L-band Lithium Niobate-On-Insulator SH0 Resonators Avoiding Transverse Spurious Modes through Electrode Apodization

This work discusses how apodization of the interdigital transducers (IDT) of shear horizontal (SH
0
) acoustic wave resonators allows to avoid the presence of spurious modes between resonances and enhance the achieved electromechanical coupling and quality factor. Achieving spurious-free resonators is fundamental to face the design of ladder filters with flat passbands. The proposal is demonstrated with manufactured resonators on a 30°YX-cut lithium niobate-on-insulator (LNOI) structure comparing the performance of apodized and non-apodized devices. Clean resonators without transverse spurious modes have been obtained showing an improvement in quality factor and electromechanical coupling coefficient, attaining $k_{t}^2=17 \%, Q_{\max } \sim 800$ and impedance ratios of 69 dB.