[atmdw]

I posted the following on Physics Forum and was unable to get a suitable answer so I would like try it here.

I am interested in learning the nature of the interaction between a incident photon and the atoms in a medium that has undergone population inversion that causes stimulated emission. Wikipedia states it as well as the pubs I have read; "When light of the appropriate frequency passes through the inverted medium, the photons stimulate the excited atoms to emit additional photons of the same frequency, phase, and direction, resulting in an amplification of the input intensity". My question is how does an incident photon (some or all of which presumably are not absorbed) induce additional photon emission from excited state atoms. I have read that there is some type of momentum transfer but I don't understand how that occurs without a change in energy (and frequency and phase) in the incident photon. A few good articles (but surprisingly few) are available on coherence but the conservation of momentum argument is confusing to me. I'm presuming that Einsteins thought experiment didn't involve a photon drive by.

Thanks
An Organic Chemist

[Juan R.]

I posted the following on Physics Forum and was unable to get a suitable answer so I would like try it here.

I am interested in learning the nature of the interaction between a incident photon and the atoms in a medium that has undergone population inversion that causes stimulated emission. Wikipedia states it as well as the pubs I have read; "When light of the appropriate frequency passes through the inverted medium, the photons stimulate the excited atoms to emit additional photons of the same frequency, phase, and direction, resulting in an amplification of the input intensity". My question is how does an incident photon (some or all of which presumably are not absorbed) induce additional photon emission from excited state atoms. I have read that there is some type of momentum transfer but I don't understand how that occurs without a change in energy (and frequency and phase) in the incident photon. A few good articles (but surprisingly few) are available on coherence but the conservation of momentum argument is confusing to me. I'm presuming that Einsteins thought experiment didn't involve a photon drive by.

Thanks
An Organic Chemist

You do not explain us neither what you consider "a suitable answer" nor what physicists already said you, but the process is the following

γ + A*   2γ + A

Evidently, this reaction must conserve energy and since that for photons E(γ)=pc, this implies

pc + E(A*) = 2pc + E(A)

that is

(E(A*) - E(A)) / c = p

One could say that there was a transfer of momentum p from the excited atom, but I prefer to think in an overall transfer of energy (pc) from it

E(A*) - E(A) = pc