  Higher-order quasi-longitudinal (QL) Lamb waves have been reported for sensor
application in liquid media at a higher thickness-to-wavelength (h/l) 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 (K2) 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 S5 and S6 QL Lamb wave
modes in liquid media based on a two-port delay line device on a thick GaAs
substrate. While the K2 values are lower for QL modes at higher-order numbers,
the signal amplitude of S21 transmission parameter at the resonant frequency is
still distinguishable from the noise floor. The result demonstrates the
feasibility of higher-order modes at a higher h/l ratio for sensor application
in liquid media, thus allowing further reduction in the wavelength and the
total size of the sensor device.
