We are familiar with the fact that in vacuo light may only be focused to an area of the same order as the wavelength. However metal surfaces support plasma modes that couple to incident light, and the focusing of surface plasma modes is not restricted by the free space wavelength. Such nanofocusing offers the possibility of huge concentrations of radiative energy in very small volumes, impossible to achieve with conventional focusing with lenses. For very modest power input, local concentrations of energy may be great enough to excite nonlinear effects that depend on the intensity.
In fact most nanostructured metal surfaces will produce such focusing as evidenced by the Surface Enhanced Raman effect which is known to be due to intense local concentrations of electric field. Further evidence of intense local field concentrations is dramatically demonstrated in recent experiments by Ebbesen et al., who studied transmission of light at a wavelength of 1500 nanometres, through a 200-nanometre-thick silver film into which cylindrical holes with a diameter of 150 nanometres had been cut. In this system light plays this Houdini-like trick by coupling to surface modes of the metal that are very closely confined to the surface and therefore find it much easier to filter through the tiny holes, as demonstrated in a recent theoretical paper.