Reduced Noise Temperatures of a THz NbN Hot Electron Bolometer Mixer

In this article, we measure the double sideband (DSB) receiver noise temperature ( $T_{\text{rec}}^{\text{DSB}})$ of an NbN hot electron bolometer (HEB) mixer at three local oscillator frequencies of 1.6, 2.5, and 5.3 THz. The HEB has cleaned contact interfaces with a 200-nm-thick Au layer. The measured $T_{\text{rec}}^{\text{DSB}}$ values are 530 ± 11 K, 640 ± 18 K, and 2190 ±150 K at 1.6, 2.5, and 5.3 THz, respectively, using an air setup with total optical losses of 2.60 ± 0.04, 2.63 ± 0.16, and 4.70 ± 0.24 dB, respectively. We derived low mixer noise temperatures ( $T_{\text{mixer}}^{\text{DSB}})$ of 240 ± 6 K at 1.6 THz and 290 ± 13 K at 2.5 THz, achieving over 30% improvement compared to published NbN HEB mixers. This enhancement can reduce the integration time of a heterodyne instrument by roughly a factor of 2. At 5.3 THz, $T_{\text{mixer}}^{\text{DSB}}$ is 620 ± 55 K, showing limited improvement due to nonoptimized antenna geometry. These results also contribute to understanding the device physics of a wide HEB (4 μm) at high frequencies. The mixer was developed for the proposed Orbiting Astronomical Satellite for Investigating Stellar Systems and Single Aperture Large Telescope for Universe Studies (concept) missions.