Self-probing of metal nanotips by rescattered electrons reveals the nano-optical near-field

Self-probing of atoms or molecules by their own electrons is a powerful tool for exploring structural dynamics with high spatial and temporal resolution. Spectra from high-harmonic generation or photoelectron rescattering reveal the quantities of interest, such as the electronic structure of the system under scrutiny. Recently, we have observed the rescattering effect at solids, in particular at metal nanotips. Here we use the self-probing approach in order to investigate an effect inherent to nano-scale systems: laser light incident on nanotips generates strongly enhanced optical near-fields that crucially depend on their geometry and material. In our experiment, we determine the strength of the near-field of tungsten and gold tips from the high-energy cut-off of rescattered electrons. Detailed experimental results are compared with a numerical solution of the time-dependent Maxwell equations.