Stimulated Phonon Emission

Inversion, through direct absorption of far-infrared laser light, of a three-level electronic system of ${\mathrm{V}}^{4+}$ ions in dilute solution in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ leads to stimulated emission of 24.7-${\mathrm{cm}}^{-1\mathrm{}}$ phonons. Stimulated phonon emission manifests itself in the presence of ballistic flow of longitudinal phonons along the $c$ axis of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$, but this flow is not observed under spontaneous phonon emission.