Direct imaging of nanogap-mode plasmon-resonant fields.

We perform direct local-field imaging of a plasmon-resonant gold nanoparticle pair separated by a gap of several nanometers using a scattering-type near-field optical microscope with a sharp silicon tip of atomic force microscope. The sharp tip allows the access for the nanogap and the high spatial resolution. Our results provide experimental evidence that the nanogap structure produces an optical spot with the size of a single nanometer (<10 nm). This is not only of fundamental importance in the field of nanophotonics, but also provide significant information for the development of plasmonic devices with the nanogap structures.