Phonon spectroscopy. I. Spectral distribution of a phonon pulse

Abstract

A vibronic-sideband spectrometer has been developed capable of resolving the spectral, spatial, and temporal evolution of a phonon distribution propagating in a solid. As a first application of the spectrometer, a determination has been achieved of the spectral distribution of a phonon pulse which has been generated by Joule heating in a metal film and which has crossed an interface between the film and a crystalline solid. The spectral distribution observed at low electrical input power agrees with existing theory, but the agreement becomes progressively worse as the input power exceeds 12 W/${\mathrm{mm}}^2$. In order to account for the experimental results an empirical frequency-dependent filtering action is defined, which hinders the transport into the crystal of phonons with frequencies above 1 THz. It is shown that a more complete treatment of the phonon transport would have to assay the effects in the heater film of umklapp processes, of the breakdown of the deformation-potential model of electron-phonon coupling, of phonon decay, and the effect of the frequency dependence of interface mismatch. The phonon spectrometer has a wider applicability than demonstrated in this paper and in the paper immediately following. The main characteristics, and limitations, of the spectrometer are discussed in some detail.