Direction-resolved nanoscale optical imaging with near-nanometer resolution by emerging infrared torsional force microscopy.

Abstract

Resolving nanoscale light-matter interactions requires both high spatial resolution and anisotropic sensitivity to in-plane and out-of-plane optical responses. We introduce torsional force microscopy-infrared microscopy, a new optical imaging technique that combines cantilever torsional dynamics with a nonlinear frequency-mixing scheme to map both in-plane and out-of-plane photothermal signals. Using birefringent mica as a model system, we resolve distinct in-plane and out-of-plane vibrational responses and reconstruct the anisotropic strain distribution of nanobubbles, in excellent agreement with simulation. Furthermore, we demonstrate near-nanometer ( ~ 1 nm) spatial resolution in optical imaging of twisted bilayer graphene, enabling site-resolved spectroscopy within individual moiré cells. Energy-dependent imaging further reveals intra-unit cell optical features, highlighting the role of competing physical processes in a moiré lattice modulating its optical properties. By providing a direct, anisotropy-resolved view of nanoscale optical heterogeneities, this emerging infrared torsional force microscopy establishes a powerful platform for probing, understanding, and ultimately engineering light-matter interactions in complex quantum materials.