  The electrical generation and detection of elastic waves are the foundation
for acousto-electronic and acousto-optic systems. For surface-acoustic-wave
devices, micro-/nano-electromechanical systems, and phononic crystals,
tailoring the spatial variation of material properties such as piezoelectric
and elastic tensors may bring significant improvements to the system
performance. Due to the much smaller speed of sound than speed of light in
solids, it is desirable to study various electroacoustic behaviors at the
mesoscopic length scale. In this work, we demonstrate the interferometric
imaging of electromechanical power transduction in ferroelectric lithium
niobate domain structures by microwave impedance microscopy. In sharp contrast
to the traditional standing-wave patterns caused by the superposition of
counter-propagating waves, the constructive and destructive fringes in
microwave dissipation images exhibit an intriguing one-wavelength periodicity.
We show that such unusual interference patterns, which are fundamentally
different from the acoustic displacement fields, stem from the nonlocal
interaction between electric fields and elastic waves. The results are
corroborated by numerical simulations taking into account the sign reversal of
piezoelectric tensor in oppositely polarized domains. Our work paves new ways
to probe nanoscale electroacoustic phenomena in complex structures by
near-field electromagnetic imaging.
