Lamb waves sensor in liquid media utilizing higher-order quasi-longitudinal S₅ and S₆ modes

Higher-order Lamb waves with quasi-zero surface displacement components on a GaAs piezoelectric plate

“…Despite this limitation, the reduction in IL when the surface is in contact with liquid can be qualitatively calculated in QZ-PAW with 3.7 h λ −1 . Furthermore, at the h/λ ratio of 3.7, the GaAs device will also excite the S 6 mode with dominantly longitudinal displacement at the surface (QL-S 6 mode) with the v ph close to LBAW v ph [20]. The detail on the QL-6 mode is described in supplementary material 4 (S4).…”

“…A general limitation of higher-order modes is the reduction of the electromechanical coupling coefficient (K 2 ) when a thicker plate or a higher-order number is used [20]. Furthermore, since the conversion of electrical to acoustic energy occurs at the surface where the IDT is located, it is expected that the QZ-PAW will have an even smaller K 2 .…”

“…Furthermore, since the conversion of electrical to acoustic energy occurs at the surface where the IDT is located, it is expected that the QZ-PAW will have an even smaller K 2 . The deposition of the metal layer at the opposite surface of the IDT changes the electrical boundary condition and can potentially increase the value of K 2 [20,21]. Figures 5(a) and (b) show the K 2 of A 1 , S 1 , A 2 and S 2 modes with Plate-IDT (S-IDT) and Metal-Plate-IDT (M-P-IDT) coupling configuration, respectively, vs the h/λ ratio at the range where QZ-PAWs exist.…”

“…For example, as shown in table 2, when the h/λ ratio is reduced to 3.7, the difference between A 2 and S 2 modes is increased from 170 to 319 kHz to separate the resonance peaks, with a trade-off in the increase of with u surf /u max and IL of A 1 mode to 14% and 1.79 dB. Another advantage of using the h/λ ratio of 3.7 is to obtain the QL S 6 mode with a dominantly longitudinal displacement [20], to compare the IL between QZ-PAW and QL-LW, which will be demonstrated in the experimental results.…”