Chaotic ultrasound generation using a nonlinear piezoelectric microtransducer

We report on a piezoelectric micromachined ultrasonic transducer (PMUT) driven in a nonlinear regime, generating chaotic amplitude modulated ultrasonic waves. At large enough drives, the PMUT enters in the Duffing regime which opens a hysteresis with two available states. By modulating the frequency of the driving signal, the system may switch between both states, and selecting the appropriate modulation frequency enables to enter in the chaotic regime. The chaos is then imprinted as a modulation of the PMUT's amplitude. We characterize this regime in the three accessible domains: electrical, mechanical and acoustic, and demonstrate they are fully correlated. We then focus on the generated acoustic signals and demonstrate that the chaotic modulation propagates according to the PMUT's linear regime. Remarkably, the detected acoustic waves are strongly correlated to the on-chip piezoelectric measurements, regardless of the acoustic beam profile. The frequency spectrum of the chaotic modulation spreads around the ultrasonic carrier, mimicking a noise modulated carrier signal. We exploit this property for jamming applications where the chaotic PMUT is used to mask surrounding acoustic waves. Unlike most jamming applications, our approach does not require driving signals with a broad frequency spectrum, the noisy pattern arising directly from the structure's dynamics. Using two PMUTs, one in the linear and the other in the nonlinear regime, we realize a proof-of-concept where the ultrasound generated by the first PMUT is drowned out by the chaotic PMUT signal. We demonstrate that the carrier frequency of the jamming PMUT does not need to match perfectly the one of the linear PMUT. This chaos generation is generic and could be adapted to any PMUT, and thanks to the rich frequency spectrum of the chaotic modulation, the frequency of the signal to jam does not need to be precisely known. (paper)