Microwave impedance microscopy imaging of acoustic topological edge mode on a patterned substrate

TL;DR

This study visualizes and confirms the existence of topological edge acoustic modes in a honeycomb phononic crystal using microwave impedance microscopy, demonstrating potential for topological surface acoustic wave devices.

Contribution

It provides the first microwave impedance microscopy imaging of gigahertz topological edge modes in a nanopillar-based phononic crystal on a substrate.

Findings

Clear visualization of topological edge modes between inverted phononic crystals.

Edge modes evolve with frequency as bulk modes are suppressed.

Potential for developing topological SAW devices using simple pillar structures.

Abstract

We have studied acoustic topological edge modes in a honeycomb phononic crystal composed of metallic nano-pillars on a LiNbO$_3$ 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 realized clear visualization of the gigahertz topological edge mode between two mutually inverted topological phononic crystals. A frequency-dependent image showed 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.