Observation of Quasidiffusive Phonon Propagation in Silicon
Photoexcitation of Si is expected to produce a broad range of phonon frequencies as a byproduct of electronic relaxation. These phonons should propagate through the crystal quasidiffusively.1 In contrast to these expectations, previous experiments on Si immersed in liquid helium observed phonon pulses with no long tail from scattered phonons and sharp phonon caustics.2 These observations imply that the detected phonons are relatively low frequency and propagate ballistically from a region very near the focused laser source. Our experiments attempt to isolate the factors responsible for this discrepancy between expectation and experiment. In particular, we find that the phonon pulses are very sensitive to the presence of liquid helium at the excitation surface and to the excitation level.