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 the ground state to transition into the excited state, then what is called a population inversion is created. When light of the appropriate frequency passes through the inverted medium, the photons are either absorbed by the atoms that remain in the ground state or the photons stimulate the excited atoms to emit additional photons of the same frequency, phase, and direction. Since more atoms are in the excited state than in the ground state then an amplification of the input intensity results. The population inversion, in units of atoms per cubic meter, is where "g" and "g" are the degeneracies