Abstract: We report the study on the quasioptical coupling efficiency and the gain bandwidth of NbN hot-electron bolometer mixers developed for the 4.7 THz channel of the German receiver for Astronomy at THz-frequencies (GREAT) and for security screening at subterahertz frequencies. Radiation coupling efficiency and directive properties of integrated lens antennas with log-spiral, log-periodic and double-slot planar feeds coupled to a hot-electron bolometer were experimentally studied at frequencies from 1 THz to 6 THz and compared with simulations based on the method of moments and the physical-optics ray tracing. For all studied antennas the modeled spectral dependence of the coupling efficiency fits to the experimental data obtained with both Fourier transform spectroscopy and noise temperature measurements only if the complex impedance of the bolometer is explicitly taken into account. Our experimental data did not indicate any noticeable contribution of the quantum noise to the system noise temperature. The experimentally observed deviation of the beam pattern from the model prediction increases with frequency and is most likely due to a non- ideality of the presently used lenses. Study of the intermediate frequency mixer gain at local oscillator (LO) frequencies between 2.5 THz and 0.3 THz showed an increase of the gain bandwidth at low LO frequencies that was understood as the contribution of the direct interaction of magnetic vortices with the radiation field. We have found that the non- homogeneous hot-spot model more adequately describes variation of the intermediate frequency bandwidth with the applied local oscillator power than any of uniform mixer models. The state-of-the-day performance of the GREAT 4.7-THz channel and the 0.8-THz security scanner will be presented.

Abstract: A mixer development for the HIFI instrument of the Herschel Space Observatory has come to the final stage. In our paper and conference presentation we will describe the most important details of the Band 6 Low and High Mixer Unit design. Special attention will be given to the optimization of the hot- electron bolometer mixer chip, which is based on 3.5nm NbN superconducting film on silicon. As the HEB’s local oscillator power requirements depend on the bolometer size, we have compared mixer noise temperature for different bolometer width- to- length ratio. A trade- off between mixer performance and local oscillator power requirements results in the mixer units equipped with optimized mixer chips, providing the largest coverage of the Band6 RF band with the lowest possible receiver noise. A short account of the beam pattern measurements of Band6 mixers will be given as well.

Abstract: NbN phonon cooled HEB’s are one of the most promising bolometer mixer technologies for (near) future (space) applications. Their performance is usually quantified by mea- suring the receiver noise temperature at a given IF frequency, usually around 1 – 2 GHz. However, for any real applications it is vital that one fully knows all the relevant properties of the mixer, including LO power, stability, direct detection, gain bandwidth and noise bandwidth, not only the noise temperature at low IF frequencies. To this aim we have measured all these parameters at the optimal operating point of one single, small volume quasioptical NbN HEB mixer. We find a minimum noise temperature of 900 K at 1.46 THz. We observe a direct detection effect indicated by a change in bias current when changing from a 300 K hot load to a 77 K cold load. Due to this effect we overestimate the noise temperature by about 22% using a 300 K hot load and a 77 K cold load. The LO power needed to reach the optimal operating point is 80 nW at the receiver lens front, 59 nW inside the NbN bridge. However, using the isothermal technique we find a power absorbed in the NbN bridge of 25 nW, a difference of about a factor 2. We obtain a gain bandwidth of 2.3 GHz and a noise bandwidth of 4 GHz. The system Allan time is about 1 sec. in a 50 MHz spectral bandwidth and a deviation from white noise integration (governed by the radiometer equation) occurs at 0.2 sec., which implies a maximum integration time of a few seconds in a 1 MHz bandwidth spectrometer.