Sub-Doppler cooling

Sub-Doppler Cooling is a mechanism that can be used to cool a substance to a temperature lower than the Doppler cooling limit. When atoms or molecules are cooled to reach the Doppler cooling limit, the natural decay induced repulsion is balanced with the cooling and trapping force induced by doppler cooling. The substance would meet the case that temperature can't be cooled to lower temperature. There are methods to break the doppler cooling limit and reach the sub-doppler cooling.

The methods to get sub-doppler cooling includes Dark Magneto-Optical Trap, Optical molasses, and Sisyphus cooling.

Dark Magneto-Optical Trap

A magneto-optical trap (MOT) is commonly used for cooling and trapping a substance by doppler cooling. In the process of doppler cooling, the red detuning light would be absorbed by atoms from one certain direction and reemitted at a random direction. The electron of the atoms would decay to an alternative ground states if the atoms have more than one hyperfine ground levels. There is the case of all the atoms in the other ground states rather than the ground states of doppler cooling, then system can't cool the atoms anymore. In order to solve this problem, the other repumping light would be incident into the system to repopulate the atoms to restart doppler cooling process. This would induced higher fluorescence reemitted from atoms and the fluorescence can be absorbed by atoms which would act as a repulsion force on atoms. Due to this problem, the doppler limit would increase and easy to meet. When there is a dark spot or lines on the shape of the repumping light, the atoms in the mid of the atomic gas would not be excited by the repumping light which can decrease the repulsion force from the previous cases.

This can help to cool the atoms to a lower temperature than the normal doppler cooling limit. This is called dark magneto-optical trap (DMOT).^[1]

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