New approach to detection of guided waves with negative group velocity: Modeling and experiment

Abstract

A new method for detecting the forward and backward acoustic waves in the piezoelectric plates by using a system of the acoustically isolated interdigital transducers (IDT) with the different spatial period is developed. This method was tested on the piezoelectric acoustic modes A1 and SH1 propagating in Y-X LiNbO3 plate. Theoretical analysis has shown that the SH1 mode in the entire frequency range represented a forward wave. At that the dispersion dependence of the A1 mode near the cutoff frequency has a smooth transition from a forward wave to a backward one with decreasing frequency. In order to experimentally observe this transition 19 IDTs with different periods were deposited on a single lithium niobate plate. The measurement of the frequency dependence of the real part of the electrical impedance of these IDTs showed that for SH1 mode the resonance frequency decreased monotonically with a growth in the spatial period. This behavior corresponded to a forward wave. For A1 mode the resonance frequency initially decreased with the growth of the IDT period and then began to increase after reaching a value of ZGV frequency. This behavior is explained by the smooth transition from the region of the forward wave to the region of the backward wave. The calculation of the frequency dependences of the real part of the electrical impedance of each transducer for the waves considered, carried out by the finite element method, turned out to be in good agreement with the experiment.