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Performance at585GHzofaSlot-Ring AntennaCoupled NiobiumHEB MixerElement forImaging Applications

L. Liu, Haidi Xu, Q. F. Xiao, A. W. Lichtenberger, M. Weikle, Charles L. BrownDepartment

Published Jan 1, 2005• 0 citations
OpenAlex

Abstract

GHz diffusion-cooled Nb HEB (d-HEB) mixers utilizing slot-ring antennas havebeendesigned and fabricated onsilicon wafers. Tooptimize powercoupling fromtheantennas tothemixing elements, twoimpedance matching schemes areproposed. Performance at585GHz ofthemixers including I-Vcurves, conversion gainand noise temperature arepresented.

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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.

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•Jan 1, 1995•0 citations
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.
Prototype one-dimensional (1-D) superconducting hot-electron bolometer (HEB) imaging mixer arrays with four pixels, designed for operation at 585 GHz, and based on slot-ring antennas arranged in the "reverse-microscope" configuration have been fabricated and implemented. Electromagnetic simulations with Agilent's Momentum software package have been performed to study the self- and mutual-impedances of the slot-ring antenna (SRA) array corresponding to various element spacings. Moreover, the element SRA off-axis radiation patterns (with an extended silicon lens) have been calculated using ray-tracing techniques and the imaging angular resolution has been predicted. Fabrication of the arrays and HEB elements are discussed, and measurements for a single mixer element are presented, yielding a double sideband (DSB) receiver noise temperature of approximately 650 K at 585 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.
AbstracF-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 ~mx0.15 pm 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 laser as local oscillator (LO), with rf hotkold 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 53000 K, with an estimated mixer noise temperature of = 750 K. These results demonstrate the operation of the diffusion-cooled bolometer mixer above
This thesis investigates the prospects of some different superconducting mixers integrated with planar antennas for the millimeter and submillimeter regime. Two kinds of Superconductor/Insulator/Superconductor (SIS) tunnel junctions for use in mixers have been fabricated; Nb/PbBi edge junctions and Nb trilayer junctions. One of the very high quality trilayer mixers is currently operating in a 350 GHz radio-telescope heterodyne-receiver. The reduced sensitivity of SIS mixers for frequencies above the gap frequency (?700 GHz) of the superconductors has directed the interest towards another type of superconducting mixer device, the superconducting Hot Electron Bolometer (HEB). Various types of this device have been studied; Nb devices for scale-measurements at 20 GHz to more easily verify some of the theoretical background of the HEB, NbN waveguide devices for 100 GHz, and NbN devices integrated with planar antennas for a quasi-optical mixer at 350 GHz. Two types of planar antennas have been used; the endfire "Broken" Linearly Tapered Slotline Antenna (BLTSA) on a thin dielectric membrane, and the double dipole antenna on a quartz substrate lens. The beam pattern of the BLTSA has been examined theoretically and measured at 40 GHz with antennas supported by Kapton foil, and at 350 and 800 GHz with antennas on thin dielectric SiOxNy membranes. Measurements of the 20 GHz Nb HEB show a mixer conversion loss close to 0 dB for IF frequencies below 40 MHz. The maximum IF bandwidth given by the electron temperature relaxation time corresponds to a frequency of about 80 MHz. A DSB mixer noise temperature of only a few hundred Kelvin has been measured. Corresponding measurements of NbN devices at 100 and 350 GHz give a mixer conversion loss of about 6-10 dB and an IF bandwidth of about 700 to 1000 MHz. The output noise is found to be dominated by electron temperature fluctuations with an equivalent noise temperature of about 100 K, and a frequency dependence equal to that of the IF signal. Phenomenological expressions for conversion loss, noise, and IF impedance have been compared measurements and shows good agreement. The quasioptical NbN HEB mixers are still under development with respect to film quality and coupling to radiation, why the mixer performance of these is expected to improve.