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Unsuspended phononic integrated circuits have been proposed for on-chip acoustic information processing. Limited by the operation mechanism of a conventional interdigital transducer, the excitation of the quasi-Love mode in GaN-on-sapphire is inefficient, and thus, a high-efficiency Rayleigh-to-Love mode converter is of great significance for future integrated phononic devices. Here, we propose a high-efficiency and robust phononic mode converter based on an adiabatic conversion mechanism. Utilizing the anisotropic elastic property of the substrate, the adiabatic mode converter is realized by a simple tapered phononic waveguide. A conversion efficiency exceeding 96% with a 3 dB bandwidth of 1.44 GHz can be realized for phononic waveguides working at GHz frequency band, and excellent tolerance to the fabrication errors is also numerically validated. The device that we proposed can be useful in both classical and quantum phononic information processing, and the adiabatic mechanism could be generalized to other phononic device designs. [ABSTRACT FROM AUTHOR]

Thin-film gallium nitride (GaN) is a promising platform for phononic integrated circuits that hold great potential for scalable information processing processors. Here, an unsuspended traveling phononic resonator based on a high-acoustic-index-contrast mechanism is realized in GaN-on-Sapphire with a frequency up to 5 GHz, which matches the typical superconducting qubit frequency. A sixfold increment in quality factor is found when temperature decreases from room temperature (Q = 5000) to 7 K (Q = 30 000), and thus, a frequency-quality factor product of 1.5 × 10 14 is obtained. Higher quality factors should be available when the fabrication process is further optimized. Our system shows great potential in hybrid quantum devices via the so-called circuit quantum acoustodynamics. [ABSTRACT FROM AUTHOR]