Characteristics of ultrasonic Lamb waves in 128/spl deg/ rotated Y-cut lithium niobate

The characteristics of ultrasonic Lamb waves propagating along the X-direction of a 128/spl deg/ rotated Y-cut lithium niobate plate are investigated. The first higher-order antisymmetric mode, the A/sub 1/ mode, is found to exhibit an anomalous behavior. The velocity of this mode remains nearly constant for all values of h//spl lambda/, where h is the plate thickness and /spl 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//spl lambda/ tends to zero. The electromechanical coupling coefficient of the wave has a value of k/sup 2/=0.78/spl times/10/sup /spl minus/2/ at h//spl lambda//spl cong/0. The coupling decreases as h//spl lambda/ increases, becoming negligible for h//spl lambda/>1. The velocity and coupling coefficient of the mode have been measured for various values of h//spl lambda/, and are found to be in fair agreement with theoretical calculations.<>