REFLECTION OF ELECTROACOUSTIC WAVES IN PIEZOELECTRIC SOLIDS - MODE CONVERSION INTO 4 BULK WAVES

In the analysis of the reflection of plane electroacoustic waves at the boundary of a piezoelectric half-space, four partial waves are required in the reflection field in order that the mechanical and the electrical boundary conditions can be satisfied. It is usually assumed that at least one of these waves is evanescent. The purpose of this paper is to point out that in strongly piezoelectric solids all four reflected waves may, for certain reflection geometries, be homogeneous bulk waves. This will be the case when the longitudinal sheet of the acoustic slowness surface possesses negatively curved regions, thus permitting a re-entrant longitudinal mode. Calculations are reported on Rochelle salt and Ba2NaNb5O15, which show that for an incident longitudinal wave the reflection coefficient for the fourth bulk wave may be as large as unity, depending on the angle of incidence.