Coupling-controlled low-dispersion quasi-longitudinal wave and its application in highly robust acoustic resonators

Abstract

Acoustic waves in thin films always suffer from high dispersion, leading to significant sensitivity to film thickness and corresponding device instability in practical applications. The quasi-longitudinal (QL) wave, conventionally classified as a symmetric Lamb wave, is generally considered a low-dispersion plate wave. However, the mechanism responsible for its low dispersion remains underexplored, limiting the excitation and control of QL waves in complex structures beyond simple plates. Here, we show that the QL wave inherits the non-dispersive nature of the longitudinal bulk wave, whereas its dispersion is introduced by coupling with highly dispersive Lamb waves. The coupling strength is governed by their displacement similarity such that reducing their similarity suppresses the coupling-induced dispersion. Based on this mechanism, a surface-wave-like QL wave is excited in an Al/LiNbO3/SiC heterostructure. The similarity between the QL wave and Lamb waves is reduced from 0.57 to 0.16, resulting in extremely weak coupling and a nearly non-dispersive QL wave with a dispersion curve slope of only −2.6 m/s. Acoustic resonators based on this structure show less than 0.8% variation in resonant frequency corresponding to the QL wave when the LiNbO3 thickness varies from 700 to 1100 nm. Our work not only clarifies the fundamental physics of the QL wave and then isolates a nearly non-dispersive QL wave from Lamb waves but also reports a highly robust acoustic resonator that overcomes the notorious sensitivity of resonant frequency to the piezoelectric film thickness.