Bichromatic laser cooling in a three-level system

We report a 1D study of optical forces on atoms in a two-frequency laser field. The light couples two ground state hyperfine structure levels to a common excited state of ⁸⁵Rb, thus forming a {Λ} system. We observe a new type of sub-Doppler coupling with blue-tuned light that uses neither polarization gradients nor magnetic fields, efficient heating with red tuning, and the spatial phase dependence of these. We observed deflection from a rectified dipole force and determined its velocity dependence and capture range. We report velocity selective resonances associated with Raman transitions. A simplified semiclassical calculation agrees qualitatively with our measurements.