Investigation of YBCO Thin-Film Surface Impedance for THz Microscopy Using Josephson Junctions

Josephson junctions based on high-temperature superconductors enable the detection of terahertz (THz) radiation at frequencies up to 5 THz. Their current-voltage characteristics provide information on the incident intensity and frequency of gigahertz and terahertz radiation, with coupling efficiency enhanced through the use of integrated antennas. This detector, called a Josephson cantilever, is employed in our THz scanning microscope to map three-dimensional far-field patterns. The cantilevers are fabricated on lanthanum aluminate-strontium aluminum tantalate (LSAT) or magnesium (MgO) substrates with epitaxially grown yttrium barium copper oxide (YBCO) thin films. Focused-helium ion beam direct writing is used to write Josephson junctions in the antenna feed points. These narrowband u-shaped antennas are designed for specific resonant frequencies, requiring the determination of the YBCO’s surface impedance. The temperature-dependent surface impedance of YBCO was determined using terahertz spectroscopy (300 GHz to 2 THz, 10 K to 300 K) and validated against measured antenna resonances that were compared to finite integration simulations in CST Studio Suite 2023. Finally, spatially resolved measurements at 1.4 THz demonstrate the imaging capabilities with symmetric Gaussian beam profiles.