Generation of High-Order Modes in Resonators Based on Aperiodically Poled Piezoelectric Film Stacks

Aperiodically poled piezoelectric film (APPF) stacks have recently been proposed as an extension of periodically poled piezoelectric film (P3F) structures, a promising platform for the expansion of radio frequency (RF) filters into centimeter- and millimeter-wave (mm-wave) frequency bands. Variations in the thicknesses between the layers of the APPF stacks enable the generation of higher order modes typically absent in P3F structures and provide additional options for the design of high-frequency filters. In this study, a simple model is proposed to optimize the thickness ratio in a three-layered structure for the generation or suppression of any mode. Its validity was confirmed by the rigorously simulated admittance functions of the resonators based on optimal structures and comparison with experimental data. The coupling coefficients of modes A1-A11 laterally excited in the 128°YX LN-based three-layered stacks were calculated as functions of the continuously varying thickness ratios. These dependencies can be used for the selection of APPF stacks with simultaneous enhancement of the primary mode and suppression of the undesired modes. In the analyzed lithium niobate (LiNbO3, LN) plates of thickness 600 nm, the electromechanical coupling coefficients of the modes A5 and A7 generated at the frequencies 14.6 and 20.4 GHz increased from 2.4% to 17% and from 1.2% to 9.9%, respectively, when the inverted middle layer of the optimal thickness was introduced, whereas the nearest modes were suppressed. The appearance of spurious symmetric modes in the previously reported experimental P3F structures was explained, and the geometry of the stacks required to avoid these spurious modes is described.