Gigahertz Phononic Integrated Circuits Based on Overlay Slot Waveguides

Harnessing the properties of phonons on a chip, phononic integrated circuits are playing an increasingly important role in optoelectronic integration. To date, most of such phononic waveguides are realized by etching a piezoelectric layer and forming a wire, which not only involves complicated fabrication processes but also limits integration with electronic and photonic elements on the same chip. To overcome these difficulties, here we propose and demonstrate a type of overlay slot phononic waveguide obtained by patterning a thin silicon cladding layer on an unetched gallium-nitride-on-sapphire substrate. We experimentally demonstrate the guiding and power splitting of gigahertz surface acoustic waves in such waveguides, with a measured propagation loss of about 1.80 dB/mm for the Rayleigh mode. We also theoretically predict that the Love mode in such waveguides can turn into a phononic bound state in the continuum under special conditions. Such waveguides introduce an alternative paradigm for phononic integrated circuitry and will enable applications in information processing, environmental monitoring, and noninvasive manipulation of biomolecules on a chip.