  Terahertz (THz) imaging provides cutting edge technique in biology, medical
sciences and non-destructive evaluation. However, due to the long wavelength of
the THz wave, the obtained resolution of THz imaging is normally a few hundred
microns and is much lower than that of the traditional optical imaging. We
introduce a sub-wavelength resolution THz imaging technique which uses the THz
radiation generated by a femtosecond laser filament in air as the probe. This
method is based on the fact that the femtosecond laser filament forms a
waveguide for the THz wave in air. The diameter of the THz beam, which
propagates inside the filament, varies from 20 {\mu}m to 50 {\mu}m, which is
significantly smaller than the wavelength of the THz wave. Using this highly
spatially confined THz beam as the probe, THz imaging with resolution as high
as 20 {\mu}m (~{\lambda}/38) can be realized.
