Excitation of higher order modes in Lamb wave delay lines

The excitation of higher-order ultrasonic Lamb wave modes by an interdigital transducer deposited on a piezoelectric plate is investigated both theoretically and experimentally. It is found that depending on the value of the ratio h/λ (h=plate thickness, λ=acoustic wavelength), a number of higher-order modes can be efficiently excited. The phase velocity of higher-order modes is greater than the surface acoustic wave (SAW) velocity, while their group velocity is lower than that of the SAW. Thus, these modes have a higher resonant frequency and also a longer time delay than the surface acoustic wave. It is found that in acoustically thin plates (h/λ<1) the interdigital transducer (IDT) can efficiently couple to higher-order Lamb waves modes having resonant frequencies greater than ten times the fundamental SAW resonant frequency