Stimulated emission energy of an absorbed photon causes an identical but opposite atomic transition: from the lower level to a higher energy level. In normal media at thermal equilibrium, absorption exceeds stimulated emission because there are more electrons in the lower energy states than in the higher energy states. However, when a population inversion is present, the rate of stimulated emission exceeds that of absorption, and a net optical amplification can be achieved. Such a gain medium, along with an optical resonator, is at the heart of a laser or maser. Lacking a feedback mechanism, laser amplifiers and superluminescent sources also function
Sound amplification by stimulated emission of radiation energy associated with the vibrational modes of the lattice. Such levels can be formed by conduction and valence bands in narrow gap indirect semiconductors. The energy gap in a semiconductor under the influence of pressure or magnetic field slightly varies and thus does not deserve any consideration. On the other hand, in narrow-gap semiconductors this variation of energy is considerable and therefore external pressure or magnetic field may serve the purpose of tuning into the resonance of one-phonon interband transition. Note that interband transition is the transition between the conduction and valence band. This scheme considers of indirect semiconductors instead