Acoustic Klein Tunneling in Nanoelectromechanical Phononic Crystals*

 Acoustic Klein tunneling, analogous to the unimpeded transmission of a relativistic Dirac particle through potential barriers, offers a new way of controlling acoustic waves in metamaterial systems. Due to the lack of imaging tools with high resolution and sensitivity, however, the phenomenon has only been demonstrated in macroscopic systems with kilo-Hertz operation frequencies. Here, we report the realization of acoustic Klein tunneling in a nano-electromechanical system operating at giga-Hertz (GHz) frequencies. Using transmission-mode microwave impedance microscopy, near-unity transmission of GHz acoustic waves only within the NPN regime is vividly observed in the on-chip phononic crystals. The tunneling at tilted incidence is also studied by controlling the launching angle of acoustic wave. Our work indicates that an exotic high-energy-physics phenomenon could find practical applications in phononic integrated systems for improved signal processing.