Population inversion atoms in the ground and excited states are equal, the rate of stimulated emission is equal to the rate of absorption for a given radiation density. The critical detail of stimulated emission is that the induced photon has the same frequency and phase as the incident photon. In other words, the two photons are coherent. It is this property that allows optical amplification, and the production of a laser system. During the operation of a laser, all three light-matter interactions described above are taking place. Initially, atoms are energized from the ground state to the excited state by a process
Sound amplification by stimulated emission of radiation means that, since bosons with the same energy can occupy the same place in space, phonons and photons are force carrier particles and they have integer spins. There are more allowed states available for occupancy in a phonon field than in a photon field. Therefore, since the density of terminal states in the phonon field exceeds that in a photon field (by up to ~10), phonon emission is by far the more likely event. We could also imagine a concept where the excitation of an electron briefly leads to vibration of the lattice and thus to phonon generation. The vibration