Cryogenic Enhancement of the Phononic Four-Wave Mixing Modal Nonlinearity in AlScN/4H-SiC

Piezoelectric acoustic devices are widely utilized for filtering in classical radio frequency (RF) signal processors due to their low loss and compact footprint. These attributes make them promising candidates for implementing miniaturized quantum RF signal processing functions. The realization of efficient nonlinear phononic interactions could enable transformative capabilities for quantum acoustic information processing. Here, we study surface acoustic wave (SAW) phononic four-wave mixing at gigahertz frequencies in an aluminum scandium nitride/4H-silicon carbide (AlScN/4H-SiC) heterostructure operated at ambient and cryogenic temperatures and demonstrate a substantial enhancement in the extracted modal nonlinearity at cryogenic temperatures. The 500 nm AlScN film on 4H-SiC substrate supports guided acoustic modes with differing field distributions (Rayleigh and Sezawa), enabling comparison of their phononic nonlinearities as a function of temperature. Continuous-wave measurements reveal approximately an order-of-magnitude increase in the four-wave mixing modal nonlinearity at 4 K relative to room temperature. Furthermore, the nonlinearity exhibits a strong dependence on the acoustic mode, with the Rayleigh mode displaying 100X higher modal nonlinear coefficient than the Sezawa mode. These results lay the groundwork for leveraging cryogenically enhanced phononic nonlinearities in next-generation quantum acoustic signal processing platforms.