Classical electrodynamics describes the optical response of macroscopic
systems, where the boundaries between materials is treated as infinitesimally
thin. However, due to the quantum nature of electrons, interfaces acquires a
finite thickness. To include non-classical surface effects in the framework of
Maxwell's equations, surface-response functions can be introduced, also known
as Feibelman d-parameters. Surface response impacts systems with strong field
localization at interfaces, which is encountered in noble metal nanoparticles
supporting surface plasmon polaritons. However, studying surface response is
challenging as it necessitates sub-nanometer control of geometric features,
e.g. the gap size in a dimer antenna, while minimizing uncertainties in
morphology. In contrast, electrical gating is convenient since the static
screening charges are confined exclusively to the surface, which alleviates the
need for precise control over the morphology. Here, we study the perturbation
of Feibelman d-parameters by direct electric charging of a single plasmonic
nanoresonator and investigate the resulting changes of the resonance in
experiment and theory. The measured change of the resonance frequency matches
the theory by assuming a perturbation of the tangential surface current.
However, we also observe an unforeseen narrowing in the resonance width when
adding electrons to the surface of a plasmonic nanoresonator. These reduced
losses cannot be explained by electron spill-out within the local-response
approximation (LRA). Such an effect is likely caused by nonlocality and the
anisotropy of the perturbed local permittivity. Our findings open up
possibilities to reduce losses in plasmonic resonators and to develop ultrafast
and extremely small electrically driven plasmonic modulators and metasurfaces
by leveraging electrical control over non-classical surface effects.Comment: 10 pages, 3 figures, 15 pages Supplementar