Enhanced non-resonant light transmission through subwavelength slits in   metal
  We analytically describe light transmission through a single subwavelength slit in a thin perfect electric conductor screen for the incident polarization being perpendicular to the slit, and derive simple, yet accurate, expressions for the average electric field in the slit and the transmission efficiency. The analytic results are consistent with full-wave numerical calculations, and demonstrate that slits of widths ~100nm in real metals may feature non-resonant (i.e., broadband) field enhancements of ~100 and transmission efficiency of ~10 at infrared or terahertz frequencies, with the associated metasurface-like array of slits becoming transparent to the incident light. 
Anders Pors, Khachatur V. Nerkararyan, Khachik Sahakyan, Sergey I.   Bozhevolnyi
physics.optics