Relaxation of an optically created phonon void in dilute ruby - art. no. 174301

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) over bar(E -2) 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 heli um bath. Coupled rate equations of the level populations in the void as wel l as the phonon density throughout the crystal account for these effects. I n particular, a quartic frequency dependence of the rate for phonon transpo rt across the crystal boundary is found, in accordance with diffuse surface scattering.