The unique optical properties of metals are at the core of many areas of research and applications, including plasmonics, metamaterials, superlensing and subdiffraction focusing, optical antennas and surface enhanced Raman scattering. One important length scale inherent in metamaterials and plasmonics research activities in the microwave, terahertz, infrared, visible and ultraviolet ranges is the skin depth of metal, which remains at the submicrometre level throughout the broad spectral range. One prominent question is whether terahertz electromagnetic waves can be controlled on the nanoscale to achieve new functionalities in the sub-skin-depth regime. Here, we show that a /30,000 slit on metal film acts as a nanogap-capacitor charged by light-induced currents, enhancing the electric field by orders of magnitudes.
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We acknowledge helpful discussions with J. B. Pendry, D. Grischkowsky and K. J. Ahn. H. Kim is also thanked for discussions concerning diffractive optics correction used in data analysis and for Kirchhoff formalism. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST), the Korea Research Foundation (KRF), KICOS (GRL, K20815000003) and the Seoul R&BD Program.