THz Microscopy With Josephson Cantilevers for Characterization of Additive Manufactured Spiral Phase Plates

Josephson cantilevers from high-temperature superconductors are versatile sensors to measure frequency and power of microwave and terahertz radiation in the range of 1 GHz to about 5 THz. They consist of at least one Josephson junction, which is fabricated from the high-temperature superconductor yttrium barium copper oxide (YBCO). The Josephson cantilever is mounted on a positioning system inside a vacuum chamber to perform three-dimensional scanning measurements in the THz microscope. The frequency and the power of external radiation can be determined from the recorded data by evaluating the occurring Shapiro steps. In this work, spiral phase plates (SPPs) optimized for the terahertz regime were additively manufactured with a fused filament fabrication 3D printer. Multiple SPPs were investigated and the results of two of them are shown, each made of a different thermoplastic. The SPPs are designed to transform Gaussian light beams into twisted light beams with an orbital angular momentum. An optically pumped molecular gas far-infrared laser system with difluoromethane as lasing medium can be employed as radiation source to generate frequencies between 1 THz and 1.6 THz with powers up to 125 mW. The beam is coupled into the THz microscope via quasi-optical windows and an evacuated optical path. The SPP is placed near the far-infrared laser to measure the far-field distribution. In this work, measurement results based on superconducting Josephson cantilevers are obtained with our unique THz microscope setup to characterize SPPs. The results present an important contribution for the evaluation of the design and the fabrication process of SPPs and showcase one of the multiple measurement modes of the THz microscope.