Experimental Study of Frequency Multiplication in a Distributed Array of SIS Junctions

We report the first experimental off-chip detection of frequency multiplication in a distributed array of superconductor–insulator–superconductor (SIS) junctions. A test device consisting of series array of 68 Nb/Al-AlOx/Nb tunnel junctions was designed to study generation of the second harmonic in the 190–210 GHz band. The SIS array was exited with microwave radiation at 3-mm band using a quasi-optically coupled Gunn oscillator, and the output response of the device was studied using a double-sideband SIS mixer operating in the 163–211 GHz range with 4–8 GHz IF bandwidth. We measured extremely sharp spectral signals, associated with the $times 2$ frequency multiplication by the SIS array. Single and multi-photon processes were observed in the response of SIS tunnel junction-array to the applied microwave radiation, confirming device operation in the quantum mode. The output power of the multiplied signal increases linearly with the power of the pumping signal up to certain level and them saturates. In attempt to verify that the device produces noticeable power, the output of the test device was connected to the LO port of the SIS mixer, and an increase of 10%–20% in the SIS mixer dark current was observed. Further development of the demonstrated principle of frequency multiplication may lead to a practical frequency multiplier device

Mm-wave harmonic generation in an array of SIS junctions

We report the first experimental off-chip detection of frequency multiplication in a distributed array of Superconductor-Insulator-Superconductor (SIS) junctions. A test device consisting of series array of 68 Nb/Al-AlOx/Nb tunnel junctions was designed to study generation of the second harmonic. We measured extremely sharp spectral signals, associated with the × 2 frequency multiplication. Distinct single and multi-photon processes were observed in the test device response operated in quantum mode. The mechanism of device saturation was experimentally studied. The test device when connected to the input of an SIS mixer, and pumped, showed 10-20% increase in the SIS junction dark current

Towards quantum limited frequency multiplication

This paper presents a tentative modelling of Superconductor-Insulator-Superconductor (SIS) junction used as frequency multiplier. The model is tested against the experimental data presented earlier and demonstrates good agreement between the measured data and simulation. Such a model will improve understanding of the SIS junction used as frequency multiplier and hopefully lead us to demonstration of the performance for such multiplier

Harmonic and reactive behavior of the quasiparticle tunnel current in SIS junctions

In this paper, we show theoretically and experimentally that the reactive quasiparticle tunnel current of the superconductor tunnel junction could be directly measured at specific bias voltages for the higher harmonics of the quasiparticle tunnel current. We used the theory of quasiparticle tunneling to study the higher harmonics of the quasiparticle tunnel current in superconducting tunnel junction in the presence of rf irradiation. The impact of the reactive current on the harmonic behavior of the quasiparticle tunnel current was carefully studied by implementing a practical model with four parameters to model the dc I-V characteristics of the superconducting tunnel junction. The measured reactive current at the specific bias voltage is in good agreement with our theoretically calculated reactive current through the Kramers-Kronig transform. This study also shows that there is an excellent correspondence between the behavior of the predicted higher harmonics using the previously established theory of quasiparticle tunnel current in superconducting tunnel junctions by J.R. Tucker and M.J. Feldman and the measurements presented in this pape

Detection of 183 GHz water megamaser emission towards NGC 4945

Aim: The aim of this work is to search Seyfert 2 galaxy NGC 4945, a well-known 22 GHz water megamaser galaxy, for water (mega)maser emission at 183 GHz. Method: We used APEX SEPIA Band 5 to perform the observations. Results: We detected 183 GHz water maser emission towards NGC 4945 with a peak flux density of ~3 Jy near the galactic systemic velocity. The emission spans a velocity range of several hundred km/s. We estimate an isotropic luminosity of > 1000 Lsun, classifying the emission as a megamaser. A comparison of the 183 GHz spectrum with that observed at 22 GHz suggests that 183 GHz emission also arises from the active galactic nucleus (AGN) central engine. If the 183 GHz emission originates from the circumnuclear disk, then we estimate that a redshifted feature at 1084 km/s in the spectrum should arise from a distance of 0.022 pc from the supermassive black hole (1.6 x 10(5) Schwarzschild radii), i.e. closer than the water maser emission previously detected at 22 GHz. This is only the second time 183 GHz maser emission has been detected towards an AGN central engine (the other galaxy being NGC 3079). It is also the strongest extragalactic millimetre/submillimetre water maser detected to date. Conclusions: Strong millimetre 183 GHz water maser emission has now been shown to occur in an external galaxy. For NGC 4945, we believe that the maser emission arises, or is dominated by, emission from the AGN central engine. Emission at higher velocity, i.e. for a Keplerian disk closer to the black hole, has been detected at 183 GHz compared with that for the 22 GHz megamaser. This indicates that millimetre/submillimetre water masers can indeed be useful probes for tracing out more of AGN central engine structures and dynamics than previously probed. Future observations using ALMA Band 5 should unequivocally determine the origin of the emission in this and other galaxies.Comment: 4 pages, accepted by A&A Letter

