So I'm reading in Ashcroft and Mermin about indirect optical transitions:
So, a photon comes in, and it only excites the electron across the indirect band gap if a phonon with the appropriate wave vector can "help" the electron by giving it momentum (they point out that the momentum of the photon is negligible). First of all, does this mean that, if the photon comes in and no phonon happens to help it, it's not absorbed, but most likely reflected? Or would something else almost certainly absorb it?
Secondly, when a phonon is there to "help" the electron, does this mean the crystal is now cooler as a result, because there are fewer/less powerful lattice vibrations (because the phonon gave up some momentum)?
The crystal has certainly gained energy from the photon, but the energy is now just in the form of... the electron having a higher energy state? (Is that actually where the energy has "gone"? Do we just say that the electron is in a higher energy state, and that's the extent of it?) As opposed to kinetic energy of the crystal, which seems like it has decreased.
If this is possible (I'm ready to be told that it's not), could it be scaled up and carefully done to cool certain semiconductors?