Lasers do this. The essential idea is that an incoming photon (of some specified energy E) knocks a bit of energy (specifically also E) out of an excited partic

Lasers do this. The essential idea is that an incoming photon (of some specified energy E) knocks a bit of energy (specifically also E) out of an excited particle which leaves as another photon. To make this happen, you need more particles in an excited state than in the ground state[1] which is exactly the same condition necessary for negative temperature. [1]: https://en.wikipedia.org/wiki/Population_inversion https://en.wikipedia.org/wiki/Population_inversion