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
Stimulated emission an atomic or molecular resonance, the spectral line shape function is described as a Lorentzian distribution where formula_12 is the full width at half maximum or FWHM bandwidth. The peak value of the Lorentzian line shape occurs at the line center, formula_13. A line shape function can be normalized so that its value at formula_9 is unity; in the case of a Lorentzian we obtain Thus stimulated emission at frequencies away from formula_9 is reduced by this factor. In practice there may also be broadening of the line shape due to inhomogeneous broadening, most notably due to the Doppler effect