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Studies the intermediate frequency bandwidth of Nb diffusion-cooled superconducting hot-electron bolometer mixers for different bias voltages. Effects of voltage increases on bandwidth; Intrinsic thermal time constant of the mixer; Effect of electrothermal feedback through the intermediate frequency circuit.

We demonstrate that the performance of NbN lattice cooled hot electron bolometer mixers depends strongly on the interface quality between the bolometer and the contact structure. We show experimentally that both the receiver noise temperature and the gain bandwidth can be improved by more than a factor of 2 by cleaning the interface and adding an additional superconducting interlayer to the contact pad. Using this we obtain a double sideband receiver noise temperature T[sub N,DSB]=950 K at 2.5 THz and 4.3 K, uncorrected for losses in the optics. At the same bias point, we obtain an IF gain bandwidth of 6 GHz. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The current noise at the output of a microcalorimeter with a voltage biased superconducting transition edge thermometer is studied in detail. In addition to the two well-known noise sources: thermal fluctuation noise from the heat link to the bath and Johnson noise from the resistive thermometer, a third noise source strongly correlated with the steepness of the thermometer is required to fit the measured noise spectra. Thermal fluctuation noise, originating in the thermometer itself, fully explains the additional noise. A simple model provides quantitative agreement between the observed and calculated noise spectra for all bias points in the superconducting transition. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present an experimental study of the intermediate frequency bandwidth of a Nb diffusion-cooled hot-electron bolometer mixer for different bias voltages. The measurements show that the bandwidth increases with increasing voltage. Analysis of the data reveals that this effect is mainly caused by a decrease of the intrinsic thermal time of the mixer and that the effect of electrothermal feedback through the intermediate frequency circuit is small. The results are understood using a qualitative model, which takes into account the different effective diffusion constants in the normal and superconducting domains. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]