Terahertz-Mikroskopie mit supraleitenden Josephson-Cantilevern und optisch gepumptem Ferninfrarot-Gaslasersystem

Metrological Instrumentation

THz microscopy with superconducting Josephson cantilevers and optically pumped far-infrared gas laser system

This thesis deals with the analysis of far-infrared radiation (FIR radiation) coupled into superconducting Josephson junctions, the latter ones being extremely sensitive detectors of electro-magnetic radiation as well as of quasi-static magnetic fields. Different antennas were simulated and built to couple radiation into the Josephson junctions in an optimal way. Grain boundary junctions of the high-temperature superconductor YBa2Cu3O7 developed for this thesis were used. They have a width of only a few _m and were cooled down to a temperature of 35 K. There are two such junctions on a cantilever, which is therefore called Josephson cantilever. A micro-positioning system of the THz microscope can move the Josephson cantilever in the measurement volume inside a magnetic shield. The THz microscope had been developed in line with two earlier theses; it was modified and improved for the measurements and can also be used for FIR measurements. An optically pumped FIR gas laser system was set up as radiation source, pumped by a CO2-laser. Control loops were set up for both lasers to stabilise the output power of only a few mW in a frequency range between 462 GHz and 7104 GHz. A quasi-optical system with plastic lenses was developed, set up and analysed to transmit the radiation power from the FIR laser system to the measurement volume of the THz microscope. Thus, the Gaussian beam characteristics of the laser radiation were not lost in the transmission. Radiation transmission using an oversized round metal waveguide was also analysed. In the measurement volume, radiation and illuminated samples in the FIR range were analysed. The detector measurement data was recorded in layers in the measurement volume, resulting in a three-dimensional overview of the radiation power distribution in the near and far field. Calibration allows the determination of the power an antenna emits to the free space. Magnetic samples were also analysed. Different reasons why the THz microscope resolution is limited will be discussed.

TU Braunschweig
Ph.D. student: