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Journal of Physics Conference Series

A superconducting flux-flow oscillator of terahertz range

Nickolay V. Kinev, Kirill I. Rudakov, Lyudmila V. Filippenko, Mikhail Yu. Fominskiy, Andrey M. Baryshev, V. P. Koshelets

Published Jun 1, 2020• 2 citations
Publisher

Abstract

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.

DOI
10.1088/1742-6596/1559/1/012021

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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.
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.
comprises in one chip a planar antenna integrated with a superconductor-insulator-superconductor (SIS) mixer, a superconducting Flux Flow Oscillator (FFO) acting as Local Oscillator (LO) and a second SIS harmonic mixer (HM) for the FFO phase locking. A new generation of the SIR devices with improved FFO performance and optimized interface between FFO and SIS/HM has been developed and comprehensively tested. To overcome temperature constraints and extend operation frequency of the all-Nb SIR we have developed and studied Nb-AlN-NbN circuits with a gap voltage Vg up to 3.7 mV and extremely low leakage currents (Rj/Rn> 30). A possibility to phase lock the Nb-AlN-NbN FFO at any frequency in the frequency range 350-720 GHz has been experimentally demonstrated. Phase–locked SIR operation over frequency range 450 – 700 GHz has been realized, spectral resolution below
The results of the development and testing of superconducting integrated microcircuits based on NbTiN/Al transmission lines at the frequencies up to 1.1 THz are presented. The integrated circuits consist of a slot planar antenna made of thin film of NbTiN coupled to a NbTiN/Al microstrip line, and a superconductor-insulator-superconductor (SIS) junction based on Nb/AlN/NbN operating as a terahertz (THz) detector. Two different designs having the operating range of 0.9-1.2 THz were numerically simulated, fabricated and experimentally tested. A strong pumping of SIS detector by a signal of the backward wave oscillator (BWO) around frequency of 1.05 THz was observed, which demonstrated the applicability of fabricated transmission lines at frequencies higher than 750 GHz, where the traditionally used Nb/Nb transmission lines cannot operate.
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.>
An integrated quasioptical receiver consisting of a planar double dipole antenna, superconductor-insulator-superconductor mixer and a superconducting local oscillator (LO) with matching circuits has been designed, fabricated and tested in the frequency range 360–490 GHz. A flux-flow oscillator (FFO) based on unidirectional and viscous flow of magnetic vortexes in a long Josephson tunnel junction, is employed as a local oscillator. All components of the receiver are integrated on a 4 mm×4 mm×0.2 mm crystalline quartz substrate using a single Nb–AlOx–Nb trilayer. The lowest DSB noise temperature of 470–560 K has been achieved within a frequency range of 425–455 GHz.
To develop an efficient local oscillator (LO) integrated with a SIS mixer operating above the gap frequency of Nb, we have investigated the radiation power P of NbN-based flux-flow-type Josephson oscillators (FFO's). The designed and fabricated chip incorporates FFO's, SIS power detectors (DET's), and their coupling circuits. Both FFO's and DET's consist of epitaxial NbN/AlN/NbN junctions with high critical current density J/sub C/ (15200 nW, enough for the optimum pumping of a SIS mixer with rf-resistance of 50 /spl Omega/, is coupled to DET's for 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 of 10-100 mA. The maximum coupling frequency of the present experiment is quantitatively agreed with the theoretical one. The peak power of 1.3 /spl mu/W is detected at 0.76 THz. The dissipated power in a FFO is smaller than 500 /spl mu/W, which is 10/sup -4/ times less than that of semiconductor sources. These results indicate the feasibility of NbN-based FFO's for a practical on-chip LO.