  We report direct visualization of gigahertz-frequency acoustic waves in
lithium niobate phononic circuits. Primary propagation parameters, such as the
power flow angle (PFA) and propagation loss (PL), are measured by
transmission-mode microwave impedance microscopy (TMIM). Using fast Fourier
transform, we can separately analyze forward and backward propagating waves and
quantitatively evaluate the propagation loss. Our work provides insightful
information on the propagation, diffraction, and attenuation in piezoelectric
thin films, which is highly desirable for designing and optimizing phononic
devices for microwave signal processing.
