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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

Sub-Doppler Traps in an Asymmetric Magneto-Optical Trap with Unequal Laser Detunings

Heung‐Ryoul Noh, Myung-Sun Heo, Wonho Jhe

Published Aug 1, 2007• 0 citations
Publisher

Abstract

The theoretical and experimental studies on the observation of adjustable multiple sub-Doppler traps in the six-beam magneto-optical trap (MOT) with unequal laser detunings are presented. When the detuning of the longitudinal lasers along the magnetic field axis is different from that of the transverse lasers, we obtain an array of three sub-Doppler traps; one usual but suppressed trap at the origin and two additional traps symmetrically but oppositely located at equal and controllable distances from the origin. We could explain the observed sub-Doppler traps and their behavior by the calculation of the sub-Doppler forces where multiple oscillating frequencies are involved.

DOI
10.1109/cleopr.2007.4391775

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The theoretical and experimental studies on the observation of adjustable multiple sub-Doppler traps in the six-beam magneto-optical trap (MOT) with unequal laser detunings are presented. When the detuning of the longitudinal lasers along the magnetic field axis is different from that of the transverse lasers, we obtain an array of three sub-Doppler traps; one usual but suppressed trap at the origin and two additional traps symmetrically but oppositely located at equal and controllable distances from the origin. We could explain the observed sub-Doppler traps and their behavior by the calculation of the sub-Doppler forces where multiple oscillating frequencies are involved.
We have realized adjustable multiple sub-Doppler traps (SDT's) in a six-beam magneto-optical trap (MOT) under asymmetric trap conditions. In the simplest case, one obtains an array of three SDT's, one usual trap at the origin and two additional traps symmetrically but oppositely located at equal and controllable distances from the origin, in good agreement with theoretical calculations. The easily adjustable array of SDT's readily available in the asymmetric MOT may open the possibility of novel atom optics or quantum optics experiments such as atom interferometer or quantum information.
We present a semiclassical theory of the sub-Doppler forces in an asymmetric magneto-optical trap where the trap-laser frequencies are unequal to one another. To solve the optical Bloch equations, which contain explicit time dependence, unlike in the symmetric case of equal laser detunings, we have developed a convenient and efficient method to calculate the atomic forces at various oscillating frequencies for each atomic density matrix element. In particular, the theory provides a qualitative understanding of the array of sub-Doppler traps (SDTs) recently observed in such an asymmetric trap. We find that the distances between SDTs are proportional to the relative detuning differences, in good agreement with experimental results. The theory presented here can be applied to a dynamic system with multiple laser frequencies involved; the number of coupled equations to solve is much reduced and the resulting numerical calculation can be performed rather simply and efficiently.
Since the first proposal and the first experiment on magneto-optical trapping of atoms, magneto-optical traps (MOT) became the most popular traps for cold atoms. Despite the numerous successful applications of MOT it is still unclear what ultimate parameters could be expected for atoms trapped in MOT. We develop the theory of MOT for cold atoms under the multilevel dipole interaction schemes. Theory is based on consideration of one-dimensional motion of atoms which have three ground-state sublevels (F/sub g/=1) and five upper-state sublevels (F/sub e/=2) and are in inhomogeneous magnetic field and in the field of two counter-propagating circular polarized laser waves. It is shown that in multilevel interaction schemes two-photon processes are responsible for formation of a narrow velocity and spatial structure in the radiation force. It is found that two-photon coherent process enhances considerably the friction force and the oscillation frequency at negative detuning. Single-photon and two-photon processes are shown to produce the two-component structure of the MOT potential and accordingly the two-component coordinate distribution.
Magneto-optical traps (MOTs) of Er and Dy have recently been shown to exhibit populationwide sub-Doppler cooling due to their near degeneracy of excited- and ground-state Land\'e ggg factors. We discuss here an additional, unusual intra-MOT sub-Doppler cooling mechanism that appears when the total Dy MOT cooling laser intensity and magnetic quadrupole gradient increase beyond critical values. Specifically, anisotropically sub-Doppler-cooled cores appear, and their orientation with respect to the quadrupole axis flips at a critical ratio of the MOT laser intensity along the quadrupole axis versus that in the plane of symmetry. This phenomenon can be traced to a loss of the velocity-selective resonance at zero velocity in the cooling force along directions in which the atomic polarization is oriented by the quadrupole field. We present data characterizing this anisotropic laser cooling phenomenon and discuss a qualitative model for its origin based on the extraordinarily large Dy magnetic moment and Dy's near degenerate ggg factors.
We have measured the trap frequency as well as the damping coefficient of a magneto-optical trap by using a transient oscillation method. The dependence of such trap properties on the various experimental parameters such as the cooling laser intensity, detuning, and magnetic field gradient is investigated. We find that the measured trap frequency is in excellent agreement with the simple rate-equation analysis based on the Doppler cooling theory. In contrast, the damping coefficient is about twice as large as the calculated one, which is attributed to the existence of the sub-Doppler trap near the trap center.
We observed efficient sub-Doppler cooling of 85Rb^{85}\mathrm{Rb}85Rb atoms under a nonzero magnetic field in a moving optical lattice formed in a bichromatic magneto-optical trap. Trap laser detunings were biased or set differently for the counterpropagating laser beams so that the atoms could be trapped where the magnetic field was nonzero. We investigated the center position and the temperature of the atomic cloud. We found that the sub-Doppler cooling effect, known to decrease as the nonzero magnetic field increases in a magneto-optical trap, would in fact increase under a particular trap-laser detuning difference. We first derived the condition under theoretical considerations and then verified it experimentally by conducting nondestructive measurement of atomic temperature.