Hot-electron bolometer terahertz mixers for the Herschel Space Observatory

We report on low noise terahertz mixers $(1.4–1.9THz)$ 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 $0.5to2.5THz$⁠, with $30GHz$ resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was $750K$ [double sideband (DSB)] at $1.6THz$ and $950K$ DSB at $1.9THz$ local oscillator (LO) frequencies. Averaged across the intermediate frequency band of $2.4–4.8GHz$⁠, the mixer noise temperature was $1100K$ DSB at $1.6THz$ and $1450K$ DSB at $1.9THz$ 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 $200–500nW$ range.