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

Submillimeter wave superconducting flux-flow oscillator

K. Sakai

Published Dec 12, 2017• 0 citations
Publisher

Abstract

As a candidate for compact submillimeter oscillators, we have fabricated the flux-flow oscillator (FFO) connected with a bow-tie antenna and with a SIS detector by using the Nb/AlOx/Nb tunnel junction technology. We have obtained the guarantee of exact oscillation of the FFO in the frequency range between 160 GHz and 309 GHz by means of the observation of velocity-matching steps and the photon-assisted tunneling steps.

DOI
10.1117/12.2301676

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All-refractory material superconducting millimeter-wave oscillators have been designed and investigated experimentally with different superconductor-insulator-superconductor (SIS) mixers integrated on the same chip. Tested structures include a flux-flow oscillator (FFO) based on a long Josephson junction, a coupling section, and an SIS detector with tuned out junction capacitance. Coupling sections were designed as multistep microstrip quarter-wave impedance transformers. All junctions have been fabricated on the basis of a high-quality trilayer Nb-AlO/sub x/-Nb process. Microwave oscillations in the frequency range 75-500 GHz have been detected experimentally. The level of delivered power was estimated from the pumped I-V curve of the strongly coupled single junction detector. Coupled power levels higher than 0.1 mu W at 256 GHz were achieved. A spectral linewidth of the FFO of less than 1 MHz has been estimated experimentally. The first attempt to create an integrated receiver based on an FFO and an SIS array mixer integrated on the same chip was made in the 2-mm wavelength band.>

