Direct Visualization of Gigahertz Acoustic Wave Propagation in Suspended Phononic Circuits

We report direct visualization of gigahertz-frequency Lamb-wave propagation in aluminum nitride phononic circuits by transmission-mode microwave impedance microscopy (TMIM). Consistent with finite-element modeling, the acoustic eigenmodes in both a horn-shaped coupler and a subwavelength waveguide are revealed in the TMIM images. Using fast Fourier-transform filtering, we quantitatively analyze the acoustic loss of individual Lamb modes along the waveguide and the power-coupling coefficient between the waveguide and the parabolic couplers. Our work provides insightful information on the propagation, mode conversion, and attenuation of acoustic waves in piezoelectric nanostructures, which is highly desirable for designing and optimizing phononic devices for microwave-signal processing and quantum-information transduction.