Relaxation of an optically created phonon void in dilute ruby
P. A. van Walree, A. F. M. Arts, and H. W. de Wijn
Faculty of Physics and Astronomy, and Debye Institute, Utrecht
University, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
A local frequency-selective depletion of phonons is created in dilute ruby
at 1.8 K by optical pumping into the Zeeman-split metastable
E(2E) doublet. This phonon "void," which is in
dynamic balance with the spin system, is observed to recover to thermal
equilibrium by an inward flow of phonons supplied from regions beyond the
void, and ultimately from the helium bath. Coupled rate equations of the
level populations in the void as well the phonon density throughout the
crystal account for these effects. In particular, a quartic frequency
dependence of the rate for phonon transport across the crystal boundary is
found, in accordance with diffuse surface scattering.