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Applied Physics Letters

Direct response of microstrip line coupled Nb THz hot-electron bolometer mixers

W. F. M. Ganzevles, J. R. Gao, Piet A. J. de Korte, T. M. Klapwijk

Published Oct 8, 2001• 2 citations
Publisher

Abstract

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.

DOI
10.1063/1.1409273

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Nov 13, 1998•1 citation
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.
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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 measure the direct response of a Nb di usion-cooled hot electron bolometer mixer in a frequency range between 0.5 THz 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. Using a far infrared (FIR) laser, the device can be fully pumped at 2.5 THz. A preliminary Y-factor measurement shows a corrected noise temperature of
Nb hot-electron bolometers (HEBs) with critical temperature T. = 5 to 6 K have previously demonstrated good heterodyne mixing performance at THz frequencies. HEBs with a lower critical temperature are predicted to have improved performance with lower noise. We present microwave (30GHz) mixing measurements on Nb HEB mixers with T, between 1.4 and 5.3 K. The reduced T, is obtained by application of a magnetic field, or by use of a bi-layer microbridge of Nb and Au with 'F e = 1.6 K. The mixer output noise and voltage range of low mixer noise are observed to decrease approximately linearly with reduction of T. The Nb-Au mixer has a mixer noise temperature TM = 50K (DSB) with reasonable dynamic range, and thus is promising for future single pixel and array receivers for THz spectroscopy.
Nb-based superconductor - insulator - superconductor (SIS) mixers with Nb tuning circuits have demonstrated good results up to the Nb gap frequency. Above the gap frequency the performance is expected to degrade quickly because RF loss in Nb becomes significant. In this paper we present the results of an effort to extend Nb-based SIS mixers to THz frequencies by employing lower-loss normal-metal Al wiring and tuning structures. The SIS mixer has two Nb/Al-oxide/Nb junctions connected by an Al microstrip inductor. The direct detection response of the device was measured by a Fourier transform spectrometer. A double-side-band receiver noise temperature of 840 K was obtained at 1042 GHz when the device was operated at 2.5 K.
Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range from 1.1 THz to 2.5 THz. The mixer device is a 0.15 - 0.3 micrometer microbridge made from a 10 nm thick Nb film. This device employs diffusion as a cooling mechanism for hot electrons. The double sideband noise temperature was measured to be less than or equal to 3000 K at 2.5 THz and the mixer IF bandwidth is expected to be at least 10 GHz for a 0.1 micrometer long device. The local oscillator (LO) power dissipated in the HEB microbridge was 20 - 100 nW. Further improvement of the mixer characteristics can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies. HEB receivers are planned for use on the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) and the ESA Far Infrared and Submillimeter Space Telescope (FIRST). The prospects of a submicron-size YBa2Cu3O7-(delta ) (YBCO) HEB are discussed. The expected LO power of 1 - 10 (mu) W and SSB noise temperature of approximately equals 2000 K may make this mixer attractive for various remote sensing applications.