Excitation of ultrasonic Lamb waves in piezoelectric plates

The excitation of ultrasonic Lamb waves by an interdigital transducer (IDT) deposited on a piezoelectric plate is analyzed using the Green’s-function method. The amplitudes of the generated Lamb waves are obtained in terms of the charge density on transducer electrodes. An electrostatic analysis that neglects piezoelectric coupling is used to relate this charge density to the voltage applied across the IDT. This is then used to calculate the radiation conductance of the transducer. Experimentally measured characteristics of interdigital transducers deposited on 128° Y-X and Y-Z lithium niobate plates are found to be in fair agreement with theoretical calculations. The analysis can also be used to calculate the electromechanical coupling between the IDT and various Lamb-wave modes. It is found that when the plate is more than a few acoustic wavelengths thick, the lowest-order symmetric and antisymmetric Lamb modes are almost equally generated. The slight difference in the velocities of these modes gives rise to the beating effect, whereby energy launched by the IDT on one surface transfers periodically back and forth between the two surfaces as a sinusoidal function of the distance traveled. A 65-MHz Lamb-wave delay line fabricated on a 0.25-mm-thick Y-Z lithium niobate plate shows the beating effect, with a beat wavelength of 14.7 mm, in good agreement with theoretical calculations.