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Physical Review A

Cooling using velocity or mode selection for alkali-metal atoms

Ralph Dum

Published Oct 1, 1996• 3 citations
Publisher

Abstract

Velocity (mode) selection accumulates atoms in long-lived atomic (dark) states thereby reducing the entropy and energy. We present a scheme using a two-frequency laser field acting on two separate dipole transitions that supports dark atomic states in a spatially arbitrary three-dimensional (3D) field configuration. Monte Carlo simulations of a 1D model of velocity selection demonstrate a reduction of the velocity spread of the atomic ensemble to less than 1 nK. Schemes to combine selection and trapping of atoms to reduce phase space density in 3D by mode selection are discussed. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.54.3299

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arXiv:atom-ph/9606002•Physical Review A•Nov 1, 1996•6 citations
Velocity (mode) selection accumulates atoms in long-lived atomic (dark) states thereby reducing the entropy and energy. We present a scheme using a two-frequency laser field acting on two separate dipole transitions that supports dark atomic states in a spatially arbitrary three-dimensional (3D) field configuration. Monte Carlo simulations of a 1D model of velocity selection demonstrate a reduction of the velocity spread of the atomic ensemble to less than 1 nK. Schemes to combine selection and trapping of atoms to reduce phase space density in 3D by mode selection are discussed. \textcopyright{} 1996 The American Physical Society.
A laser-cooling scheme for molecules is presented based on repeated cycle of zero-velocity selection, deceleration, and irreversible accumulation. Although this scheme also employs a single spontaneous emission as in [Raymond Ooi, Marzlin, and Audretsch, Eur. Phys. J. D 22, 259 (2003)], in order to circumvent the difficulty of maintaining closed pumping cycles in molecules, there are two distinct features which make the cooling process of this scheme faster and more practical. First, the zero-velocity selection creates a narrow velocity-width population with zero mean velocity, such that no further deceleration (with many stimulated Raman adiabatic passage (STIRAP) pulses) is required. Second, only two STIRAP processes are required to decelerate the remaining hot molecular ensemble to create a finite population around zero velocity for the next cycle. We present a setup to realize the cooling process in one dimension with trapping in the other two dimensions using a Stark barrel. Numerical estimates of the cooling parameters and simulations with density matrix equations using OH molecules show the applicability of the cooling scheme. For a gas at temperature T=1T=1T=1 K, the estimated cooling time is only 222 ms, with phase-space density increased by about 30 times. The possibility of extension to three-dimensional cooling via thermalization is also discussed.
In this paper we propose a new and simple velocity selective method, which combines the velocity selective coherent population trapping of atoms in a light field and the kinetic energy coupling between the dark state and coupled state(s) during free flight, in order to reach a very narrow momentum distribution in a short period of time. This technique combined with force-assisted velocity selective coherent population trapping can produce very efficient cooling and trapping. It can also be used to overcome the cooling limit caused by the leakage of the dark state.
We describe a numerical implementation of our semiclassical laser-cooling theory [J. Javanainen, Phys. Rev. A 44, 5857 (1991)] for an arbitrary multilevel atom and light field. There is satisfactory agreement between experiments and temperatures obtained from Langevin-equation simulations of the motion of an atom subject to light-pressure force and the accompanying diffusion in three-dimensional (3D) optical molasses in which there are three orthogonal pairs of counterpropagating waves with a common linear polarization for the two beams in a pair, but orthogonal polarizations between the pairs. However, the velocity distribution of the atoms is anisotropic and may deviate strongly from a Gaussian, heterodyne spectra may have a sideband, and the atomic density may be a maximum at the maximum of the potential of the dipole forces of the molasses beams. Subsequent velocity-linearized analysis of atomic damping and diffusion shows that the damping tensor may have a negative eigenvalue corresponding to heating in large regions of space, even though the light is red tuned for cooling. It is also found that the position-averaged damping and diffusion tensors make a poor predictor of the temperature obtained in Langevin-equation simulations and real experiments. Based on additional simulations of a simple 1D model, we formulate a hypothesis that connects our findings. The key idea is that an atom trapped in a minimum of the potential of the dipole forces is subject to a weaker-than-average damping, and therefore tends to boil out of the trap.
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.
We present a novel optical cooling scheme capable of loading and cooling atoms directly inside deep optical dipole traps utilizing hyperfine dark states. In the presence of strong light shifts of the upper excited states, this allows the velocity selective dark-state cooling of atoms into the conservative potential without loss of atoms. We report the lossless optical cooling inside the trap with a seven-fold increase in the number of atoms loaded. Our findings open the door to all-optical cooling of trapped atoms and molecules which lack the closed cycling transitions normally needed to achieve low temperatures and the high initial densities required for evaporative cooling.
We discuss the role of light absorption by pairs of atoms (radiative collisions) in the context of a model for velocity selective coherent population trapping with emphasis on its application to metastable triplet helium. Our main interest is the application of such dark-state cooling schemes to the regime where individual quantum states get multiply occupied, a regime which is sometimes referred to with the term atom laser. We show that, because of radiative collisions, for positive detuning of the driving light fields from an atomic resonance the conditions for proliferation of the occupation numbers of the lowest lying states can only be marginally met. Multiple occupation of these states only occurs if a very efficient sub-Doppler precooling mechanism is operative. In the case of negative frequency detuning the requirements on this sub-Doppler mechanism are less restricting, provided one avoids molecular resonances. \textcopyright{} 1996 The American Physical Society.