Large optical field enhancement for nanotips with large opening angles
  We theoretically investigate the dependence of the enhancement of optical near-fields at nanometric tips on the shape, size, and material of the tip. We confirm a strong dependence of the field enhancement factor on the radius of curvature. In addition, we find a surprisingly strong increase of field enhancement with increasing opening angle of the nanotips. For gold and tungsten nanotips in the experimentally relevant parameter range (radius of curvature $\geq 5\,$nm at $800\,$nm laser wavelength), we obtain field enhancement factors of up to ${\sim}35$ for Au and ${\sim}12$ for W for large opening angles. We confirm this strong dependence on the opening angle for many other materials featuring a wide variety in their dielectric response. For dielectrics, the opening angle dependence is traced back to the electrostatic force of the induced surface charge at the tip shank. For metals, the plasmonic response strongly increases the field enhancement and shifts the maximum field enhancement to smaller opening angles. 
Sebastian Thomas, Georg Wachter, Christoph Lemell, Joachim   Burgd\"orfer, Peter Hommelhoff
physics.optics