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IEEE Transactions on Applied Superconductivity•Jun 1, 1999•8 citations
As a candidate for compact submillimeter oscillators, we have fabricated the flux-flow oscillator (FFO) connected with a bow-tie antenna and with a SIS detector by using the Nb/AlOx/Nb tunnel junction technology. We have obtained the guarantee of exact oscillation of the FFO in the frequency range between 160 GHz and 309 GHz by means of the observation of velocity-matching steps and the photon-assisted tunneling steps.
A reliable technique for wide band measurements of the spectral linewidth of superconducting oscillators integrated on-chip with superconductor-insulator-superconductor (SIS) detectors has been developed. The spectral linewidth of flux-flow oscillators (FFO) based on the unidirectional and viscous flow of magnetic vortices in a long overdamped Josephson tunnel junction was measured in the frequency range 250–580 GHz, and a linewidth as low as 200 kHz was obtained at 450 GHz. Also stable frequency locking of a FFO to very high (≤60th) harmonics of an external microwave reference source has been demonstrated. The proposed technique may improve the sensitivity, frequency resolution, and stability of the fully superconducting integrated submillimeter wave receiver.
Abstract We have elaborated, fabricated and tested a THz source radiating to open space based on the superconducting flux-flow oscillator (FFO). In this concept, the oscillator is integrated with the transmitting lens antenna based on a slot structure in Nb film with a thickness of ~200 nm located on the same chip. The slot planar antenna is matched to the oscillator (by input) and to the semielliptical Si lens with a diameter of 10 mm (by output) providing a narrow output beam of THz emission. A harmonic mixer based on the superconductor-insulator-superconductor junction embedded in the “FFO and antenna” integrated structure has been used for the phase locking of the oscillator. Several designs of antenna coupled with the oscillator by microstrip lines have been numerically simulated, and the batches of experimental samples based on Nb-AlN-NbN superconducting trilayers with Rn ·A ~ 20 Ω·μm2 (jc ~ 10 kA/cm2) have been fabricated and tested. Two different setups were used for experimental study: a THz spectrometer based on the SIS receiver with a high spectral resolution (better than 0.1 MHz) and a Si bolometer. The overall operating range of 250 to 700 GHz is covered by all the developed designs.
All-refractory material superconducting millimeter-wave oscillators have been designed and investigated experimentally with different superconductor-insulator-superconductor (SIS) mixers integrated on the same chip. Tested structures include a flux-flow oscillator (FFO) based on a long Josephson junction, a coupling section, and an SIS detector with tuned out junction capacitance. Coupling sections were designed as multistep microstrip quarter-wave impedance transformers. All junctions have been fabricated on the basis of a high-quality trilayer Nb-AlO/sub x/-Nb process. Microwave oscillations in the frequency range 75-500 GHz have been detected experimentally. The level of delivered power was estimated from the pumped I-V curve of the strongly coupled single junction detector. Coupled power levels higher than 0.1 mu W at 256 GHz were achieved. A spectral linewidth of the FFO of less than 1 MHz has been estimated experimentally. The first attempt to create an integrated receiver based on an FFO and an SIS array mixer integrated on the same chip was made in the 2-mm wavelength band.>
The first object of this work is to theoretically study the effects of tunnel junction parameters on flux-flow Josephson oscillators (FFOs) by numerical simulation and to experimentally study the rf properties of FFOs by integrating them together with an SIS (superconductor-insulator-superconductor) mixer/detector on a chip. By using a CAD modeled integrated sub-mm wave receiver, the composite linewidth of two FFOs was measured to be less than 2.1 MHz in the band 280-330 GHz. The receiver consists of two identical FFOs and an SIS mixer. The linewidth was determined by mixing the two signals in the SIS mixer and analyzing the intermediate frequency. The output power coupled to a 10 .OMEGA. microstripline was about 0.5 µW, and could be adjusted by changing the current bias of the oscillator. The FFOs are connected to the SIS mixer via interdigital capacitors. The FFOs and the SIS element are made of Nb/NbOx/PbBi junctions by the "reactive ion beam oxidation" technique with the critical current density jc controlled by the oxidation time. Lift-off stencils for defining the junction sizes and the top-electrodes were made by using electron-beam writing on PMMA/Copolymer double layer resist. The jc of a long Josephson junction (LJJ) determines the fluxon motion. For LJJs which had jc > 1 kA/cm2, velocity-matching (VM, where the phase velocity of propagating fluxons equals that of the electromagnetic wave in the LJJ) steps could be tuned linearly from about 0.2 to 1.4 mV by an applied magnetic field. Coupled radiation at around 100 GHz (about 3 nW), 261 GHz (4.6 nW), and 330 GHz (0.43 µW), from flux-flow oscillations, was observed. From the step's steepness, the junction quality factor QJ is determined. QJ is between 4 to 7 in the voltage range of 0.4 to 1 mV. Coupled radiation (> 3 nW) at around 100 GHz corresponding to resonant soliton oscillation in low jc samples (250 to 1000 A/cm2) was detected. Simulations show that, for a uniform bias feed, the height of the VM step .gammam increases almost linearly with the external magnetic field .beta.e and the quasiparticle loss .alpha., but is less dependent on the surface loss .beta.. An approximate expression .gammam = a + .alpha(1+b).beta.e agrees with simulations. The maximum ac output voltage decreases with increasing dampings .alpha. and .beta.. It is important to include the .beta.-term in the mathematical model. The second part of this work is devoted to rf properties of long YBa2Cu3O7-.delta. (YBCO) grain boundary junctions made on 0- 32 °. [001]-tilt YSZ bicrystals. Josephson flux-flow resonances and Fiske resonances were observed. The first resonances could be tuned from about 0.5 to 2.0 mV in a 30 µm long junction at 16 K, and we determined the ratio between barrier thickness and relative dielectric constant td/.epsilon.r=2.9 Å and the loss .alpha.= 1.3 at 16 K. Simulations support the experimental observation that Josephson vortices can propagate unidirectionally in an overdamped, homogeneous junction up to .alpha.Ã? 2. From the Fiske resonances at 256-638 GHz, their Q-values, and the Steward-McCumber parameter, we find td/.epsilon.r=4 Å, the London penetration depth .gamma.L=145±5 nm and the surface resistance RS ~20 m.OMEGA. at 280 GHz. We conclude that the boundary is dielectric and the YBCO grains are well superconducting within 5-20 nm from the boundary. The dielectric between the grains could be part of a tunnel barrier and/or a matrix for filamentary conduction. Josephson flux-flow transitors based on long bicrystal junctions were made with an Au loop used as the control line. The line modulated the critical current Ic. Current gain larger than unity was obtained below 70 K. The gain increased with the junction length and varied roughly like the square root Ic when adjusting the temperature. A transresistance larger than 5 .OMEGA. was obtained at 15 K. Better performance is expected from junctions with high current densities.
A Superconducting Integrated Receiver (SIR) comprises in a single chip a planar antenna combined with a superconductor-insulator-superconductor (SIS) mixer, a superconducting Flux Flow Oscillator (FFO) acting as a Local Oscillator (LO) and a second SIS harmonic mixer (HM) for the FFO phase locking. In this report an overview of the FFO development and optimization is presented. In order to overcome temperature constraints and extend operation frequency of the fully Nb SIR we have developed and studied Nb-AlN-NbN circuits with a gap voltage Vg up to 3.7 mV and extremely low leak currents (Rj/Rn > 30). Continuous tuning of the frequency due to bending and overlapping of the Fiske steps and a possibility to phase lock the Nb-AlN-NbN FFO at any frequency in the range 350-750 GHz has been experimentally demonstrated. After optimization of the FFO design the free-running linewidth between 7 and 1.5 MHz has been measured in the frequency range 350 - 750 GHz, which allows to phase-lock from 50 to 95 % of the emitted FFO power. New designs of the FFO intended for further improvement of its parameters are under development, but even at the present state the Nb-AlN- NbN FFOs are mature enough for practical applications. These achievements enabled development of a 500 - 650 GHz integrated receiver for the Terahertz Limb Sounder (TELIS) project intended for atmosphere study and scheduled to fly on a balloon in 2008.
The Josephson Flux-Flow Oscillator (FFO) has been used as an on chip local oscillator at frequencies up to 650 GHz. The FFO linewidth of about 1 MHz was measured in the resonant regime V<915 /spl mu/V for niobium-aluminum oxide-niobium tunnel junctions, while considerably larger values were reported at higher voltages. To overcome this fundamental linewidth broadening we propose a novel on chip Cherenkov radiation flux-flow oscillator (CRFFO). It consists of a long Josephson junction and a superconducting slow wave transmission line that modifies essentially the junction dispersion relation. Two SIS detectors are connected both to the long Josephson junction and the transmission line to evaluate available microwave power. The output power coming both from the long junction and the transmission line is estimated at different bias conditions.