The relation between crystallographic symmetry and the bidirectionality of SAW generation by IDT's

The paper discusses the effect of crystallographic symmetry on the directionality of an IDT with single symmetrically shaped electrode per period. Four families of substrate orientations have been found for which such an IDT launches SAW or leaky SAW to the right and left with equal amplitudes. The bidirectionality follows from the fact that the stop-bands of short and open-circuited infinite gratings have a common edge for those cuts. The coalescence of a pair of cut-off frequencies has been established through the analysis of the COM equations and a numerical algorithm conventionally used to compute cut-off frequencies and dispersion curves.