system. It is shown that the mechanisms of Sisyphus cooling and velocity-selective coherent population trapping can co-exist in such a configuration, and under certain conditions the majority of the laser-cooled atoms are in the Rydberg state.
Two-dimensional laser cooling based on velocity-selective coherent population trapping is investigated theoretically for the J = 1 ↔ J = 1 atomic transition. Wave vectors and polarizations of three laser beams are chosen to realize a coherent superposition of three degenerate ground states. For the first time forces related to Doppler and interference effects are combined with coherent population trapping to realize a very efficient cooling. Numerical solutions are presented for laser cooling of helium atoms.
We study a class of laser cooling schemes that combine velocity-selective coherent population trapping (VSCPT) with Doppler cooling mechanisms. The Doppler mechanism prevents the atoms from diffusing into the region of high momentum, thereby removing the characteristic long-time asymptotic behaviour of VSCPT where, in the end, all the atoms escape from the trap. It is shown that the transition Jg = 2 to Je = 1 while maintaining the simple field configuration present in the original VSCPT case provides the additional Doppler mechanism and leads to virtually zero temperature without the loss of atoms that characterize the original VSCPT dynamics.
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 present a new laser-cooling scheme based on velocity-selective optical pumping of atoms into a nonabsorbing coherent superposition of states. This method has allowed us to achieve transverse cooling of metastable 4He^{4}\mathrm{He}4He atoms to a temperature of 2 \ensuremath{\mu}K, lower than both the usual Doppler cooling limit (23 \ensuremath{\mu}K) and the one-photon recoil energy (4 \ensuremath{\mu}K). The corresponding de Broglie wavelength (1.4 \ensuremath{\mu}m) is larger than the atomic-transition optical wavelength.