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 We note that acoustic waves of 100 GHz to 1 THz have wavelengths in nanometre range. Sound amplification according to the experiment taken in the University of Nottingham could be based on an induced cascade of electrons in semiconductor superlattices. The energy levels of electrons are confined in the superlattice layers. As the electrons hop between gallium arsenide quantum wells in the superlattice they emit phonons. Then, one phonon going in, produces two phonons coming out of the superlattice. This process can be stimulated by other phonons and then give rise to an acoustic amplification. Upon the addition of electrons,