Theory of sub-Doppler cooling of three-level Λ atoms in standing light waves

A general theory of cooling of three-level Λ atoms in two standing light waves between which there is a relative spatial phase shift is presented for arbitrary ratios of the intensity and detuning. It is shown that in the case of equal detuning of the light waves, deep (sub-Doppler) cooling of three-level atoms occurs for any values of the spatial phase shift. For zero spatial phase shift, the atoms are strongly cooled due to coherent population trapping in the given atom-field interaction scheme. On the other hand, for the case of different frequency detunings, sub-Doppler cooling of Λ atoms is possible only with a nonzero relative phase shift; it is shown that this is associated with the so-called open-quotes Sisyphusclose quotes cooling mechanism. The authors underscore that in their scheme neither a polarization gradient of the exciting waves nor a magnetic field is required to achieve this type of cooling pattern; two standing waves acting on different transitions of a Λ atom are sufficient. 13 refs., 10 figs