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.
Bibliographical noteFunding Information:
This research was supported by the Korea Science and Engineering Foundation (KOSEF) (SRC, No: R11-2008-095-01000-0 ) and the Korea Research Foundation (KRF) grant funded by the Korea government (MEST) (No: 2009-0071309 ), KICOS (GRL, K20815000003 ), Seoul Science Fellowship and the Seoul R&BD Program ( 10543 ).
- Field enhancement
- Kirchhoff integral