Tunability of Josephson junctions along with control on quasi-particle population in superconducting microbridges open exciting prospects in the future of the superconducting-metamaterial electronics.
1. Left-handed tunable transmission lines: towards tunable Veselago lens and RF cloaking
The cloaking device - the most popular example of an interesting microwave metamaterial - is a spatial filter providing specific distribution of microwaves. We have studied microwave properties of a superconducting tunable coplanar waveguide (CPW). Pairs of Josephson junctions, SQUIDs, provide tunable inductance, which varies in the range of 0.08-0.5 nH by changing an external dc magnetic field. The circuit is designed to have left- and right-handed transmission properties separated by a variable rejection band; the band edges can be tuned by the magnetic field. We present transmission measurements on CPWs based on up to 120 Nb-AlOx-Nb Josephson junctions at 8-11 GHz [1]. The experimental data provide a basis for extension of the approach towards 2-D transmission medium with controllable refraction needed for Veselago / cloaking effects.
2. Compact Multi-SQUID Split Ring Resonators
Tunable microwave filters are the basic devices in the high-frequency technology. We have proposed and studied experimentally compacted yet tunable superconducting split-ring resonators (SRRs) [2]. A fraction of SRR is replaced by Nb-AlOx-Nb SQUIDs, which inductance is sensitive to the external magnetic field. We present successful results of extensive EM-simulation and experimental data for the JSRR weakly coupled to a transmission line within frequency range 11-13 GHz. The suggested modification of traditional SRR into JSRR brings at least two advantageous features: i) the metallization of JSRR can be reduced at least by 6 times making SRR more compact at given frequency, thus reducing the density of metal in the artificial electromagnetic media, ii) the new device can be tuned by at least 10% in frequency using small magnetic fields.
3. Microwave-based TES circuits for electronic imaging
The electronic imaging at microwave and at far infrared is the fast growing field limited with capacity of their readout circuit. To improve the frequency division multiplexing in TES imaging arrays, it is suggested to replace commonly used SQUID amplifiers with a semiconductor high-frequency cooled amplifier. This would result in a single 10-GHz bandwidth amplifier serving the array of more than 1000 detectors. We suggested TES as a load for a high-Q resonator, weakly coupled to a microwave transmission line. The NEP as low as 2×10 19 W/Hz0.5 is our goal at temperature of 300 mK for submicron-size TES absorber made of Ti. To verify the new concept, prototype TES devices made of Nb are developed and tested above 4 K [3]. The NEP of about 2×10 14 W/Hz0.5 is estimated for the experimental micron-size prototype devices made of Nb at 4.5 K. Extending the initial concept we have design a superconducting 2-D meta-surface suitable for sensitive detection and imaging of small signals in frequency range 600-700 GHz [4]. This 2-D medium contains compact planar antennas densely distributed over hexagonal spot and coupled to individual resonators, providing a possibility for simultaneous readout of the pixel-antennas using FDM technique. The numerical results demonstrate the feasibility of such devices for application at THz. A patent application is currently in preparation.
4. Active superconducting metamaterials: array oscillator and amplifier
We develop two concepts for active superconducting metamaterials, which can generate and amplify the GHz-range signals, both are based on 2-D circuits with Josephson junctions. The superconducting oscillator is formed from a network of underdamped tunnel junctions tuned by applied magnetic field. The overdamped tunnel junctions are arranged into the distributed 2-D array thus creating the amplifying media [5]. Both circuits form a series-parallel connection of the Josephson junctions that is the way of improvement in both the saturation level and the impedance of a network-like media. It is estimated via a detailed electromagnetic model that the high-efficiency oscillator can perform at least up to 0.7 THz with tunability of about 2%; the quantum-limited-noise distributed rf SQUID-amplifier anticipated to operate over the band 8-12 GHz with gain as large as 10 dB. A patent application is currently in preparation.
[1] E. A. Ovchinnikova, S. Butz, P. Jung, V. P. Koshelets, L. V. Filippenko, A. S. Averkin, S. V. Shitov, A. V. Ustinov. Design and experimental study of superconducting left-handed transmission lines with tunable dispersion // Supercond. Sci. Techn.26 (2013) 114003, arXiv: 1309.7557 (2013).
[2] A. Vidiborskiy, V. P. Koshelets, L. V. Filippenko, S. V. Shitov, A. V. Ustinov. Compacted tunable split-ring resonators // Applied Physics Letters 103, 162602 (2013) http://dx.doi.org/10.1063/1.4826255, arXiv: 1308.4891 (2013)
[3]A. Kuzmin, S. V. Shitov, A. Scheuring, J. M. Meckbach, K. S. Il’in, S. Wuensch, A. V. Ustinov, M. Siegel. Development of TES Bolometers with High-Frequency Readout Circuit // IEEE Transactions on Terahertz Science and Technology. – January 2013. – DOI: http://dx.doi.org/10.1109/TTHZ.2012.2236148 .
[4] E.V. Erhan, S. V. Shitov, and A. V. Ustinov, Superconducting metamaterial for electronic imaging: Conceptual development, Extended abstract of the 7th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics - Metamaterials 2013, Bordeaux, France, 16-21 September 2013.
[5] S. V. Eismont, S. V. Shitov, and A. V. Ustinov, Superconducting active metamaterials: Conceptual development, Extended abstract of the 7th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics - Metamaterials 2013, Bordeaux, France, 16-21 September 2013.