Measurement of THz Spot Sizes with a λ/200 Diameter in the Near-Field of a Metal Tip

We report on a method to obtain asub-wavelength resolution in terahertz time-domain imaging. In our method,a sharp copper tip is used to locallydistort and concentrate the THz electricfield. The distorted electric field, presentmainly in the near field of the tip, iselectro-optically measured in an (100)oriented GaP crystal. By raster scanning the tipalong the surface of the crystal we find asmallest THz spot size of 10 μm forfrequencies from 0.1 to 2.5 THz. For ourpeak frequency of 0.15 THz this corresponds to aresolution of λ/200. Our setup has thepotential to reach a resolution down to afew μm, and is a promising candidate tostudy single, living cells in the THzfrequency range.