The Influence of LO Power Heating of the Tunnel Junction on the Performance of THz SIS Mixers

We describe the performance of a superconductor- insulator-superconductor (SIS) mixer operating in the frequency range of 780-950 GHz. Unlike most SIS mixers, the tunnel junction employs two different superconductors, a niobium nitride top and a niobium bottom electrode sandwiching an aluminum nitride barrier layer, fabricated on a niobium titanium nitride ground plane. The mixer was tested in a pulse tube cryostat, with all the optical components, in the signal path, mounted inside the vacuum environment to avoid attenuation of the RF signal as it propagates from the hot/cold loads to the mixer. With this setup, we have measured an RF-corrected noise temperature of ~220 K. In this article, we focus on investigating the influence of local oscillator (LO) power heating on the performance of the terahertz mixer. The increase in the junction's physical temperature can be observed experimentally by noting the suppression of the gap voltage in the pumped current-voltage (I-V ) curve as the LO pumping level is increased. Similar observation has already been reported, and attempts were made to estimate the effective temperature of the device using equations of heat transfer between the mixer chip layers. Here, we present an experimental method of quantifying this effect by recovering the effective temperature of the junction through comparing the pumped I-V curves at different pumping levels and fixed bath temperature, with the unpumped I-V curves obtained at varying bath temperatures. We also estimate, for the first time, the effect of heating on the noise temperature as a function of bath temperature and frequency. We show that for typical experimental parameters, the LO heating can increase the double-sideband receiver noise temperature by as much as 20%, and that in the frequency range of the measurements, the effective temperature of the junction at fixed LO power increases linearly with frequency at a rate of 0.5 K/100 GHz.