(Image by ZHANG Wen)

Fig. 1 Measured gain bandwidth at the same DC bias  point but different bath temperatures. Bath temperatures are 5.4, 6 and  6.4 K, respectively. The inset shows the I-V curves at different bath  temperatures, intersection point of which is the DC bias point for gain  bandwidth measurement.

Researchers led  by Prof. SHI Shengcai , Purple Mountain Observatory, Chinese Academy of  Sciences, combine heterodyne mixing with direct detection of  superconducting hot electron bolometer, which is based on a several  nanometer thick NbN film, connected to a planar antenna. The  superconducting hot electron bolometer was assumed to have high  sensitivity at a bath temperature far below its transition temperature.  For some special applications, for example, Dome A 5m THz telescope in  Antarctica and space missions, less cooling requirement is desirable,  making heterodyne mixer operating at higher temperature. Superconducting  NbN HEB can also be used to directly detect week signal in the vicinity  of superconducting transition as a detector due to its sharp  temperature dependence of resistance.

They studied  the receiver noise temperature and intermediate frequency gain bandwidth  at different bath temperatures, and find that the corrected noise  temperature and IF gain bandwidth are 800 K at 0.5THzand 750 K 0.85 THz,  and 3 GHz at its optimum bias point. Both the receiver noise  temperature and IF gain bandwidth were found insensitive to the bath  temperature, in good agreement with those simulated with the hot-spot  model taking account of the HEB’s current-dependent resistive  transition. Furthermore, the HEB’s frequency response was measured by a  Fourier transform spectrometer at different bias points and bath  temperatures. The estimated noise equivalent power (NEP) was close to  10-13 W/Hz0.5 around the HEB’s transition temperature. The work by ZHANG  Wen, MIAO W., ZHOU K. M. , LI S. L. , LIN Z. H. , YAO Q. J. , and SHI  S.C. (corresponding author), accepted by IEEE transactions on applied  superconductivity, has been published online  ( http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5685588 ).