Single-Photon Atomic Cooling - Raizen Lab - The University of ...

The significance of Isat is that at saturation the Rabi frequency has a value
comparable to the natural decay rate Γ and that ρ22 reaches 1/2 of its max-
imum value. With the substitution I = (1/2)cǫ0E 2 0, the saturation intensity
can be written
Isat = ǫ0cΓ 2 2
4|ê · µ| 2
(2.74)
where ê is the direction of the polarization of the electric field. The value of the
saturation intensity depends on the transition strength through Γ as well as the
relative orientation of the polarization of light and transition dipole moment,
but for simplicity the minimum value is usually reported as a representative
figure. For the D2 (F = 2 → F ′ = 3) transition Isat = 3.577 13(74)mW/cm 2
for light with isotropic polarization [43].
A consequence of saturation is that the measured width of a transi-
tion increases with increasing probe intensity due to a phenomena known as
saturation broadening. The measured full width at half maximum (FWHM)
increases with intensity as [70]
2.5.2.2 Optical Molasses
∆ωFWHM = Γ 1 + I
1/2 . (2.75)
Isat
Optical Molasses is a laser cooling technique commonly used to reduce
the temperature of an atomic vapor. Because atoms in a gas move in all direc-
tions the applied laser beams must cool in all three dimensions by interacting
with the gas along three orthogonal dimensions as shown in Fig. 2.7. In this
55