New structures for high resolution THz-imaging in medical applications

For applications in medical imaging, microspectroscopy and also in ultrafast time domain spectroscopy in the THz-range often spatial resolutions in the sub-wavelength range are requested. Therefore special structures, called coupling structures, are needed to focus the incident Gaussian beam or free space mode on a probe. Traditionally for this purpose cones are used, which results in high transmission losses, especially at high resolutions. Beginning with the invention by Keilmann et al [1] this problem was solved by using different coupling structures to transform the incident wave in a TEM (transversal electromagnetic) wave without lower cut-off frequency. Further development has been lead to a class of structures with frequency independent properties and even higher resolution. An example of such structures is a logarithmic periodic coupling structure [2] inside a cone to couple the incident wave on a two-wire TEM wave guide. By designing the single dipole an adaptive frequency transmission is possible. The two-wire structure also leads to a higher field concentration, preferable for microspectroscopy. Using usual numerical methods (FEM, FDTD, MoM) for the design it leads to enormous computation effort. Also ray tracing methods (GTD, PO, UTD) could used only for parts of such structures. Furthermore such methods give no answers on design questions. Therefore an approach from group theory was used to design such structures. The presented structures are used for high spatial resolution transmission / time domain spectroscopy and experimental imaging in the THz-Range at the IRIS Beamline at BESSY and we will present some experimental results.