We demonstrate two-dimensional robust Raman cooling in a four-level tripod system, in which velocity selective population transfer is achieved by a stimulated Raman adiabatic passage. In contrast to basic two-dimensional Raman cooling with square-envelope pulses [Phys. Rev. A 83, 023407 (2011)], the technique presented here allows for a wide variation in the pulse duration and amplitude once the adiabaticity criterion is satisfied. An efficient population transfer together with attainment of a narrow spread of the resonant-velocity group leads to the narrowing of the velocity-distribution spread down to
0.1vrec, corresponding to an effective temperature equal to
0.01Trec. This robust method opens additional possibilities for cooling of neutral atoms.
It introduced several methods to transfer population,including two-level system and three-level system, and gave some comparisons on transfer efficiency and transfer velocity of them in theory. It is shown that population transfer by coherent pulse sequence is better than incoherence one. Numerical simulation results showed that when using Stimulated Raman Adiabatic Rapid Passage (STIRAP), in which the pump laser pulse fully overlaps the Stokes pulse with the Stokes preceding the pump, population can be transferred completely and rapidly. Robust control of population transfer among Zeeman sublevels in the ground states of three-level system is investigated using adiabatic rapid passage in theory. The laser pulse widths are supposed to be short compared to the lifetime of the excited level, which means adiabatic. We demonstrated that techniques of STIRAP can be found to optimize population transfer in the ground states of three-level Λ system.