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Annual Meeting Optical Society of America

Velocity-selective coherent population trapping in conjunction with pol-grad cooling in a folded three-level system under standing wave excitation

D. P. Katz, M. S. Shahriar, Philip Hemmer, Mara Goff Prentiss, N. P. Bigelow

Published Jan 1, 1993• 0 citations
Publisher

Abstract

We identify the conditions under which velocity selective coherent population trapping (VSCPT) takes place when a three level Λ system is excited by a pair of Raman resonant standing waves.
DOI
10.1364/oam.1993.tudd.1

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In recent years, there has been a great deal of interest in velocity selective coherent population trapping (VSCPT).1 However, since VSCPT occurs via a random walk in momentum space, in order to achieve significant VSCPT in three dimensions, it is necessary to precool and confine atoms dose to the recoil momentum. Shahriar et al.2 recently predicted that this type of cooling and velocity confinement assisted VSCPT can be achieved in one, two, or three dimensions when a Λ-system atom is excited by a pair of blue detuned Raman resonant standing waves, with a phase difference of θ = π/4.
We have demonstrated velocity-selective coherent population trapping (VSCPT) in a two-level system created by circularly polarized light driving the $2{}^{3}{S}_{1}\ensuremath{\rightarrow}3{}^{3}{P}_{2}$ transition in metastable helium. It is quite different from the usual VSCPT because there need be no consideration of selection rules, polarization, or internal atomic states. This most primitive case elicits the simple nature of VSCPT as a special kind of quantum interference, and demonstrates the presence of VSCPT in a system that has only two internal levels. It is readily observed for this transition because the ratio of the recoil frequency to the natural linewidth is 0.22, two orders of magnitude larger than for most laser cooling experiments. Our trapped state is fed by Doppler cooling, which is unusually effective here because of the large recoil, and is totally absent in previously described VSCPT experiments.
One-dimensional velocity-selective coherent population trapping (VSCPT) in a cascade atomic configuration, with the second excited level a Rydberg state, is investigated through numerical calculations. The trapping efficiency for the case of travelling wave lasers is derived from the solution of the optical Bloch equations. For excitation by bichromatic standing waves, the energy-band approach is applied. Although the cooling process is limited by the spontaneous emission decay of the Rydberg state, the coexistence of polarization-gradient cooling and VSCPT in the case of standing waves allows the preparation of the majority of the atomic population in a cold Rydberg state.
ABSTRACT In this work we present an investigation of velocity-selective coherent population trapping (VSCPT) in asym- metric three-level schemes. The light force in such schemes is of an intrinsically transient character and shouldnot be described in a quasiclassical approach. Instead, results of the full quantum-mechanical calculations are presented with emphasis on their meaning to the efficiency of subrecoil laser cooling. 1 INTRODUCTION Laser cooling of neutral atoms has received significant advances by the introduction of velocity-selectivecoherent population trapping ((VSCPT) some years ago.1 Since it leads to atomic temperatures well below therecoil limit, studies of ultracold atoms and atomic interactions may be improved by magnitudes.The effect of VSCPT relies on optical pumping of atoms into a state WNC) which is a trap state for a certaincenter-of-mass velocity vO. This trap state is created by a destructive superposition of atomic states within amultilevel quantum system. Once being in such a state LWNC) the laser light does not interact anymore with theatom which leads to a decoupling with the rest of the system at vo. Atoms located outside this small velocityregion get a chance to be trapped by the momentum diffusion due to spontaneous emission of photons. Aftera sufficient long interaction time nearly all atoms are trapped inside a narrow momentum distribution whichcan be interpreted as cooling. The theoretical and experimental investigation of VSCPT is usually consideredby use of a three-level A-system as shown in Fig.1. Such a system can be found for example in a symmetrical
We show that an imperfection of velocity-selective coherent population trapping (VSCPT) in three-level atoms excited by standing light waves causes a rectified force on cooled atoms. The rectified force as well as the cooling force are calculated both analytically and numerically for and cascade three-level systems. Combination of these forces with the VSCPT mechanism can lead to localization of very cold atoms in potential wells created by the rectified force. This effect should be taken into account in experiments with VSCPT in standing waves, and can be used for realizing superlattices of cold atoms, in particular, cold Rydberg atoms.