Excitation of higher-order ultrasonic Lamb wave modes in piezoelectric plates

The excitation of higher-order ultrasonic Lamb wave modes by an interdigital transducer (IDT) deposited on a piezoelectric plate is investigated both theoretically as well as experimentally. Green’s function method of analysis is used to calculate the coupling of the IDT to the various modes. This is then used to calculate the performance of a Lamb wave delay line. Experimental results obtained on delay lines fabricated on 128° rotated Y-cut, X-propagating lithium niobate plates are compared with theoretical calculations. The resonant frequencies and time delays of the modes are found to be in fair agreement with calculated values. It is shown that delay lines fabricated on acoustically thin plates (h/λ<1, where h=plate thickness, and λ=acoustic wavelength) can efficiently excite Lamb waves at frequencies greater than 10 times the resonant frequency of the lowest-order mode.