THz near-field measurement of a square hole

We report on a technique to perform broadband near-field optical microscopy (ANSOM) in the THz domain, in which all components of the THz near-field are measured directly using electro-optic detection in GaP electro-optic crystals with a focused near-IR probe beam. The spatial resolution of our technique is much smaller than the THz wavelengths. This enables us to measure with great accuracy the time-dependent distortion of the field around objects, such as slits, holes or spheres, placed in the THz beam. This technique also provides temporal resolution far below the THz oscillation period. As an example, we present measurements of all components of the electric field in a plane underneath a 200 mum by 200 mum square hole made by gold deposition on a silicon sample, and by deposition directly onto the GaP detection crystal. On the shadow side of the gold layer, we observe a strong evanescent field for the z-component at the edges of the hole, even for wavelengths more than 20 times longer than the hole dimensions. Our spatial resolution can be two-orders of magnitude below the wavelength limit. Using our ability to measure all components of the time- dependent electric field, we are able to present a 2D vector plot of the electric field near the hole, in a plane below the metal film.