This paper is a contribution to the Physical Review Applied collection titled Phononics and Metamaterials.

Conventional A1-mode Lamb-wave resonators exhibit high electromechanical coupling coefficients ($K_2$) but suffer from the presence of numerous spurious modes. In this study, we propose a circular A1-mode Lamb-wave resonator that effectively addresses this issue. The design transforms the interdigital transducer (IDT) configuration from a traditional rectangular layout to a circular geometry. This approach achieves the cancellation of spurious modes utilizing their anisotropic in-plane acoustic velocities on Z-cut lithium niobate, while maintaining an effective excitation of the A1 mode due to its isotropic acoustic velocity on the surface. In addition, the variation of the IDT pitch and metallization ratio along the radial direction results in the excitation of independent spurious modes that conflict with each other, further enhancing the suppression effects. Experimental results demonstrate that the proposed resonator achieves a $K_2$ as high as 32.14% and a quality factor (Q) of 524. These findings highlight a promising design strategy for the development of high-performance, spurious-mode-free A1-mode Lamb-wave resonators.

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• Acoustic wave phenomena
• Elastic waves
• Piezoelectric devices
• Acoustic metamaterials
• Devices
• Micromechanical & nanomechanical oscillators
• Transducers
• Radio frequency techniques