Raman cooling

-Works for three-level atoms
Uses stimulated Raman transitions.
Level scheme:

Two laser beams with different frequencies $\omega_{1}, \omega_{2}$ from opposite directions
$\Rightarrow$ absorption from one beam, stimulated emission into the other
$\Rightarrow$ total momentum transfer= $\hbar (k_{1}+k_{2})$
Very small linewidth of Raman transitions
$\Rightarrow$ only talk to narrow velocity class of atoms around v₀
$\Rightarrow$ control v₀ by detuning $\omega_{1}-\omega_{2}$
Cooling scheme:

1.

Sequence of pulses with different negative detunings $\omega_{1}-\omega_{2}$ from left, giving atoms with v<0 kick to the right.

2.

optically pump atoms back with pulse of light at real transition. Spontaneous emission randomizes velocity distribution

3.

repeat sequence 1) with positive detunings from the right, giving atoms with v>0 kick to the left

4.

optically pump atoms back

By using different pulse sequences (detuning, power, and pulse shapes) one can tailor a velocity-dependent excitation probability for atoms around v=0. Same situation achieved as in VSCPT:
Excitation probability P(v) with dip at v=0 and randomizing diffusion process that allows atoms to fall into the (v=0)-trap. Note: Trap is not a real trap (only v-dependence, no dependence on x)