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 emission takes place in the structure under electrical pumping. A bias voltage is applied to a weakly coupled n-doped GaAs/AlAs superlattice and increases the amplitude of the coherent hypersound oscillations generated by a femtosecond optical pulse. An evidence of hypersound amplification by stimulated emission of phonons emerges, in a system where the inversion of the electron populations for phonon-assisted transitions exists. This evidence is provided by the bias-induced amplitude increase and experimentally observer spectral narrowing of the superlattice phonon mode with a frequency of 441 GHz. The main target of this type of experiments is to highlight the realization probability