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IEEE Transactions on Applied Superconductivity

Fabrication of all-NbCN flux-flow oscillators with a tapered slotline antenna and off-chip detection of their radiation power

Shogo Kiryu, Satoshi Kohjiro, Alexandre Shoji, Satoshi Kodaira, K. Kawaguchi, Junji Inatani

Published Jun 1, 1995• 2 citations
Publisher

Abstract

All-NbCN flux-flow oscillators (FFO) with tapered slotline antenna (TSA) were fabricated on MgO substrates. A five-step microstrip transformer was used to couple an FFO, which has a low impedance, to a TSA having a high impedance, The microstrip transformer is designed to have a return loss more than 20 dB at the center frequency. We have measured submillimeter wave radiation from the oscillators using an off-chip silicon-composite bolometer. Radiation from a fabricated FFO was detected in a bias voltage range from 1.18 mV to 1.48 mV, which corresponds to a frequency range from 570 GHz to 710 GHz.>

DOI
10.1109/77.403212

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IEEE Transactions on Applied Superconductivity•Mar 1, 1993•67 citations
All-NbCN flux-flow oscillators (FFO) with tapered slotline antenna (TSA) were fabricated on MgO substrates. A five-step microstrip transformer was used to couple an FFO, which has a low impedance, to a TSA having a high impedance, The microstrip transformer is designed to have a return loss more than 20 dB at the center frequency. We have measured submillimeter wave radiation from the oscillators using an off-chip silicon-composite bolometer. Radiation from a fabricated FFO was detected in a bias voltage range from 1.18 mV to 1.48 mV, which corresponds to a frequency range from 570 GHz to 710 GHz.>
Flux-flow-type Josephson oscillators consisting of epitaxial NbCN/MgO/NbCN junctions were fabricated and their current-voltage (I-V) characteristics were measured in various magnetic fields to evaluate their oscillation characteristics. On the I-V curve for a fabricated oscillator, clear current steps were observed up to 3 mV, indicating that oscillation occurs up to 1.4 THz. Using a theoretical model, available output power for the oscillator was calculated to be in the range from 20 mu W to 40 mu W in the frequency range from 500 GHz to 1.2 THz.>
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.
To develop an efficient local oscillator (LO) operating above 0.7 THz, the gap frequency of Nb, on a same chip with a SIS mixer, we have investigated the radiation power P of NbN-based flux-flow-type Josephson oscillators (FF0s). The designed and fabricated chip incorporates FF0s, SIS power detectors (DETs), and their coupling circuits. Both FFOs and DETs consist of epitaxial NbNIA1N/NbN junctions with high critical current density (15 200 nW, enough for the optimum pumping of a SIS mixer with if-resistance of 50 Q, is coupled to DETs in the frequency range of 0.5-0.9 THz. The coupling bandwidth is larger than 20% of its central frequency. In the band, the radiation frequency is tuned by the control current through the FFO in the range of 10-100 mA. The peak power >1 [AV is detected at 0.76THz. In addition, the dissipated power in a FFO is smaller than 500 [LW, which is less than 10 -4 of that of semiconductor sources. These experimental results indicate that FFOs with high-fc NbN/A1N1NbN junctions are applicable for an on-chip LO above 0.7 THz.
Radiation from a linear array oscillator with a spiral antenna has been measured using an off-chip coupled bolometer. The array oscillator has a distributed array structure with 300 resistively shunted junctions which are placed in the groups of 6 junctions. The array oscillator was fabricated on a Si wafer with a two-arm spiral antenna. The chip was attached to a Si lens with an anti-reflection coating. Rather large power (>100 nW) was detected at a number of array bias voltages V/sub bias/ from 115 mV to 196 mV, which correspond from 185 GHz to 315 GHz. The maximum measured power was 550 nW at V/sub bias/=125 mV (200 GHz).
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.>