Terahertz field enhancement by a metallic nano slit operating beyond the skin-depth limit

The unique optical properties of metals are at the core of many areas of research and applications, including plasmonics 1 , 2 , 3 , 4 , metamaterials 5 , 6 , superlensing and subdiffraction focusing 7 , 8 , 9 , 10 , optical antennas 11 , 12 , 13 , 14 and surface enhanced Raman scattering 15 . One important length scale inherent in metamaterials and plasmonics research activities in the microwave 5 , 16 , terahertz 17 , 18 , 19 , infrared 20 , 21 , visible 22 and ultraviolet ranges 7 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 λ /30,000 slit on metal film acts as a nanogap-capacitor charged by light-induced currents, enhancing the electric field by orders of magnitudes. The effect of a tiny gap in a metal substrate on incident terahertz radiation in the regime where the gap's dimensions are smaller than the metal's skin-depth are investigated. The results and theoretical analysis show that the gap acts as a capacitor charged by light-induced currents, and dramatically enhances the local electric field.