Coherent phonon avalanches

Avalanches of resonant acoustic phonons are observed following population i nversion of the Zeeman-split (E) over bar(E-2) doublet in dilute ruby by se lective optical pumping. In confined geometries only a single stage of stim ulated emission is seen, whereas longer crystals form an acoustic cavity al lowing multiple passage of the phonon pulse through the active zone. Cohere nt equations of motion for the lattice displacement and the spin Bloch vect or excellently account for these results. (C) 1999 Elsevier Science B.V. Al l rights reserved.