Low-noise heterodyne mixing with NbN microbolometers at 800 GHz

Abstract

Mixer performance of NbN transition edge microbolometers was investigated in a frequency range from 795 to 813 GHz. The devices consist of a parallel array of 3 NbN lines with submicron length patterned by e-beam lithography in a self-aligned process. The heterodyne mixer response was proven by means of two superposed oscillator signals at the receiver input and the observation of a discrete line in the output spectrum. Receiver noise temperatures, Trec, were determined by the hot/cold load Y-factor method in a quasioptical receiver at 4.2 K. The accessible intermediate frequency (IF) band was 1000 to 1750 MHz. We have compared devices with a NbN film thickness of 100 and 50 Å. Lowest receiver noise temperatures were obtained at 800 GHz. Uncorrected Trec values around 900 K were measured for both devices at a 1000 MHz IF frequency. For higher IF frequencies, Trec increased much less in the 50 Å device than in the 100 Å device. At 800 GHz and 1750 MHz IF frequency, Trec was only 1180 K. This result indicates that in an optimized NbN bolometric mixer the IF frequency band can extend to reasonable high values for radioastronomical applications.