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IEEE MTT-S International Microwave Symposium digest

Low noise direct detector at submillimeter wavelength using SIS junction [Nb/AlO/sub x//Nb]

Alexandre Karpov, J. Blondel, M. Voß, K. H. Gundlach

Published Nov 22, 2002• 0 citations
Publisher

Abstract

We have demonstrated the feasibility of a broad band SIS direct detector with a bandwidth up to 70 GHz around 330 GHz. The SIS detector responsivity approaches the quantum limit. The optical noise of 13 mK/spl radic/s measured with the new direct detector receiver is close to the noise of bolometer receiver developed for radioastronomy at the same wavelength. The SIS direct detector works at 4.5 K instead of 0.1-0.3 K required for bolometer receiver.

DOI
10.1109/mwsym.1997.602949

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We have demonstrated the feasibility of a broad band SIS direct detector with a bandwidth up to 70 GHz around 330 GHz. The SIS detector responsivity approaches the quantum limit. The optical noise of 13 mK/spl radic/s measured with the new direct detector receiver is close to the noise of bolometer receiver developed for radioastronomy at the same wavelength. The SIS direct detector works at 4.5 K instead of 0.1-0.3 K required for bolometer receiver.
A new type of ultra broad band SIS low noise radiometer has been developed. The SIS receiver instantaneous band of about 30% is significantly increased compared to traditional designs. We avoid a restriction related to the limited band of the Intermediate Frequency (IF) low noise amplifier by using a multifrequency heterodyne power source. In this regime the frequency mixing is performed simultaneously in a big number of the subbands and the IF signals are combined in a common amplifier. A low noise operation in this regime is possible due to the quantum nature of the frequency mixing in SIS quasi particle tunnel junctions. The SIS receiver covers 25 GHz band centered at 90 GHz with an equivalent noise temperature of about 50 K. The receiver uses a fixed tuned SIS mixer with Nb/Al Ox/Nb junctions. The critical Josephson current density is about 2.6 KA/cm/sup 2/ and the junction area is of about 1.5 /spl mu/m/sup 2/. The limits to further improvement of the radiometer and the possibilities for extension of the technique to the sub millimeter band are discussed.
An ultra-low noise single sideband SIS receiver has been prepared for radio astronomy at the sub millimeter wavelength /spl lambda//spl ap/0.85 mm. The minimum single sideband receiver noise temperature of 48 K corresponds to 3 h/spl omega//k or equivalent number of 3 photon of noise. The minimum single sideband SIS mixer noise temperature is about 20 K, close to 1.2 h/spl omega//k, or one photon of noise. The Nb/AlOx/Nb junctions with a Josephson critical current density of 9 KA/cm/sup 2/ and with the area of about 0.9 /spl mu/m/sup 2/ were used. The receiver has been tested at the 30 meter IRAM radio telescope in the winter seasons of 1997 and 1998. The observations at the radio telescope are speeded up by a factor of two to three with the new receiver.
We describe quasi-optical SIS mixers operating in the submillimeter band (500-750 GHz) which have very low noise, around 5 h/spl nu//k/sub B/ for the double-sideband receiver noise temperature. The mixers use a twin-slot antenna, Nb/Al-Oxide/Nb tunnel junctions fabricated with optical lithography, a two-junction tuning circuit, and a silicon hyperhemispherical lens with a novel antireflection coating to optimize the optical efficiency. We have flown a submillimeter receiver using these mixers on the Kuiper Airborne Observatory, and have detected a transition of H/sub 2//sup 18/O at 745 GHz. This directly confirms that SIS junctions are capable of low-noise mixing above the gap frequency.>
We study noise and conversion gain in the SIS mixers with Nb/MgO/NbN junctions at the millimeter and submillimeter wavelengths in order to estimate the possibility to extent the low noise operation towards the theoretical limit of 4/spl Delta//h=2.4 THz. We present as preliminary result the lowest achieved receiver noise with NbN SIS junctions of 65 K with conversion gain of -6 dB at 162 GHz and 230 K receiver noise with conversion gain of -10 dB at 306 GHz. The junction area is 2 /spl mu/m/sup 2/ and Josephson critical current density is 1.4 KA/cm/sup 2/. The optimum R/sub N//spl omega/C of NbN junction for mixers is estimated as 600 GHz/V. In preliminary tests the junctions R/sub N//spl omega/C is 10-20 time superior to the optimum and a significant improvement of NbN SIS mixers may be expected. The noise sources in the NbN SIS junctions are studied and a possible mixer sensitivity improvement is discussed. The thermal properties of the SIS NbN are studied. The frequency limit of the low loss integrated tuning structure in NbN is estimated as at least as 1.5 THz.
We have designed, fabricated and evaluated a quasi-optical SIS receiver with NbN/AlN/NbN tunnel junctions in the submillimeter-wave region. The receiver consists of a planar self-complementary log-periodic antenna and MgO hyperhemispherical lens with an anti-reflection cap for RF optics, NbN SIS junctions and a radial short stub tuner to resonate out its capacitance for RF matching. The prepared NbN SIS junction has a current density of 20 kA/cm/sup 2/ and is about 1 /spl mu/m in diameter, supplying a small /spl omega/C/sub J/R/sub N/ product (/spl omega/C/sub J/R/sub N/=3 at 300 GHz). From 297 to 356 GHz the average double sideband (DSB) receiver noise temperature measured by using the standard Y-factor method was about 200 K. The lowest receiver noise temperature, 167 K (DSB), was obtained at around 303 GHz. These results suggest that our NbN SIS junctions can be used for terahertz mixer elements instead of Nb SIS junctions.
We report on the development of low-noise quasi-optical SIS mixers for the frequency range 400-850 GHz. The mixers utilize twin-slot antennas, two-junction tuning circuits, and Nb-trilayer junctions. Fourier-transform spectrometry has been used to verify that the frequency response of the devices is well predicted by computer simulations. The 400-850 GHz frequency band can be covered with four separate fixed-tuned mixers. We measure uncorrected double-sideband receiver noise temperatures around 5h/spl nu//k/sub B/ to 700 GHz, and better than 540 K at 808 GHz. These results are among the best reported to date for broadband heterodyne receivers.