Giant field enhancement by funneling effect into sub-wavelength slit-box   resonators
  Inspired by the acoustic Helmholtz resonator, we propose a slit-box electromagnetic nanoantenna able to concentrate the energy of an incident beam into surfaces a thousand times smaller than with a classical lens. This design gives birth to giant field intensity enhancement in hot volume, throughout the slit. It reaches $10^4$ in the visible up to $10^8$ in the THz range even with focused beams thanks to an omnidirectional reception. These properties could target applications requiring extreme light concentration, such as SEIRA, non-linear optics and biophotonics. 
Paul Chevalier, Patrick Bouchon, Riad Haidar, Fabrice Pardo
physics.optics