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 $\sim$220 K. In this article, we focus on investigating the influence of local oscillator (LO) power heating on the performance of the terahertz mixer. The increase in the junction's physical temperature can be observed experimentally by noting the suppression of the gap voltage in the pumped current–voltage ($I\text{--}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 $I\text{--}V$ curves at different pumping levels and fixed bath temperature, with the unpumped $I\text{--}V$ 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.

中文翻译：

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隧道结LO功率加热对太赫兹SIS混频器性能的影响

我们描述了在 780-950 GHz 频率范围内工作的超导体-绝缘体-超导体 (SIS) 混频器的性能。与大多数 SIS 混合器不同，隧道结采用两种不同的超导体，氮化铌顶部电极和铌底部电极夹在氮化铝阻挡层之间，在铌钛氮化物接地平面上制造。混频器在脉冲管低温恒温器中进行了测试，信号路径中的所有光学组件都安装在真空环境中，以避免射频信号从热/冷负载传播到混频器时衰减。通过这种设置，我们测量了经过 RF 校正的噪声温度$\sim$220 K。在本文中，我们重点研究本地振荡器 (LO) 功率加热对太赫兹混频器性能的影响。通过注意到泵浦电流-电压中间隙电压的抑制，可以通过实验观察结物理温度的增加（$I\text{--}V$) 曲线随着 LO 泵浦电平的增加而变化。已经报道了类似的观察结果，并尝试使用混合器芯片层之间的热传递方程来估计器件的有效温度。在这里，我们提出了一种量化这种效应的实验方法，该方法通过比较泵浦来恢复结的有效温度。$I\text{--}V$ 不同泵送水平和固定浴温下的曲线，未泵送的 $I\text{--}V$在不同浴温下获得的曲线。我们还首次估计了加热对噪声温度的影响，作为浴温和频率的函数。我们表明，对于典型的实验参数，LO 加热可以使双边带接收机噪声温度增加多达 20%，并且在测量的频率范围内，固定 LO 功率下结的有效温度随频率为 0.5 K/100 GHz。