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

Higher-order quasi-longitudinal (QL) Lamb waves have been reported for sensor application in liquid media at a higher thickness-to-wavelength (h/λ) ratio than the fundamental symmetric modes, with the main advantage of having higher operating frequency and reduction in the total size of the device without reducing the plate thickness. However, the trade-off in the reduction of the electromechanical coupling coefficient (K
²
) is a hinder in utilizing QL Lamb waves with higher-order numbers. Here we performed finite element simulation and verified experimentally the possibility of utilizing S
5
and S
6
QL Lamb wave modes in liquid media based on a two-port delay line device on a thick GaAs substrate. While the K
²
values are lower for QL modes at higher-order numbers, the signal amplitude of S
21
transmission parameter at the resonant frequency is still distinguishable from the noise floor. The result demonstrates the feasibility of higher-order modes at higher h/λ ratio for sensor application in liquid media, thus allowing further reduction in the wavelength and the total size of the sensor device.