Optical molasses at high intensity (forces in OL's)

[carlosbgois]

Hey there.

My goal is to understand the origin of the main forces that keep together an optical lattice created by two identical counter-propagating laser beams.

I quote a small excerpt about the forces in optical molasses with high intensity, which I'm having trouble in understanding:

The force that actually slows the atoms derives from the light shift which is the reversible exchange of momentum between the atoms and the light field via absorption followed by stimulated emission.

[...]

However, in the absence of the spontaneous emission that causes the velocity dependent damping force, atoms that move through the light field experience no average force because these stimulates processes of momentum exchange between atom and field can occur in either direction with equal likelihood.

As I understand, the first paragraph is talking about the dipole force, and the second is about the scattering force, which is velocity dependent.

The problem is that they seem to contradict each other, by saying that "the force that actually slows the atoms is the dipole force," and later that "in the absence of the scattering force the atoms moving through the light field experience no average force."

Can someone give me a hint on how to interpret this correctly?

 

[UltrafastPED]

The key phrase is "stimulated emission", which is also of the utmost importance for lasers to operate.

An atom may absorb a photon coming in from any direction, it will also emit the photon in any direction: essentially random directions.

But stimulated emission occurs when a photon passing "nearby" an already excited atom stimulates the emission of a photon; this is no longer a random process! The stimulated photon will have the same energy and momentum as the passing photon.

Now when you analyze the description above you will see that the atoms must be chosen so that there are available excited states which can be reached with the laser being used.