A Technology Demonstrator for 1.6–2.0 THz Waveguide HEB Receiver with a Novel Mixer Layout

In this paper, we present our studies on a technology demonstrator for a balanced waveguide hot-electron bolometer (HEB) mixer operating in the 1.6–2.0 THz band. The design employs a novel layout for the HEB mixer combining several key technologies: all-metal THz waveguide micromachining, ultra-thin NbN film deposition and a micromachining of a silicon-on-insulator (SOI) substrate to manufacture the HEB mixer. In this paper, we present a novel mixer layout that greatly facilitates handling and mounting of the mixer chip via self-aligning as well as provides easy electrical interfacing. In our opinion, this opens up a real prospective for building multi-pixel waveguide THz receivers. Such receivers could be of interest for SOFIA, possible follow up of the Herschel HIFI, and even for ground based telescopes yet over limited periods of time with extremely dry weather (PWV less than 0.1 mm)

Superconducting 4-8-GHz Hybrid Assembly for 2SB Cryogenic THz Receivers

We present here the design and characterization of an intermediate frequency (IF) assembly comprising a compact 90 hybrid chip (coupled line coupler - Lange coupler- coupled line coupler), two bias-T circuits for biasing the superconductor-insulator-superconductor (SIS) mixers, and two transmission-line circuits. Specifically, the miniaturized three-section hybrid chip fabricated using thin-film technology utilizes superconducting Niobium (Nb) transmission lines, air bridges to connect the fingers of the Lange coupler (middle section), and is complemented with two bias-T circuits with integrated MIM capacitors. The assembly was designed to ensure amplitude and phase imbalances better than 0.6 dB and +/- 2 degrees, respectively. Experimental verification of the assembly at 4 K shows good agreement between the measurements and simulations with amplitude imbalance of 0.5 dB and maximum phase imbalance of +/- 2 degrees. The ALMA band-5 (163-211 GHz) receiver will include such assembly. The receiver tests shows sideband rejection ratio better than 15 dB over the entire RF band, i.e., a systematic improvement of 3-9 dB as compared with the previously reported results

Terahertz components packaging using integrated waveguide technology

We present an integrated waveguide based packaging solution compatible with different THz component technologies, both for room temperature and cryogenic operations, employing space-qualified wire-bonding for electrical contacts. The proposed waveguide packaging relies on the combination of all-metal micro-machined THz waveguide and active component chip layouts suitable for the realization of systems from 200 up to 5000 GHz. It provides possibility of making 3-dimensional structures via facilitating of multi-level (layered) designs. The surface roughness of the fabricated THz waveguide structure was demonstrated to be 20 nm, while a 2 μm alignment accuracy of the active component chip was verified. \ua9 2011 IEEE

Dependence of the scatter of the electrical properties on local non-uniformities of the tunnel barrier in Nb/Al-AlOx/Nb junctions

In this paper, we study the effect of the tunnel barrier thickness non-uniformity in Nb/Al-AlOx/Nb tunnel junctions using the measurement results of the junctioncapacitance (C) and the normal resistance (Rn). The local thickness distribution of the AlOx tunnel barrier in Nb/Al-AlOx/Nb trilayer (RnA ∼ 30 Ω μm2) was studied by high resolution transmission electron microscopy. The specific resistance (RnA) values of the measured junctions range from 8.8 to 68 Ω μm2. We observed scatter in both the junction specific resistance and capacitance data, which is considerably higher than the measurement uncertainty. We also observed noticeable scatter in the RnC product, which does not stem from junction area estimation uncertainties. We discuss the possible reasons that contribute to this scatter. We suggest that the local thickness non-uniformity of the tunnel barrier significantly contributes to the scatter in the RnC product. We confirm this conclusion through an illustrative model based on the barrier imaging data, which results in the variation of the RnC data consistent with the measurements in this paper

Ultra-Wideband 90 degrees Waveguide Twist for THz applications

We report on the design, fabrication, and characterization of a novel 90\uba waveguide step twist with 56% fractional bandwidth. The proposed twist provides 90\uba field rotation in the frequency range 210-375 GHz. The experimental results are in good agreement with the electromagnetic simulations showing an insertion loss below 0.3 dB and a return loss better than 20 dB over most of the band. The ultra-wideband performance, tolerance to fabrication inaccuracies and compactness makes the proposed design attractive for various mm and sub-millimeter applications. The twist geometry is optimized for simple fabrication through direct milling. Furthermore, the design remains highly compact since both steps are fabricated on a single washer