Stimulated emission of phonons in an acoustical cavity
P. A. Fokker, J. I. Dijkhuis, and H. W. de Wijn
Faculty of Physics and Astronomy, and Debye Research Institute, Utrecht
University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
Stimulated emission is studied of phonons resonant with the metastable
Zeeman-split E(2E) doublet in ruby. Initial
population inversion of this two-level system is prepared by selective
pulsed optical pumping, and the ensuing stimulated emission of resonant
phonons is observed via an accelerated growth of the luminescence
intensity emanating from the lower level. Under favorable conditions, a
periodic recurrence of the process of stimulated emission is observed,
which points to a phonon cavity formed by the end faces of the crystal in
combination with a limited interaction length of the resonant phonons. To
analyze these phenomena in detail, use is made of rate equations for the
phonon occupation and the level populations with explicit account of their
position dependences. With plausible assumptions an analytic solution of
these equations is achieved, which excellently fits the data. The effect
of lattice anharmonicity is shown to depend on the phonon energy in
agreement with the theoretical prediction.