One-dimensional laser cooling below the Doppler limit

A theoretical analysis is given for one-dimensional laser cooling below the Doppler limit of J=1/2 ground-state atoms. The laser field consists of a pair of counterpropagating, linearly polarized, low-power beams, whose polarization directions differ by an angle θ (0≤θ≤π/2). For θ≪1, the effective optical-pumping time is shown to increase strongly near the nodes of the standing wave, and the cooling force can be much larger than that for θ=π/2. Moreover, for θ≪1, it can be shown that the stimulated part of the atomic diffusion is reduced considerably as compared with that for θ=π/2. As a consequence it is possible to achieve an equilibrium atomic distribution that, for θ≪1, is characterized by a mean kinetic energy that is lower than that predicted to occur for θ=π/2. The equilibrium velocity distribution is not necessarily Maxwellian, and thus the temperature of the atomic ensemble may not be well defined. The achievable kinetic energy is so small that the cooled atoms may be trapped in the vicinity of the laser-field nodes.