Antenna Designs for Efficient Coupling to Josephson Junctions for THz Microscopy

Josephson junctions from high-temperature superconductors allow the characterization of frequency and power of high-frequency electromagnetic radiation in the THz range by evaluation of their current-voltage characteristic. This feature makes a Josephson junction suitable to be used as a sensor for the mapping of near and far-field distributions. The sensitivity of such a so-called Josephson cantilever is improved by using antennas to couple the radiation to the Josephson junction. In this work, the Josephson cantilever is fabricated with epitaxial yttrium barium copper oxide (YBa ₂ Cu ₃ O ₇ ) on a magnesium oxide (MgO) bicrystal substrate. A model for the surface impedance of YBa ₂ Cu ₃ O ₇ in the THz regime is presented, which is required for highly efficient antenna designs. This model depends, among other properties, on the quality and thickness of the thin film. Also, the sensitivity of the Josephson cantilever is strongly dependent on the gain of the antenna and the impedance mismatch of the Josephson junction to the antenna's feed point. This work investigates suitable antenna designs for the coupling of $ {\mathbf{1.4}\,\text{THz}}$ radiation to Josephson junctions, whilst considering the low Josephson junction impedance and the electrical properties of the YBa ₂ Cu ₃ O ₇ and substrate.