Nb superconducting hot electron bolometer mixers coupled with microstrip lines

The measured direct and heterodyne response of a quasi-optically coupled superconducting Nb hot-electron bolometer mixer (HEBM) at 2.5 THz is reported. This mixer exploits a novel coupling circuit in which the microstrip line transformer is used to feed the rf signal from the twin slot antenna to the microbridge. The microstrip line is made of Au/SiO/sub 2//Al. Using a Fourier transform spectrometer, the frequency response of such a device is measured. We find a peak response at a frequency of 1.9 THz, which is 20% lower than predicted, and a bandwidth of 1.6 THz as we expect. By applying 2.5 THz radiation from a far infrared laser, we measure an uncorrected noise temperature T/sub N,rec/ of 4200 K.