Lamb wave propagation in 128° Y-X lithium niobate plates

This paper is concerned with the propagation of ultrasonic Lamb waves along the X-direction of a 128° rotated Y-cut lithium niobate plate. In this material, it is found that one of the modes, the first higher-order antisymmetric mode (called the A₁ mode), shows an unusual and interesting behavior. The velocity of this mode remains nearly constant for all values of h/λ, where h is the plate thickness and X is the acoustic wavelength. The electromechanical coupling coefficient, K² is negligible for h/λ greater than 1, but increases as h/λ decreases, reaching a value of 0.78% as h/λ tends to zero. Thus the IDT can couple to this mode if the plate is acoustically thin (h/λ<1). Another interesting property of this mode is that for small values of h/λ, its particle displacement is predominantly shear-horizontal (SH) in nature. Since the component of particle displacement normal to the free surface is very small, this quasi-SH wave will not suffer excessive attenuation even when the propagation surface is in contact with a liquid medium. This wave can therefore be used for liquid-phase sensing. The velocity and coupling coefficient of the mode have been measured for various values of h/λ and are found to be in fair agreement with theoretical calculations