Spectroscopy of high-frequency phonons

It has recently become possible to observe experimentally the spectral, spatial and temporal evolution of non-equilibrium phonon distributions in solids. The author reviews those techniques which are applicable to high-frequency (>or approximately=200 GHz) phonon propagation. Each technique is analysed for the physical principles involved, its characteristic parameters, and its range of applicability. The physical phenomena, to which the techniques have been applied, are discussed and the experimentally obtained results are compared to theoretical predictions whenever possible. It is shown that, although no universally applicable phonon spectrometer has so far been devised, those spectrometers presently in use are applicable to a broad range of phenomena inherent in phonon transport.