An Ultra-Low-Noise 600-700 GHz Heterodyne Terahertz Receiver for Ground-Based Astronomy Observations

We present a room-temperature, heterodyne terahertz detection system, specifically developed for ground-based astronomy applications. This system employs an optical local oscillator and a plasmonic photomixer to down-convert radiation within the $600-700 \text{GHz}$ atmospheric window into radio frequencies. The heterodyne receiver incorporates an optical phase-locked loop, which provides exceptional frequency stability and high spectral resolution. Additionally, an optical alignment stabilization system is implemented to minimize signal fluctuations during prolonged observation periods. To significantly enhance the signal-to-noise ratio, we have developed a spectral lock-in detection scheme, resulting in record-low detection sensitivities while operating at room-temperature. Our measurements demonstrate double-sideband noise temperature values ranging from 91 K, with a 1 -second integration time, down to 2.5 K for an integration time of 320 seconds. These results exhibit sensitivities surpassing those of conventional heterodyne receivers that require cryogenic cooling, underscoring the superior performance of this system in practical observations.