  Classical electrodynamics describes the optical response of systems using
bulk electronic properties and infinitesimally thin boundaries. However, due to
the quantum nature of electrons, interfaces have a finite thickness.
Non-classical surface effects become increasingly important as ever smaller
nanoscale systems are realized and eventually dominate over volume-related
phenomena. Investigating the response of surface electrons in such systems,
therefore, becomes imperative. One way to gain control over non-classical
interface effects and study them is through electrical gating, as the static
screening charges reside exclusively at the surface. Here, we investigate the
modulation of the surface response upon direct electric charging of a single
plasmonic nanoresonator by measuring the resulting changes in resonance. We
analyze the observed effects within the general framework of surface-response
functions and provide a basic model derived from electron spill-out within the
local-response approximation (LRA). Our observed change in resonance frequency
is well accounted for by assuming a modulation of the in-plane surface current.
Surprisingly, we also measure a change in the resonance width, where adding
electrons to the surface leads to a narrowing of the plasmonic resonance, i.e.,
reduced losses. The description of such effects requires considering nonlocal
effects and the inclusion of a possible anisotropy of the perturbed surface
permittivity. Our experiment, therefore, opens a vast field of investigations
on how to gain control over the surface response 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.
