A cloud of ultracold atoms confined in a magneto-optical trap (MOT) has a temperature greater than that of a low-density vapour in the same light field. It has been observed (Drewsen M. et al., AppL Phys. B, 59 (1994) 283) that this temperature excess is proportional to the cube root of the total number of trapped atoms (N1/3). We present an explanation for this effect in terms of the scattering which arises when photons spontaneously emitted by atoms have an appreciable probability of being reabsorbed within the cloud. This hypothesis has been tested by time-of-flight measurements of the temperature of clouds of atoms released from a MOT into l-dimensional and 3-dimensional optical molasses in the g + -r -and lin I lin configuration.
We have calculated the long-range part of the momentum difFusion coefficient of two atoms in a radiation field using an S-matrix expansion to first order in the dipole-dipole interaction between the atoms. This perturbative treatment is limited to the low-saturation regime and large detunings (~b~) & p). The physical processes arise from the interaction with the coherent and incoherent parts of the scattered spectrum. We find that in the regime of a constant density the extra difFusion term when averaged over the trap volume depends on the total number of trapped atoms to the third power and is independent of the atom laser detuning.PACS number(s): 32.80. Pj, 42.50.Vk
The authors discuss the radiation force in a one-dimensional sigma +- sigma - laser standing wave, for transitions J to J+ Delta J, for any J. and Delta J=+or-1.0. in the limit of low saturation of the atomic transition. The interaction between the radiation field and the atom produces a motion-induced orientation of the atomic ground state which grows rapidly with J. The radiative force on the atom is sensitive to this orientation. For J to J+1 the dissipative part of the total force is much larger than the reactive part, at low velocities, while for the other cases the two contributions are roughly equal and opposite. By using a simple method of calculation, the authors gain much physical insight into the mechanism of induced orientation, and give quantitative results for the size of the radiation force, including both the low-velocity friction coefficient and the velocity capture range.
We present a fully quantum analysis of velocity selective coherent population trapping (VSCPT) in two schemes for which there is a force which assists in the accumulation of atoms in the trapping state. In the first scheme, the Fg=2 to Fe=2 transition was studied where the existence of a semiclassical polarization gradient force even for a pure sigma +- sigma - configuration has been demonstrated which gives cooling for blue detuning. We also verify that cooling may be obtained for the F=2 to 1 and F=3 to 2 transitions; our fully quantum mechanical calculations yield a result which is consistent with recent experimental results of Valentin et al. (1992) but differ from previous semiclassical calculations. Secondly, we discuss a scheme of force-assisted VSCPT which combines VSCPT on the Fg=1 to Fe=1 transition with the coupling to another transition for which a strong sub-Doppler cooling force is present. This scheme allows velocity selective coherent population trapping and the polarization-gradient force to work under optimum conditions simultaneously. We find that both of these two schemes lead to efficient cooling of atoms.
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