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 3dB bandwidth of 1.44GHz 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.

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