High-order near-field imaging of low-dimensional materials at infrared wavelengths.

Near-field imaging provides insight into the fundamental light-matter interactions on a nanometer scale. Scattering-type scanning near-field optical microscopy (s-SNOM) is a powerful technique capable of overcoming the diffraction limit and achieving spatial resolutions below 10 nm (sub-10 nm). However, constrained by the working mechanisms, the signal-to-noise ratio of the imaging is highly affected by undesired background scattering light, which is found to be associated with the optical mode and excitation wavelength, especially for samples with a large specific surface area. Here, we propose a high-resolution method with high-order near-field modes at the infrared range to measure low-dimensional materials. With this technique, we reveal the excitation and propagation of the surface plasmon polaritons in graphene and carbon nanotubes, which was impossible with the low-order imaging approach. Besides, the imaging quality for gold nanoparticles on gold thin film is much better than the AFM results. This paper offers an advanced approach for high-resolution measurement of low-dimensional materials with s-SNOM, owning great potential for sensitive nanoscale imaging.