Hot-electron bolometer terahertz mixers for the Herschel Space Observatory

TLDR

The study reports on low‑noise terahertz mixers (1.4–1.9 THz) developed for the Herschel Space Observatory’s heterodyne spectrometer. The mixers are double‑slot‑antenna superconducting hot‑electron bolometers fabricated from thin NbN films, characterized by Fourier‑transform spectroscopy over 0.5–2.5 THz and discrete noise‑temperature measurements, and their heterodyne receiver stability was compared to direct‑detection mode. The lowest recorded noise temperatures were 750 K DSB at 1.6 THz and 950 K DSB at 1.9 THz, averaging 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz over a 2.4–4.8 GHz IF band, with optimal LO power in the 200–500 nW range.

Abstract

We report on low noise terahertz mixers (1.4-1.9 THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5 to 2.5 THz, with 30 GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750 K [double sideband (DSB)] at 1.6 THz and 950 K DSB at 1.9 THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4-4.8 GHz, the mixer noise temperature was 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200-500 nW range.