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Applied Physics Letters

Direct response of twin-slot antenna-coupled hot-electron bolometer mixers designed for 2.5 THz radiation detection

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

Published May 29, 2000• 28 citations
Publisher

Abstract

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.

DOI
10.1063/1.126614

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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
The measured direct and heterodyne response of a quasi-optically coupled superconducting Nb hot-electron bolometer mixer (HEBM) at 2.5 THz is reported. This mixer exploits a novel coupling circuit in which the microstrip line transformer is used to feed the rf signal from the twin slot antenna to the microbridge. The microstrip line is made of Au/SiO/sub 2//Al. Using a Fourier transform spectrometer, the frequency response of such a device is measured. We find a peak response at a frequency of 1.9 THz, which is 20% lower than predicted, and a bandwidth of 1.6 THz as we expect. By applying 2.5 THz radiation from a far infrared laser, we measure an uncorrected noise temperature T/sub N,rec/ of 4200 K.
The noise performance of a Nb hot-electron bolometer mixer at 2.5 THz has been investigated. The devices are fabricated from a 12-nm-thick Nb film, and have a 0.30 μm×0.15 μm in-plane size, thus exploiting diffusion as the electron cooling mechanism. The rf coupling was provided by a twin-slot planar antenna on an elliptical Si lens. The experimentally measured double sideband noise temperature of the receiver was as low as 2750±250 K with an estimated mixer noise temperature of ≈900 K. The mixer bandwidth derived from both noise bandwidth and IF impedance measurements was ≈1.4 GHz. These results demonstrate the low-noise operation of the diffusion-cooled bolometer mixer above 2 THz.
We report on the development of a quasioptical Nb hot-electron bolometer mixer for a 2.5 THz heterodyne receiver. The devices are fabricated from a 12 nm thick Nb film, and have a 0.30 /spl mu/m/spl times/0.15 /spl mu/m in-plane size, thus exploiting diffusion as the electron cooling mechanism. The rf coupling was provided by a twin-slot planar antenna on an elliptical Si lens. A specially designed 2.5 THz system, using a CO/sub 2/-pumped FIR laser as local oscillator (LO), with rf hot/cold loads enclosed in vacuum to avoid atmospheric absorption, was used in the experiment. The experimentally measured double sideband (DSB) noise temperature of the receiver was as low as /spl les/3000 K, with an estimated mixer noise temperature of /spl ap/750 K. These results demonstrate the operation of the diffusion-cooled bolometer mixer above 2 THz.
We demonstrate a quasi-optical NbN hot electronbolometer (HEB) mixer using a twin-slot antenna on a Si lens tocouple terahertz radiation. The mixer shows a receiver noisetemperature of 1150 K at 2.5 THz, which is expected based on atemperature of 1150 K at 2.5 THz, which is expected based on amodel that includes quantum noise. The measured direct response.is understood by taking into account the main beam efficiency andthe parasitic reactance due to the geometric change betweenbolometer and transmission line. The measured beam of the mixeris nearly collimated and has a Gaussian beam efficiency of 90 %with side-lobes below -16 dB.
Two type of Nb diffusion-cooled HEB mixers have been studied. One uses a twin-slot antenna designed for 700 GHz, and has a bridge length of 200 nm. This device is fabricated using an in situ-process for the interface between the bridge and the cooling pads. An uncorrected receiver noise temperature of 1200 K at 640 GHz and an IF bandwidth of 5 GHz have been measured. The second device uses a log-periodic antenna designed for a wide RF bandwidth. However, there is a misalignment of the Nb bridge with respect to the Au cooling pads. Receiver noise temperatures at 0.64, 1.9, and 2.5 THz have been measured. We are able to link the geometry feature to the measured properties.