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Optics Letters

Coherent trapping of atomic populations

H. R. Gray, R. M. Whitley, Carlos R. Stroud

Published Dec 1, 1978• 768 citations
Publisher

Abstract

It is often desirable in laser spectroscopy and isotope separation to extract as much as possible of an atomic or molecular population that is distributed among a number of ground-state sublevels and low-lying metastable levels. We describe a form of coherent trapping that occurs when multiple resonant laser beams are used to couple the various ground states to a common upper level. This effect prevents the extraction of the entire population. We then study the effect with two dye lasers and an atomic beam and suggest possible ways to maximize the pumping efficiency.

DOI
10.1364/ol.3.000218

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It is often desirable in laser spectroscopy and isotope separation to extract as much as possible of an atomic or molecular population that is distributed among a number of ground-state sublevels and low-lying metastable levels. We describe a form of coherent trapping that occurs when multiple resonant laser beams are used to couple the various ground states to a common upper level. This effect prevents the extraction of the entire population. We then study the effect with two dye lasers and an atomic beam and suggest possible ways to maximize the pumping efficiency.
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Light-induced drift (LID) is a transport phenomenon in a gas mixture with dramatic manifestations such as the optical piston1 and applications in atomic isotope separation.2 The induced drift velocity is proportional to the excited state fraction, and for Na in Xe high drift velocities have been achieved using two lasers.3 Here one laser excites atoms from one hyperfine ground state and the second laser atoms from the other, avoiding hyperfine pumping. Excited in this way, the Na atom represents a Λ type three-level system, where coherent effects can be important.
As we have shown recently /1/, the efficiency of an optically pumped laser is strongly sensitive to the structure of the atomic (or molecular) levels involved in the amplifying process, in particular to M-degeneracy. Specifically, when the common level b shared by the pump and laser transitions in a typical three-level configuration (Fig.1) has two sublevels m=±1 and the pump and laser fields are both linearly polarized (such as in the case, for instance, of the ammonia farinfrared laser /2/), there is a large difference in the dynamics of the laser when the pump and laser field polarizations are parallel or orthogonal /1,2/.
We propose a scheme to achieve one-dimensional velocity-selective coherent population trapping in a five-level system under two-frequency laser excitation. It may be applied to either D line of alkali-metal atoms. The trapped state is a time-dependent superposition of three sublevels from two different hyperfine ground states. Stable trapping may be produced by correctly detuning the laser beams to compensate for different hyperfine and kinetic energies of these ground states. We present numerical results for a five-level model, as well as for the complete level systems appropriate for alkali-metal atoms.
The authors demonstrate the effect of atomic coherence on the generic example of two lossless micromasers coupled in series by a common pump beam of excited two-level atoms. The fields are studied via conditional measurements on the final state of the atoms. They consider the two simplest sequences: in the energy preserving (transferring) scheme they require each atom to be detected in its upper (lower) state. Due to the two paths that the atoms can follow to reach the same final state correlation between the two macroscopically separated fields can arise. The authors discuss a scheme which leads to the generation of entangled trapping states of the two fields of the form, mod n,n+M)+or- mod n+M,n), starting from number states mod n, n). Starting from initial coherent states arbitrary steady-state superpositions of the two fields can be generated by switching from the energy transferring to the preserving scheme at an optimum number of atoms. In the absence of dissipations, both methods can produce steady-state coherent superpositions of arbitrary number states of two macroscopically separated fields (nonlocal 'Schrodinger-cats'). A scheme is discussed where entanglement can be transferred from the fields to two atomic beams. Finally, the authors briefly discuss the effect of injected atomic coherence on lasers.
We report an experimental observation of coherent population transfer in a multilevel system by adiabatic following through a trapped state. Raman transitions between ground-state sublevels are induced by two partially overlapping ${\mathrm{\ensuremath{\sigma}}}^{\ifmmode\pm\else\textpm\fi{}}$ polarized laser beams, tuned to the hyperfine transition F=4\ensuremath{\rightarrow}F'=4 of the cesium D2{\mathit{D}}_{2}D2​ line. An efficiency of more than 50% is observed for the mF{\mathit{m}}_{\mathit{F}}mF​=+4\ensuremath{\rightarrow}mF{\mathit{m}}_{\mathit{F}}mF​=-4 population transfer. The influence of other excited hyperfine levels on the transfer efficiency is pointed out. The feasibility of the technique with counterpropagating laser beams is also demonstrated.