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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Performance at 585 GHz of a Slot-Ring Antenna Coupled Niobium HEB Mixer Element for Imaging Applications

Lian Liu, H. Xu, Q. Xiao, Arthur W. Lichtenberger, Robert M. Weikle

Published Jan 18, 2006• 0 citations
Publisher

Abstract

585 GHz diffusion-cooled Nb HEB (d-HEB) mixers utilizing slot-ring antennas have been designed and fabricated on silicon wafers. To optimize power coupling from the antennas to the mixing elements, two impedance matching schemes are proposed. Performance at 585 GHz of the mixers including I-V curves, conversion gain and noise temperature are presented.

DOI
10.1109/icimw.2005.1572510

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•Jan 1, 1995•0 citations
585 GHz diffusion-cooled Nb HEB (d-HEB) mixers utilizing slot-ring antennas have been designed and fabricated on silicon wafers. To optimize power coupling from the antennas to the mixing elements, two impedance matching schemes are proposed. Performance at 585 GHz of the mixers including I-V curves, conversion gain and noise temperature are presented.
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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.
A quasi-optical SIS mixer designed for efficient radiation coupling is described. The mixer uses a twin-slot antenna which has the advantages of a good beam pattern and a low impedance. The radiation and impedance characteristics of the antenna were obtained from a moment-matched calculation. Tapered superconducting microstrip transmission lines are used to carry the radiation from the slot antennas to the tunnel junction. The effective impedance seen by the tunnel junction is quite low, about 4 Omega , which allows micron-size junctions to be used at 500 GHz. The mixers have been fabricated using Nb/Al-oxide/Nb tunnel junctions and a receiver noise temperature of 420 K (DSB) was measured at 490 GHz, which is the best yet obtained for a quasi-optical mixer at this frequency. The comparatively large junction area increases the mixer saturation power and allows strong suppression of noise from the Josephson effect by the application of a magnetic field of modest strength.>
We are developing improved versions of a slot antenna SIS mixer which we have previously described. The initial work demonstrated a double sideband noise temperature of 420 K for a 500 GHz quasi-optical SIS mixer employing a twin-slot antenna on a quartz dielectric substrate. A quartz hyperhemispherical lens is used to focus the incoming radiation onto the twin-slot antenna. The advantages of a twin-slot antenna include a low impedance (35 omega) and a clean, symmetric beam pattern into the dielectric with a 70 percent efficiency. In our original mixer, the radiation was coupled from the two slot antennas onto superconducting microstrip lines which fed the SIS junction. By performing an impedance transformation using tapered lines and by feeding the radiation from the two slots to the junction in parallel, the effective (real) impedance seen by the junction was reduced to just 4 omega. This very low impedance allowed a junction area of 2.3 sq micron to be used at 500 GHz, which was manufactured using optical lithography. However, no attempt was made to tune out the junction capacitance. We estimate that this capacitance reduces the impedance coupling efficiency to eta(sub Z) approx. equals 0.23, for our junction with omega R(sub N) C = 5.3 at 500 GHz. The recent development of techniques using electron-beam lithography to manufacture junctions with very small areas (approx. equals 0.1 sq microns) now allows considerably more flexibility in the design of SIS mixer circuits. We have redesigned the slot-antenna mixer to take advantage of this possibility. In particular, we have included a novel circuit which allows the junction capacitance to be tune out over a broad bandwidth. For instance, mixers designed for 800 GHz using NbN/MgO/NbN junctions with realistic parameters achieve a 3 dB impedance bandwidth of nearly 400 GHz. Furthermore, our circuit uses only short lengths of microstrip and should be less sensitive to RF losses than other designs. The improved impedance match should give a large reduction in noise temperature as compared to our previous mixer. The new devices are currently under fabrication. Further details of the design and any available experimental results are discussed.
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.