Superfluorescence and amplified spontaneous emission of 29-${\mathrm{cm}}^{\mathrm{-}1}$ phonons are observed following inversion of the E¯${(}^2$E)-2A¯${(}^2$E) acoustic transition of ${\mathrm{Cr}}^{3+}$ in ruby. The conditions for both of these manifestations of phonon avalanches are selected via the ${\mathrm{Cr}}^{3+}$ concentration. Superfluorescence is described in terms of a pendulum equation of the acoustic Bloch vector. Rate equations of the level and phonon populations account for amplified spontaneous emission.

• Received 24 July 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2015