Sound amplification by stimulated emission of radiation pumping energy into the system we can have a stimulated emission of radiation—which means that the pump forces the system to release a big amount of energy at a specific time. A fundamental characteristic of lasing, like the population inversion, is not actually possible in a two-level system and therefore a two level-laser is not possible. In a two-level atom the pump is, in a way, the laser itself. The amplification of coherent terahertz sound in a Wannier-Stark ladder superlattice has been achieved in 2009 according to a paper publication from the School of Physics and Astronomy in the University
Stimulated emission given by where the proportionality constant "B" is known as the "Einstein B coefficient" for that particular transition, and "ρ"("ν") is the radiation density of the incident field at frequency "ν". The rate of emission is thus proportional to the number of atoms in the excited state "N", and to the density of incident photons. At the same time, there will be a process of atomic absorption which "removes" energy from the field while raising electrons from the lower state to the upper state. Its rate is given by an essentially identical equation, The rate of absorption is thus proportional