This project explored the use of multiple laser wavelengths and multiple atomic transitions to laser cool and trap atoms. Traditional laser cooling relies largely on mechanical forces due to light scattering from a single color laser. Multiple wavelengths and transitions provide interesting extensions to usual "single-photon" cooling: access to substantially different effective photon momenta, access to different atomic line widths and saturation intensities, the possibility of coherence and EIT effects, and the possibility to easily separate atom fluorescence from laser excitation. This approach relies on mechanical forces arising from excited state to excited state transitions. For example, replacing the single laser excitation shown in a) by the three-laser excitation in b) would result in larger light scattering forces, and the fluorescence would be at a very different wavelength from the excitation lasers, which could be easily filtered away.
Our research project explored these ideas in magneto-optically trapped Cesium. Single photon cooling is normally performed on the 852 nm cycling transition, shown in red. Three photon cooling would operate through the three different transitions (shown as orange, blue and green in b), with wavelengths at 894 nm, 795 nm and 761 nm, respectively. (There are other possible wavelength choices, but these have convenient diode laser wavelengths.) A longer term goal (assuming early successes) would be to make a single atom surface MOT by coating the surface to reflect 894 nm, 794 nm, and 761 nm, but to transmit the fluorescing 852 nm. More speculatively, there may be interesting four-wave mixing applications, and it may be possible to demonstrate this scheme using Rydberg transitions, where the interactions between atoms are strong and long ranged.