Heating of a Superconductor–Insulator–Superconductor Mixer Under Exposure to External Terahertz Radiation

Abstract

We present the results of studying the thermal effects produced by a backward-wave oscillator signal at a frequency of 1.0–1.1 THz on a mixing element based on an Nb–AlN–NbN superconductor–insulator–superconductor (SIS) tunnel junction included in a NbTiN–SiO₂–Al microstrip transmission line. The developed receiving system consists of a slot antenna, matched microstrip transmission lines, and two SIS mixers with junction areas of about 1 μm². Under exposure to a signal at a frequency around 1 THz, a significant change in the current–voltage characteristic of the SIS junctions was observed, which manifested as a decrease in the gap voltage and indicated heating of the mixing element, significantly affecting the characteristics of the receiving system. An analytical model describing the studied structure exposed to a high-frequency signal is proposed. The mechanisms leading to direct heating of the tunnel junction and to a change in the distribution function of quasiparticles in the electrode material are analyzed. The relevant effects are identified and their influence on the heating of the junction and, as a result, on the decrease in the gap voltage is numerically estimated. The total heat generation budget and the spatial temperature distribution in the integrated circuit are calculated.