Laser cooling, friction, and diffusion in a three-level cascade system

A model is presented to calculate optical forces, friction, and diffusion for three-level atoms in a ladder configuration. This model is then applied to the 2³S₁-2³P₂-3³D₃ cascade in metastable helium. It is demonstrated that metastable helium atoms can be decelerated within a much shorter distance using cascade excitation by overlapping traveling waves. The equilibrium temperature representing the final width of the velocity distribution is calculated as a function of laser detuning and intensity. This equilibrium temperature is also calculated for the situation of overlapping standing waves (one-dimension), and is found to be well above the Doppler limit for two-level 2³S₁-2³P₂ excitation. Numerical results are compared with existing experimental data.