Sound amplification by stimulated emission of radiation was first experimentally demonstrated in the Gigahertz range in 2009. Announced in 2010, two independent groups came up with two different devices that produce coherent phonons at any frequency in the range megahertz to terahertz. One group from the University of Nottingham consisted of A.J. Kent and his colleagues R.P. Beardsley, A.V. Akimov, W. Maryam and M. Henini. The other group from the California Institute of Technology (Caltech) consisted of Ivan S. Grudinin, Hansuek Lee, O. Painter and Kerry J. Vahala from Caltech implemented a study on Phonon Laser Action in a tunable two-level system. The University of Nottingham device
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