Abstract
The observed time traces for a heat pulse experiment on vitreous silica decay exponentially with a characteristic decay time of 30 µs in the power density range from 10-3 to 10-2 W/mm2, suggesting quasidiffusive phonon propagation. The experimental observation of quasidiffusion in Si requires a low excitation density in order to avoid the emergence of a localized source of low-frequency phonons at high density. The observed density is more than three orders of magnitude below the excitation density in weakly photoexcited Si.