Sub-Doppler cooling

In atomic, molecular, and optical physics, sub-Doppler cooling is a class of laser cooling techniques that reduce the temperature of atoms and molecules below the Doppler cooling limit. In experiment implementation, Doppler cooling is limited by the broad natural linewidth of the lasers used in cooling. Regardless of the transition used, however, Doppler cooling processes have an intrinsic cooling limit that is characterized by the momentum recoil from the emission of a photon from the particle. This is called the recoil temperature and is usually far below the linewidth-based limit mentioned above. By laser cooling methods beyond the two-level approximations of atoms, temperature below this limit can be achieved.

Optical pumping between the sublevels that make up an atomic state introduces a new mechanism for achieving ultra-low temperatures. The essential feature of sub-Doppler cooling is the non-adiabaticity of the moving atoms to the light field. For a spatially dependent light field, the orientation of moving atoms is adjusted by optical pumping to fit the conditions of the light field. Yet the moving atoms do not instantly adjust to the light field as they move, their orientation always lags behind the orientation that would exist for stationary atoms, which determines the velocity-dependent differential absorption and hence the cooling. With this cooling process, lower temperatures can be obtained.