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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Development of THz Nb diffusion-cooled hot electron bolometer mixers

J. R. Gao, Pavel A. Yagoubov, T. M. Klapwijk, Piet A. J. de Korte, W. F. M. Ganzevles, D. Wilms Floet

Published Feb 1, 2003• 8 citations
Publisher

Abstract

We summarize our research activities on THz Nb diffusion-cooled hot electron bolometer (HEB) mixers, carried out at Space Research Organization Netherlands (SRON) and Delft University of Technology. This paper will include our understanding on the device physics of diffusion-cooled HEB mixers, noise and IF bandwidth measurements of waveguide mixers around 0.7 THz, and in particular recent measurements of Nb quasi-optical mixers at 0.64 and 2.5 THz. The waveguide devices demonstrate a receiver noise temperature of 900 K at 0.7 THz. The quasi-optical mixers show 1200 K at 0.64 THz and 4500 K at 2.5 THz and a maximum IF bandwidth of at least 5 GHz.

DOI
10.1117/12.459713

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We summarize our research activities on THz Nb diffusion-cooled hot electron bolometer (HEB) mixers, carried out at Space Research Organization Netherlands (SRON) and Delft University of Technology. This paper will include our understanding on the device physics of diffusion-cooled HEB mixers, noise and IF bandwidth measurements of waveguide mixers around 0.7 THz, and in particular recent measurements of Nb quasi-optical mixers at 0.64 and 2.5 THz. The waveguide devices demonstrate a receiver noise temperature of 900 K at 0.7 THz. The quasi-optical mixers show 1200 K at 0.64 THz and 4500 K at 2.5 THz and a maximum IF bandwidth of at least 5 GHz.
Diffusion-cooled Nb hot-electron bolometer (HEB) mixers have the potential to simultaneously achieve high intermediate frequency (IF) bandwidths and low mixer noise temperatures for operation at THz frequencies (above the superconductive gap energy). We have measured the IF signal bandwidth at 630 GHz of Nb devices with lengths L = 0.3, 0.2, and 0.1 micrometer in a quasioptical mixer configuration employing twin-slot antennas. The 3-dB EF bandwidth increased from 1.2 GHz for the 0.3 gm long device to 9.2 GHz for the 0.1 gm long device. These results demonstrate the expected 1/L squared dependence of the IF bandwidth at submillimeter wave frequencies for the first time, as well as the largest EF bandwidth obtained to date. For the 0.1 gm device, which had the largest bandwidth, the double sideband (DSB) noise temperature of the receiver was 320-470 K at 630 GHz with an absorbed LO power of 35 nW, estimated using the isothermal method. A version of this mixer with the antenna length scaled for operation at 2.5 THz has also been tested. A DSB receiver noise temperature of 1800 plus or minus 100 K was achieved, which is about 1,000 K lower than our previously reported results. These results demonstrate that large EF bandwidth and low-noise operation of a diffusion-cooled HEB mixer is possible at THz frequencies with the same device geometry.
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 employs diffusion as a cooling mechanism for hot electrons. The double sideband receiver noise temperature was measured to be approximately equals 2750 K at 2.5 K at 2.5 THz; and mixer IF bandwidths as high as 9 GHz are achieved for 0.1 micrometers long devices. The local oscillator power dissipated in the HEB microbridge was in the range 20- 100 nW. Further reductions in LO power and mixer noise can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. A distributed-temperature model has been developed to properly describe the operation of the diffusion-cooled HEB mixer. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies.
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.
New advances in Hot Electron Bolometer (HEB) mixers have resulted in record low receiver noise temperatures at THz frequencies recently. We have developed quasi-optically coupled NbN HEB mixers and measured noise temperatures up to 1.56 THz, as described in this paper. We project the anticipated future performance of such receivers to have even lower noise temperature and LO power requirement as well as wider gain and noise bandwidths. We introduce a proposal for integrated focal plane arrays of HEB mixers which will further increase the detection speed of THz systems.
We highlight the progress on NbN hot electron bolometer (HEB) mixers achieved through fruitful collaboration between SRON Netherlands Institute for Space Research and Delft University of Technology, the Netherlands. This includes the best receiver noise temperatures of 700 K at 1.63 THz using a twin-slot antenna mixer and 1050 K at 2.84 THz using a spiral antenna coupled HEB mixer. The mixers are based on thin NbN films on Si and fabricated with a new contact-process and-structure. By reducing their areas HEB mixers have shown an LO power requirement as low as 30 nW. Those small HEB mixers have demonstrated equivalent sensitivity as those with large areas provided the direct detection effect due to broadband radiation is removed. To manifest that a HEB based heterodyne receiver can in practice be used at arbitrary frequencies above 2 THz, we demonstrate a 2.8 THz receiver using a THz quantum cascade laser (QCL) as local oscillator.
New advances in hot electron bolometer (HEB) mixers have recently resulted in record-low receiver noise temperatures at terahertz frequencies. We have developed quasi-optically coupled NbN HEB mixers and measured noise temperatures up to 2.24 THz, as described in this paper. We project the anticipated future performance of such receivers to have even lower noise temperature and local-oscillator power requirement as well as wider gain and noise bandwidths. We introduce a proposal for integrated focal plane arrays of HEB mixers that will further increase the detection speed of terahertz systems.