Spectral-Spatial Diffusion of Resonantly Trapped Phonons

The observation of spectral and spatial diffusion of phonons in an ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ crystal containing both inelastic and resonance-elastic scattering centers (two different species of impurity ions) is reported. Phonons were generated by pulsed far-infrared excitation at a fixed frequency and monitored by time-resolved detection at a different frequency. Analysis of the data leads to the determination of the basic parameters of spectral and spatial diffusion.