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Journal of Experimental and Theoretical Physics

Atom cooling by VSCPT: Accumulation plus filtering

Fam Le Kien, V. I. Balykin

Published Feb 1, 1999• 1 citation
Publisher

Abstract

We study laser cooling by velocity-selective coherent population trapping (VSCPT) in a double-Λ scheme with decay beyond the working levels. We show that this additional decay channel filters diffused atoms from trapped ones and provides an ultrasharp atomic momentum distribution.
DOI
10.1134/1.558791

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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.
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
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.
One-dimensional laser cooling based on velocity-selective coherent population trapping (VSCPT) has been investigated numerically through the solution of the optical Bloch equations and through a Monte Carlo analysis. The 1\ensuremath{\rightarrow}1 and 2\ensuremath{\rightarrow}2 transitions have been examined as a function of the atomic recoil frequency, the spontaneous-emission decay rate, and the Rabi frequency of the cooling laser. It has been found that for a large set of those parameters, the VSCPT cooling process may be described through scaling-law relations. The scaling laws are not valid at long atom-laser interaction times or large Rabi frequencies, where the atomic Doppler shift plays a significant role in the atomic motion evolution. Similar results for two atomic transitions suggest the validity of the scaling law for any one-dimensional VSCPT process. \textcopyright{} 1996 The American Physical Society.
We show that an ensemble of atoms in highly excited states with narrow center-of-mass momentum distribution (cold Rydberg atoms) can be obtained with velocity-selective (pi) - pulses and laser cooling by velocity-selective coherent population trapping (VSCPT). Both methods use three-level cascade systems with the uppermost state being the Rydberg one where the atoms are excited and cooled. Very high fraction of atoms can be cooled to subrecoil temperatures by the use of VSCPT in standing light waves.