Stimulated Phonon Emission

Inversion, through direct absorption of far-infrared laser light, of a three-level electronic system of V⁴⁺ ions in dilute solution in Al₂{O}₃$ leads to stimulated emission of 24.7-${cm}^{{-}1}$ phonons. Stimulated phonon emission manifests itself in the presence of ballistic flow of longitudinal phonons along the $c$ axis of ${Al}₂O₃, but this flow is not observed under spontaneous phonon emission.