Toward efficient GHz frequency acoustic wave injection in <b> <i>μ</i> </b> m-scale unsuspended geometries: A case study in ScAlN on silicon-on-sapphire

Focusing GHz frequency acoustic fields into and out of wavelength (μm) scale unreleased waveguide geometries with near-unity efficiency is of potential interest in a variety of problems ranging from building chip-scale microwave to optical photon transducers and integrated RF front-ends. In scenarios requiring co-propagating light fields or extremely low mechanical dissipation, the acoustic fields are routed in a high refractive index semiconductor layer-like silicon, with a piezoelectric overlayer for acoustic field generation. We study acoustic focusing in silicon-on-sapphire substrates with a scandium-doped aluminum nitride (ScAlN) piezoelectric overlayer, and outline the tradeoffs involved with efficient acoustic field generation into Sezawa modes, where the electromechanical coupling strength can be enhanced by increasing Sc%, and their injection and focusing into μm-scale silicon waveguides. In contrast to integrated photonics, strong confinement critically affects both generation and focusing efficiency, thereby imposing stronger constraints on material platforms that can provide a viable alternative to device suspension. We show that sapphire does not provide sufficient acoustic velocity contrast to silicon for shear waves, and a switch to higher contrast silicon carbide substrates is necessary to approximate the efficiencies that can be achieved in suspended devices.