Design and Analysis of Guided Surface Acoustic Waves in ScAlN on Sapphire for Phononic Integrated Circuits

This study presents a comprehensive dispersion analysis and characterization of guided surface acoustic waves (SAWs) in 30% scandium aluminum nitride (ScAlN) alloy thin films on sapphire. The solidly mounted platform, which supports the fundamental Rayleigh and Sezawa SAW modes, offers mechanical robustness and high electromechanical coupling ( $k_{t}^{2}$ ), while maintaining high confinement of the acoustic modes. Numerical modeling, coupled with experimental results, showcases the characteristics of focusing interdigitated transducers (FIDTs) for injecting acoustic energy into piezoelectric etch-defined acoustic waveguides and highlights their advantages over conventional uniform aperture transducers. Identity mapping of boundary conditions significantly reduces degrees of freedom (DoFs) in modeling energy injection into acoustic waveguides. The theory of Gaussian beams in optics is applied to the FIDTs to model the physical response of the transducers accurately and emphasize their high-intensity focusing nature. This work also demonstrates the ability of FIDTs to facilitate phononic devices and phononic integrated circuit (PnICs) applications in slow-on-fast piezoelectric platforms.