  Giant field enhancement and field singularities are a natural consequence of
the commonly employed local-response framework. We show that a more general
nonlocal treatment of the plasmonic response leads to new and possibly
fundamental limitations on field enhancement with important consequences for
our understanding of SERS. The intrinsic length scale of the electron gas
serves to smear out assumed field singularities, leaving the SERS enhancement
factor finite even for geometries with infinitely sharp features. For silver
nano-groove structures, mimicked by periodic arrays of half-cylinders (up to
120 nm in radius), we find no enhancement factors exceeding ten orders of
magnitude (10^10).
