▎ 摘　　要

Acoustic graphene plasmons (AGPs) hold promise for nanophotonics and sensing applications. Here, the authors observe enhanced in-plane scattering of mid-infrared AGPs caused by atomic steps on the substrate surface, suggesting potential strategies for controlling their propagation via substrate engineering. Acoustic graphene plasmons (AGPs) have ultrastrong field confinement and low loss, which have been applied for quantum effect exploration and angstrom-thick material sensing. However, the exploration of in-plane scattering of AGPs is still lacking, although it is essential for the manipulation of ultraconfined optical fields down to atomic level. Here, by using scattering-type scanning near-field optical microscopy (s-SNOM), we show that the mid-infrared AGPs can be strongly scattered by atomic level height steps, even though the step height of the scatterer is four orders of magnitude smaller than the incident free wavelength. This effect can be attributed to larger back scattering of AGPs than that of the traditional graphene plasmons. Besides, the scattering of AGPs by individual scatterers can be controlled via electrical back gating. Our work suggests a feasible way to control confined optical fields with atomic level height nanostructures, which can be used for ultra-compacted strong light-matter interactions.