Aspects of using acoustic plate modes of higher orders for acoustoelectronic sensors

The results from experimental studies of the sensor properties of acoustic modes in LiNbO3 plates with thicknesses h/λ from 0.7 to 5 (where h is thickness and λ is wavelength) are discussed. It is shown that the mode responses are not always equal when liquid is applied to the upper and lower surfaces of a plate. When h/λ ≤ 1, the higher harmonics of lower order modes n are localized near the surface by transducers and interfere with the fundamental harmonics of higher order modes, violating the response symmetry. When h/λ ≥ 1, no such interference is observed, since the higher order modes are localized in the central part of the plate while the lower order modes are localized near the surface with transducer, again violating the response symmetry. The development of sensors for gases and liquids based on higher order modes thus becomes possible only with thin plates at frequencies that exclude interference.