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 difference in energies between the two states as a photon. The photon will have frequency "ν" and energy "hν", given by: where "h" is Planck's constant. Alternatively, if the excited-state atom is perturbed by an electric field of frequency "ν", it may emit an additional photon of the same frequency and in phase, thus augmenting the external field, leaving the atom in the lower energy state. This process is known as stimulated emission. In a group of such atoms, if the number of atoms in the excited state is given by "N", the rate at which stimulated emission occurs is