Surface-enhanced Raman spectroscopy (SERS): nonlocal limitations
  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). 
Giuseppe Toscano, S{\o}ren Raza, Sanshui Xiao, Martijn Wubs,   Antti-Pekka Jauho, Sergey I. Bozhevolnyi, N. Asger Mortensen
physics.optics cond-mat.mes-hall