We measure the direct response of a Nb diffusion-cooled hot-electron bolometer mixer in a frequency range between 0.5 and 3.5 THz. The mixer consists essentially of a twin-slot antenna, a co-planar waveguide transmission line and a Nb superconducting bridge. It is designed for use in receivers with astronomical and atmospherical applications around 2.5 THz. We calculate the impedance of the antenna, the transmission line, and the bridge separately using models which are developed for frequencies below 1 THz and predict the direct response of the mixer. We demonstrate that these models can be applied to much higher frequencies. However, the measured central frequency is 10%-15% lower than predicted.
We summarize our research activities on THz Nb diffusion-cooled hot electron bolometer (HEB) mixers, carried out at Space Research Organization Netherlands (SRON) and Delft University of Technology. This paper will include our understanding on the device physics of diffusion-cooled HEB mixers, noise and IF bandwidth measurements of waveguide mixers around 0.7 THz, and in particular recent measurements of Nb quasi-optical mixers at 0.64 and 2.5 THz. The waveguide devices demonstrate a receiver noise temperature of 900 K at 0.7 THz. The quasi-optical mixers show 1200 K at 0.64 THz and 4500 K at 2.5 THz and a maximum IF bandwidth of at least 5 GHz.
We study the direct response of a superconducting Nb hot-electron bolometer mixer centered at 2.5 THz that incorporates an Au/SiO2/Al microstrip transmission line. The direct response is studied using a Fourier transform spectrometer. We find a peak response at a frequency of 2.1 THz, which is 20% lower than predicted based on a model that neglects the inductance effect in the microbridge.
A6slvacL-The measured direct and heterodyne response of a quasi-optically coupled superconducting Nb hotelectron bolometer mixer (HEBM) at 2 . 5 THa is reported. This mixer exploits a novel coupling circuit in which the microstrip linc transformer is used to feed the rf signal from the twin slot antenne to the microbridge. The microstrip line is made of Au/SiOa/Al. Using a Fourier transforin spectrometer, the frequency response of such a device is measured. We flnd a peak response at a frequency of 1.9 THz, which is 20% lower than predicted, and a bandwidth of 1.6 THz as wc expect. B y applying 2.5 THa radiation from a far infrared laser, we measure an uncorrected noise temperature Keywords-Superconducting device, hot-electron bolorneter mixer, microstrip line, submillimeter radio astronomy. of 4200 K.
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