Tunable phononic frequency combs in curved piezoelectric micromachined ultrasound transducers enabled by 2:1 internal resonance and a ferroelectric thin film

Phononic frequency combs have been demonstrated in various MEMS resonators through intermodal coupling, analogous to their optical counterparts. This work presents the generation of tunable frequency combs through nonlinear intermodal coupling based on 2:1 internal resonance and a ferroelectric thin film in curved circular piezoelectric micromachined ultrasound transducers (PMUTs). Our experiments reveal strong 2:1 resonance coupling between the first asymmetric and second symmetric modes of the circular PMUT and its critical role in comb evolution. Notably, robust comb behavior with varying comb spacing is observed at both coupled modes as a function of applied DC voltage, AC actuation voltage, and single-tone drive frequency. We provide a theoretical dynamical model that explains the experimental results and highlights the role of modal coupling via 2:1 internal resonance in comb generation. Our findings underscore the significance of resonance coupling and input parameters in generating and tuning phononic frequency combs, paving the way for advancements in nonlinear phononics.