Quasi longitudinal Lamb acoustic modes along ZnO/Si/ZnO structures

A novel structure consisting of a Si plate sandwiched between two ZnO layers is suggested as propagation medium for Quasi Longitudinal (QL) acoustic Lamb waves. Considering a low-dispersive quasi-longitudinal mode as an example, the phase velocity v, electromechanical coupling constant k2 and mechanical displacements U1, U3 versus plate and ZnO layers thicknesses have been calculated starting from uncoated plate through the same plate with one layer to the plate with two layers onto opposite faces. A remarkable increase in the electromechanical coupling, together with an essential decrease in the vertical displacement U3, at the ZnO surface, have been theoretically demonstrated and experimentally verified for definite combinations of the films/plate thicknesses. This property of the structure is attractive for applications to microwave liquid sensors.