New Structures for High Resolution THz-Imaging in Medical Applications

In the past years new methods for medical and biochemical imaging and microspectroscopy in the THz-range were developed. Naturally in imaging and also in microspectroscopy high spatial resolutions are requested. This is contrary to the relatively long wavelengths in the THz-range and has been lead to nearfield microscopy methods with resolution in the sub-wavelength range. For this method special structures, called coupling structures, are needed to focus the incident Gaussian beam or free space mode on a probe, as shown at Figure 1. At the beginning cones are used for this purpose. This was connected with high transmission losses due the sub-wavelength-regime and the evanescent wave propagation inside the cones.
In a first step invented by Keilmann et al [1] the transmission losses are lowered using a wire in the centre of the cone. With this method it is possible to transform the incident wave in a TEM (transversal electro-magnetic) wave without lower cut-off fre-quency.