J. Phys. Colloques
Volume 42, Numéro C6, Décembre 1981International Conference on Phonon Physics
|Page(s)||C6-201 - C6-208|
J. Phys. Colloques 42 (1981) C6-201-C6-208
PHONON IMAGINGW. Eisenmenger
Physikalisches Institut der Universität Stuttgart, 7000 Stuttgart 80, Pfaffenwaldring 57, F.R.G.
Investigations of phonon propagation and scattering in solids use either coherent microwave phonons or incoherent phonons in the form of heat pulses1 generated by current flow through thin metallic films and bolometer detection, or monochromatic incoherent phonons generated and detected with superconducting tunneling junctions2. Applying these techniques to a perfect single crystal, quantitative measurements require knowledge on phonon propagation in anisotropic media. In contrast to optic properties the acoustic propagation in anisotropic media is much more complicated by the large number of elastic constants. An additional complication arises if the dispersion of acoustic phonons is included. Whereas the propagation of coherent phonons is simply described by the anisotropic constant-energy surfaces in q-space, the propagation of incoherent phonons (generated by a point source) is determined by the distribution of group velocities. These distributions were first calculated by Taylor, Maris and Elbaum3 with the result that specific propagation directions and modes show strong and sharply peaked intensity maxima. This phenomenon was called "phonon focussing". In later work also names as phonon channeling, phonon caustics etc. have been used. The pronounced sharpness of these distributions with strong intensity changes within angles of 1 degree raised strong experimental and theoretical interest for a more complete spacial description, or for imaging the phonon distributions. First, three-dimensional directivity patterns were obtained by computer analysis4. More recently, direct measurements of phonon intensity distributions5 and, for instance, measurements with computer-aided spacial display6 of the phonon intensity have led to impressive phonon images on crystal surfaces. Alternatively, it is also possible to obtain direct phonon images7, observing the thickness increase of a superfluid 4He-film on the crystal surface by the fountain effect in regions of high phonon intensity. In this lecture the fundamentals of phonon focussing and phonon imaging techniques will be reviewed.