  Classical nonlocality in conducting nanostructures has been shown to
dramatically alter the linear optical response, by placing a fundamental limit
on the maximum field enhancement that can be achieved. This limit directly
extends to all nonlinear processes, which depend on field amplitudes. A study
of third-harmonic generation in metal film-coupled nanowires reveals that for
sub-nanometer vacuum gaps the nonlocality enhances the effective nonlinearity
by four orders of magnitude as the field penetrates deeper inside the metal
than that predicted assuming a purely local electronic response.
