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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

New method to induce stimulated Raman transitions

David Holleville, J. Fils, N. Dimarcq

Published Jan 22, 2003• 1 citation
Publisher

Abstract

Stimulated Raman transitions are very useful tools in atomic optics. They have such different applications as sub-recoil cooling (Raman cooling and VSCPT), transverse velocity selection in some cold atom experiments, separation of wave-packets in atomic interferometry. There are several techniques to obtain a set-up, which is able to induce stimulated Raman transitions. The aim of the paper is to describe a new method based on the direct modulation of a laser diode current.

DOI
10.1109/freq.1999.840722

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Stimulated Raman transitions are very useful tools in atomic optics. They have such different applications as sub-recoil cooling (Raman cooling and VSCPT), transverse velocity selection in some cold atom experiments, separation of wave-packets in atomic interferometry. There are several techniques to obtain a set-up, which is able to induce stimulated Raman transitions. The aim of the paper is to describe a new method based on the direct modulation of a laser diode current.
The usefulness of stimulated Raman transitions has been demonstrated in a variety of different applications. These two-photon transitions have the capability of very high resolution because the transition linewidths depend on the relative jitter between two frequencies, rather than absolute jitter on either one. Recent demonstrations include laser cooling and atomic interferometry [1]. They may also be needed for improved atomic clocks [2]. Due to this high resolution, stimulated Raman transitions can be a direct replacement for a microwave cavity and are often more compatible with optical experiments because a laser beam requires less space.
A technique for cooling atoms in two dimensions to temperatures below the single photon recoil limit k{sub B}T{sub rec} = (hk){sup 2}/2M is demonstrated. This technique combines velocity selection using stimulated Raman transitions and dissipation by spontaneous emission. It is an extension of the one dimensional cooling technique that was demonstrated recently. Experimental results for sodium atoms are presented, and the limits on the minimal cooling temperatures are analyzed. Finally, the possibility of applying the Raman cooling technique to atoms in a dipole trap is discussed.
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.
Optical dipole traps can provide strong, state-independent confinement of atoms. Unfortunately, the spatially dependent light shifts of optical transitions in dipole traps can interfere with conventional laser cooling techniques. These limitations may be overcome by cooling trapped atoms with stimulated Raman transitions. In particular, the lights shifts of ground state sublevels can be equal, so that the Raman cooling transition is unperturbed. Cooling in the sideband limit, to the quantum ground state of motion, should be possible. We have confined {sup 85}Rb atoms in a far-detuned optical dipole trap, and indirectly measured extremely low heating rates of the trapped atoms. We have also driven velocity-sensitive stimulated Raman transitions of the trapped atoms. The high resolution of these stimulated Raman resonances should allow us to resolve the motional sidebands and to directly cool the trapped atoms.
It is known [1-3], that the samples of atoms with a velocity spread much lower than the recoil velocity uR= ħk/M (k is the wave number of exciting light field, M is the mass of atom) can be obtained by use of velocity selection with stimulated Raman transitions. A theoretical analysis of the velocity selection method is presented in [2,3]. So, it was shown in [2] that the formation of such narrow velocity structures is performed not only by the counterpropagating, but also by co-propagating light waves. This fact allows to form the two-dimensional velocity distribution of the widths δvx,y « uR by the velocity selection technique using laser configuration as in Fig. 1b.