CHARACTERISTICS OF ULTRASONIC LAMB WAVES IN 128-DEGREES ROTATED Y-CUT LITHIUM-NIOBATE

The characteristics of ultrasonic Lamb waves propagating along the X-direction of a 128-degrees rotated Y-cut lithium niobate plate are investigated. The first higher-order antisymmetric mode, the A1 mode, is found to exhibit an anomalous behavior. The velocity of this mode remains nearly constant for all values of h/lambda, where h is the plate thickness and lambda is the acoustic wavelength. The particle displacement of the mode tends towards that of a pure shear horizontal (SH) wave as the ratio h/lambda tends to zero. The electromechanical coupling coefficient of the wave has a value of k2 = 0.78 x 10(-2) at h/lambda congruent-to 0. The coupling decreases as h/lambda increases, becoming negligible for h/lambda > 1. The velocity and coupling coefficient of the mode have been measured for various values of h/A, and are found to be in fair agreement with theoretical calculations.