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IEEE International Conference on Terahertz Electronics (THz)

Hotspot mixing: a new principle of heterodyne mixing in a superconducting hot-electron bolometer

D. Wilms Floet, E. Miedema, J. R. Gao, T. M. Klapwijk

Published Nov 27, 2002• 0 citations
Publisher

Abstract

We present the hotspot mixing as a new principle of heterodyne mixing in a superconducting hot-electron bolometer. Applying DC and LO powers to a superconducting bridge generates a hotspot within the bridge. The size of the hotspot, and thus the resistance of the bridge, oscillates with the heating power at intermediate frequency. By solving heat-conducting equations, we can predict the mixer performances, such as the conversion efficiency, as a function of dc bias voltage, LO power, or the bath temperature.

DOI
10.1109/thz.1998.731651

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Journal of Applied Physics•Nov 15, 2009•13 citations
We present the hotspot mixing as a new principle of heterodyne mixing in a superconducting hot-electron bolometer. Applying DC and LO powers to a superconducting bridge generates a hotspot within the bridge. The size of the hotspot, and thus the resistance of the bridge, oscillates with the heating power at intermediate frequency. By solving heat-conducting equations, we can predict the mixer performances, such as the conversion efficiency, as a function of dc bias voltage, LO power, or the bath temperature.
We present a study of the resistive behavior of hot-electron bolometers in two different regimes. At bath temperatures near the critical temperature and under the condition of a low current density, the behavior can be understood in terms of charge-imbalance, Andreev reflection, and the superconducting proximity effect. At lower temperatures, a high current density and the application of LO power result in the formation of a resistive electronic hotspot. We propose to interpret heterodyne downconversion as the result of the length oscillation of the hotspot at the intermediate frequency.
Abstract — A hot spot mixer model for phonon cooled hot electron bolometers is set up to accurately determine the dependence of the bolometer resistance on bias and absorbed LO power. An one-dimensional power balance for the nonequilibrium electrons containing an electron heat conduction term along the film and a loss term due to phonon escape to the substrate is solved. In the device's operating region of interest the absorbed heating power is usually large enough to heat a part of the bolometer bridge above the critical temperature forming a normal conducting hot spot. In this model heating due to the LO is assumed to be uniform whereas bias heating takes place in the hot spot region only. Solving the nonlinear power balance for the hot spot length allows to predict current-voltage characteristics, required heating powers and regions of optimal operation. Applying a small signal model in a given operating point yields an estimate for the intrinsic conversion gain. I. LARGE SIGNAL MODEL Existing device models for hot electron bolometers [1,2] treat the bolometer as a lumped element. It is assigned an effective temperature for the phonons and another for the electrons at a given bath temperature. In terms of heating bias power and RF power are equivalent and exchangeable. From this a method [3,4] to calculate the absorbed RF power is derived. This method is referred to as "isotherm " method: Operating points with identical device resistance must have identical temperatures since R(To„,,(,,, ) is a single valued monotonous function. Identical temperature implies identical power losses and identical 592
We present theoretical analysis of the performance of a superconducting hot-electron bolometer mixer with electron cooling due to electron phonon interaction. The analysis is based on a complete set of thermal balance equations for phonons and electrons. A non-thermal action of a dc current has been considered phenomenologically using an analytical fit of isothermal current-voltage characteristics of the mixer. The operation point of the mixer has been determined as a result of the heating by both the power of a local oscillator and the power dissipated by the current. Electron temperature fluctuations, Nyquist noise, and shot noise have been supposed to only contribute to the mixer noise. The conversion gain and the noise temperature of the mixer have been evaluated taking into account an electro-thermal feedback between the mixer and an intermediate frequency circuit with an arbitrary impedance. We have obtained general expressions for the conversion gain, noise temperature and impedance which enabled us to estimate ultimate performance of a superconducting hot-electron bolometer mixer. Theoretical predictions are in good agreement with experimental results obtained for a NbN hot-electron bolometer mixer at a radiation frequency of 2.5 THz.
This paper presents the modeling and measurement of a quasioptical niobium nitride superconducting hot electron bolometer mixer at submillimeter wavelengths. The modeling is performed with a distributed hot spot model which is based on solving a heat balance equation for electron temperature along the superconducting microbridge. Particular care has been taken during the modeling concerning the temperature-dependent resistance and the bias current dependence of the critical temperature of the device. The dc and mixing characteristics of this mixer have been computed and we have observed a quite good match between the predicted and the measured results for both dc characteristics and mixing performances at submillimeter wavelengths.