ABSTRACT
In the heat pulse technique, originally introduced by R. J. von Gutfeld and A. H. Nethercot, a short-duration electrical voltage or optical excitation is applied to a thin metallic film evaporated on one face of the crystal under investigation. The heat pulse sequence generates a non-equilibrium distribution of phonons in the sample. A thin-film bolometer is evaporated on the opposite face of the sample to detect the pulses. Optical techniques can offer useful advantages over other phonon spectroscopies. For example, in optical techniques phonons are generated or detected completely within the sample, so that no influence of surface or any outside agent is encountered. Additionally, optical techniques can generate a wide range of phonon frequencies including both optic and acoustic branches. Phonon echo studies are emerging as a powerful experimental technique for studying phase transitions and other phenomena in solids.
