Superconducting Hot-Electron Mixers and Micro-Bolometers

In 1993, the HEB mixer was proposed for operation at THz frequencies. Bolometer mixers had known for a long time, but they were very slow and allowed intermediate frequencies only in the MHz range; too small for modern heterodyne spectroscopy. HEB mixers rely on the temperature dependence of the resistance of a superconducting microbridge near the superconducting transition temperature. The key element of the mixer is a lithographically defined superconductor Nb nanobridge. They are "square law" mixers, somewhat different from the SIS or Schottky diode mixers. Contrary to these mixers, bolometer mixers do not follow the signal and local oscillator electrical field instantaneously, but react to the square of the sum of the fields. The intermediate frequency is thus the beat frequency between signal and local oscillator. The device is a "simple" niobium strip. The hot electron bolometer achieves such sensitivity by exploiting the weak thermal coupling between phonons and electrons in an NIS (Normal metal-Insulator-Superconductor) junction. The incoming radiation is absorbed by the normal metal. Because of the weak coupling, a tiny incident power will raise electron temperature significantly in the metal. These 'hot' electrons then tunnel through the NIS barrier. The incident power can be measured by plotting the current voltage curve of the junction which changes with temperature. To make the bolometric response fast (for high enough intermediate frequencies of a few GHz), only the electron bath is heated above the lattice temperature (="Hot Electrons"), and the fast cooling mechanism is the diffusion of the hot electrons into normal conducting heat sinks instead of electron-phonon cooling to the substrate, which is slow for Niobium. Therefore, these new bolometers are called "diffusion-cooled HEB’s". Both requirements force the microbridge to be thin, narrow and short. Also, the upper frequency limit of the HEB mixer will not be restricted by the Nb energy gap, in contrast to Nb SIS mixers.


a) Hot-electron Mixer on quartz b) Resistance-temperature plot with substrate with a superconducting the bias point shown by dotted lines  microbridge  joining stripline at the mid-point of the transition filters, joining to the amplifier

We have already developed the technology to fabricate good quality niobium HEB’s, using electron-beam lithography, which operate on timescales of ~ 1 ns or shorter, depending on the cooling mechanism (electron-phonon scattering or electron diffusion).These have been used to perform Fourier transform spectroscopic measurements, proving that they work at least across the wavelength range 600 - 100 mm (500 - 3000 GHz). This work is aimed at developing 4 K operating temperature ultra-fast low-noise heterodyne detectors for use in astrophysics, plasma diagnostics, surface science and laboratory spectroscopy, with sensitivities close to the quantum limit and high spectral resolution (l/Dl ~ 10^6).