Propagation of optically generated acoustic phonons in Si

Abstract

This paper deals with the spectral and spatial distributions of nonequilibrium acoustic phonons produced by optical excitation of a high-purity Si crystal at low temperatures (T2 K). We report the observation of a quasidiffusion process in Si, resulting from anharmonic-decay and elastic-scattering processes. We show that the observation of quasidiffusive propagation was previously masked by the losses of high-frequency phonons into the helium bath. By removing the contact of the helium bath from the excitation surface, we have quantified the effects of the bath on the detected heat pulses. As the power density of the optical pulses is increased, qualitative changes occur in the shapes of the heat pulses, and at high density, the quasidiffusive propagation is bypassed by the emergence of a localized source of low-frequency phonons. The threshold density for the formation of the localized source is far below that calculated for a ``hot spot'' based on phonon-phonon interactions. We postulate that the photoexcited carriers, largely in the form of electron-hole droplets, are playing a dominant role in determining the frequency distribution of emitted phonons. Our experiments employ a wide variety of techniques to characterize the propagation of nonequilibrium phonons in silicon: Phonon imaging is used to gauge the size and lifetime of the phonon sources, as well as indicate the frequency distribution of the detected phonons. Comparison is made between direct photoexcitation of silicon and optical excitation of a metal film deposited on the silicon surface. These experiments and Monte Carlo simulations give insight into the diffusion of high-frequency phonons near the interface. The occurrence of a helium bubble at a point of high excitation is shown to have a marked influence on the detected heat pulses.