Towards terahertz near-field microscopy.

We have detected sub-wavelength spot sizes in the near-field of a metal tip, which is illuminated with terahertz (THz) pulses. The THz near-field is detected by using electro-optic detection in a (100) oriented GaP crystal. Contrary to conventional electro-optic detection, which uses (110) oriented detection crystals, (100) crystals only allow the detection of THz light, polarized perpendicular to our crystal surface. This component is strongly localized near the apex of the tip, which has sub-wavelength dimensions. The detection process is blind to the incident THz radiation, which is polarized parallel to the crystal surface. As a result, a sub-wavelength THz spot size with an intensity full-width half maximum (FWHM) diameter of lambda/200 is observed.