Efficient Lateral Excitation of Higher-Order Harmonics in Lithium Niobate Resonators Using Periodically Poled Piezoelectric Films (P3F)

The use of Periodically Poled Piezoelectric Film (P3F) structures as a new platform for next-generation radio-frequency acoustic filters can extend their frequencies into centimeter-wave bands. In this study, acoustic modes laterally excited in P3F structures were numerically investigated. Specifically, the efficiency of the harmonic excitation and spurious modes that deteriorate the resonator characteristics were examined in P3F structures with different numbers of layers, various crystal orientations, and thickness variations between the layers. The results revealed that the electromechanical coupling of the fundamental and higher-order harmonics was most sensitive to thickness variations. Moreover, higher-order harmonics that are not excited in P3F structures can be efficiently generated in Aperiodically Polarized Piezoelectric Film (APPF) stacks. An example of an APPF structure optimized for the generation of the A5 mode is described. This mode shows a high electromechanical coupling of 17.5%, which decreases as the thickness ratio deviates from the optimal value.