Imaging an Acoustic Topological Edge Mode on a Patterned Substrate with Microwave Impedance Microscopy

We study acoustic topological edge modes in a honeycomb phononic crystal composed of metallic nanopillars on a LiNbO3 substrate. Acoustic band calculations show that the topological surface acoustic wave (SAW) mode inhabits the edge of the honeycomb phononic crystal in spite of the hybridization with the internal acoustic modes of the substrate. Pulse-type microwave impedance microscopy realizes clear visualization of the gigahertz topological edge mode between two mutually inverted topological phononic crystals. A frequency-dependent image shows that the edge mode evolves as the bulk SAW modes are suppressed owing to the energy gap formation, consistent with the topological nature. The realization of a topological waveguide in a simple pillar structure on a substrate might pave a new path to the development of topological SAW devices for a wide range of usages such as quantum computing, sensing, and communication applications.