Achieving high spatial resolutions for imaging with terahertz (THz) waves requires near-field probes, such as a
sub-wavelength aperture probe. Bethe’s theory of transmission through a sub-wavelength aperture of size a
predicts that the transmitted electric field scales as Eαa3. This strong dependence limits the size of apertures that can be employed and hence the spatial resolution. This dependence however changes for the evanescent
field components in very close proximity (~1μm for THz waves) to the aperture, as shown by electromagnetic
simulations. To exploit this effect in a THz near-field probe, we developed a photoconductive THz near-field
detector structure, which incorporates a thinned photo-conductive detector region and a distributed Bragg
reflector between the detector and the aperture plane. Near-field probes are manufactured with different
aperture sizes to investigate transmission of THz pulses through apertures as small as 3μm. The experimental
results confirm that the transmitted field amplitude, and therefore the sensitivity, increases by about one order of
magnitude for the new probes. A 3μm aperture probe with a spatial resolution of λ/100 at 1THz is
demonstrated.
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