Small-mode-volume surface-acoustic-wave resonators with focused Gaussian beam modes.

Hybrid devices using surface-acoustic-wave (SAW) resonators offer a platform for exploring the coherent couplings between acoustic waves and other quantum systems. To increase the coupling in such systems, reducing the resonator mode volume is effective. However, conventional SAW resonators with parallel interdigitated electrodes face increasing diffraction loss with decreasing acoustic mode width. To minimize the in-plane mode volume while suppressing diffraction loss, focusing-type SAW resonators are used in analogy with the Gaussian beam in an optical resonator. It focuses the wavefront of the SAW mode and narrows the beam near the center of the device [1]. Nevertheless, the design principle of such focused resonators has not been established, and the device geometry that allows selective excitation of the fundamental resonance mode has not been clarified.

In this study, we fabricated the focused SAW resonators with electrodes of various curvatures and lengths to clarify the correspondence between the resonator designs and the resonance modes. The microwave transmission revealed multiple resonance peaks, and their number systematically correlated with the curvature of the electrodes. These modes were identified as the fundamental Gaussian mode and higher-order transverse modes. Furthermore, by adjusting the length of the electrodes, we selectively fabricated a single-mode resonator with only the fundamental mode and a multi-mode resonator with higher-order transverse modes.

[1] S. J. Whiteley et al., Nat. Phys. 15, 490–495 (2019).