Temporal dynamics of GHz acoustic waves in chipscale phononic integrated circuits

Abstract: Phononic integrated circuits, which manipulate GHz-frequency acoustic fields in {\mu}m-scale waveguides, provide new degrees of freedom for routing and manipulation of microwaves in deeply sub-wavelength geometries with associated implications for chipscale sensing and signal processing. The combination of low propagation loss, long interaction lengths and slow speed of sound put together with the large measurement bandwidths and high frequency resolution available from modern vector network analyzers (VNA) makes it feasible to visualize the temporal dynamics of propagating acoustic fields in these devices and see the device in action. Two representative examples we discuss here are pulse circulation and ringdown in an acoustic microring resonator, and the observation of (parasitic) multipath interference effects in waveguide resonator geometries. In the absence of fast 3D acoustic field imaging modalities, such time domain reflectometry based methods provide the best alternative for mapping interface reflection and loss, which becomes increasingly critical as these devices start to scale in complexity.