A closed-loop platform for the design and nanoscale imaging of GHz acoustic metamaterials

TL;DR

This paper introduces a high-resolution, broadband electrostatic force microscopy technique for real-space imaging of GHz surface acoustic waves in metamaterials, enabling detailed band structure analysis and control.

Contribution

It develops a novel EFM method that allows continuous, sub-200 nm resolution imaging of traveling SAWs across wide GHz bandwidths, bridging design and experimental validation.

Findings

Mapped full frequency range around the Dirac point in SAW graphene analog

Visualized transition from ballistic to diffusive SAW transport

Demonstrated tuning of band gaps by breaking sublattice symmetry

Abstract

Band structure engineering in surface acoustic wave (SAW) metamaterials could advance both classical telecommunications and quantum information processing. However, no imaging technique has demonstrated the necessary capability to resolve sub-$\mu$m traveling SAWs across wide GHz bandwidths. Existing methods capture only fragments of the dispersion at discrete frequencies, preventing systematic characterization and control of SAW-based metamaterials. Here, we develop electrostatic force microscopy (EFM) to enable real-space imaging of traveling SAWs in honeycomb metamaterials on LiNbO$_3$. Our application leverages sub-200 nm spatial resolution, broad GHz bandwidth, and non-contact imaging to map complex band structures with continuous frequency resolution and expanded frequency range, while preserving sub-lattice detail. Using EFM, we map the full relevant frequency range around the Dirac point of a SAW graphene analog, including the acoustic Dirac cones, and the transition from ballistic to diffusive SAW transport regime. Furthermore, by breaking sublattice symmetry, we tune the opening of a band gap at the Dirac point, and image frequency-dependent wave localization on sublattice sites. Our EFM technique closes the loop between design and real-space validation, streamlining the engineering of arbitrary SAW landscapes for next-generation applications spanning telecommunications, microfluidics, and quantum acoustics.