FORMATION OF SURFACE PHONON FOCUSING CAUSTICS IN CRYSTALS

The effect of elastic anisotropy on surface phonon focusing in the (1 1 1) surface of cubic crystals is studied theoretically and experiment ally. Calculations of the focusing factor for BaF2, NaCl, CaF2 and KCl are compared with anisotropic powder patterns created by laser-genera ted surface acoustic waves. Formation of a pair of caustics in <11(2) over bar> directions with increasing anisotropy is observed. Condition s for caustics to appear on a (1 1 1) cubic crystal surface are expres sed in terms of the elastic constant ratios.