Far field detection of terahertz near field enhancement of sub-wavelength slits using Kirchhoff integral formalism

We demonstrate that the near field enhancement of sub-wavelength slits can be experimentally determined in the far field by using a reference aperture Our simple model derived from the Kirchhoff integral formalism shows that enhancement of the near field at a slit exit with respect to the incident wave can be read in the transmitted amplitude through the slit attached on a reference aperture normalized by the transmitted amplitude through the reference aperture Furthermore the near field enhancement obtained in such a way is essentially independent of the reference aperture size By performing terahertz time domain spectroscopy we experimentally confirm the inverse frequency dependence of the near field enhancement of extremely narrow slits and measure the maximum field enhancement reaching 200 at 0 1 THz for a 500 nm-width slit (C) 2010 Elsevier BV All rights reserved