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Josephson junctions (JJs) with non-hysteretic I-V characteristic and high critical parameters are required for many applications; in particular, they are needed to create tunable THz range oscillators. We fabricated and studied shunted superconductor-insulator-superconductor (SIS) Nb-AlOx-Nb tunnel junctions with an area of about 1m2, estimated their parameters, and compared them with theoretical models. To assess the performance of shunted junctions in the terahertz range, their characteristics were studied under the influence of a high-frequency signal; the estimated value of the characteristic voltage for the shunted junction at a high frequency is about 1 mV. To improve the circuit parameters, a new type of the shunting based on application of NIS junctions was proposed, which reduces the influence of parasitic inductance, expands the frequency range of the generator, and decreases the size of the structure. The JJs with a new type of shunting have been fabricated and studied. The niobium integrated circuits intended for matching the superconducting oscillator and an SIS junction as a microwave detector were designed, fabricated, and tested. Such structures are required for studying the spectral characteristics and analyzing the operation of the oscillator. The developed circuits have demonstrated efficient operation at frequencies from 300 to 700 GHz; the experimental results are in good agreement with the simulated characteristics. The results obtained will be implemented for further development of the THz range oscillators.

The developments in the field of low-noise receiving systems in the sub-THz range, performed at the Kotelnikov Institute of Radio Engineering of the Russian Academy of Sciences in recent years and aimed at the creation of receivers with quantum sensitivity for space and ground-based radio telescopes are presented. Superconductor–insulator–superconductor (SIS) mixers based on high-quality tunnel junctions are key elements of the most sensitive sub-THz heterodyne receivers. The article presents the results of the development of SIS receivers in the 211–275 GHz and 800–950 GHz ranges with a noise temperature in the double-sideband (DSB) mode of about 20 K and 220 K, respectively. These achievements will be used to develop receiving systems for APEX and LLMA ground-based telescopes, as well as for the Millimetron space mission.

We present evidence that in the sub-THz frequency band, human skin can be considered as an electromagnetic bio-metamaterial in which its natural emission is a product of skin tissue geometry and embedded structures. Radiometry was performed on 32 human subjects from 480 to 700 GHz. Concurrently, the subjects were exposed to stress, while heart pulse rate (PS) and galvanic skin response (GSR) were also measured. The results are substantially different from the expected blackbody radiation signal of the skin surface. PS and GSR correlate to the emissivity. Using a simulation model for the skin, we find that the sweat duct is a critical element. The simulated frequency spectra qualitatively match the measured emission spectra and show that our sub-THz emission is modulated by our level of mental stress. This opens avenues for the remote monitoring of the human state.

Superconducting integrated circuits based on high-quality Nb–AlOx–Nb and Nb–AlN–NbN tunnel junctions, including a harmonic mixer with a high harmonic number, have been developed, optimized, and investigated. Details of the design, manufacturing methods, and specificities of operation of superconducting elements and circuits for the detection and study of terahertz radiation from cryogenic integrated oscillators are presented. The signal of an integral terahertz oscillator was detected, its power was estimated, and the radiation spectrum was measured with a resolution of about 1 Hz. The synchronization of a superconducting oscillator at any frequency in the range 250–750 GHz with a spectral quality higher than 50% has been realized, and the phase noise of the generator in the PLL mode has been measured.

We report on a comparative analysis of three different laboratory terahertz (THz) sources emitting into open space based on a backward wave oscillator with a frequency multiplier, a microwave frequency multiplier to large numbers of harmonics based on semiconductor superlattices, and a recently developed long Josephson junction-based oscillator. The weight‒size parameters, frequency responses, and radiation powers of the sources have been qualitatively and quantitatively compared, along with the complexity of their tuning and operation under laboratory conditions.

We present an experimental observation and a study of harmonics of radiation from a flux-flow oscillator (FFO) based on a long Josephson junction. An integrated microcircuit consisting of the FFO, the transmitting antenna and a harmonic mixer (HM) was used to provide the phase-locked emission in the terahertz (THz) range to open space. Both the FFO and the HM were made of superconductor–insulator–superconductor (SIS) trilayers based on Nb/AlOx/Nb. Two independent techniques were used for detecting of the output emission: a THz Fourier transform spectrometer with a wideband detector based on an 4.2 K silicon bolometer, and a THz spectrometer based on the heterodyne SIS receiver with a high spectral resolution. The FFO spectral composition obtained using the FTS demonstrates the main Josephson frequency and clear higher harmonics. Following that, the spectral characteristics of the 2nd harmonic at a frequency of 600–670 GHz (corresponding to the main frequency of 300–335 GHz) were carefully studied with a spectral resolution better than 0.1 MHz using the SIS receiver. To our knowledge, this is the first direct high-frequency observation of Josephson harmonics carried out at the true frequency of oscillations, which is in contrast to dc measurements.

We give a review of both our own original scientific results of the development of superconducting receivers for sub-terahertz astronomy and the main leading concepts of the global instrumentation. The analysis of current astronomical problems, the results of microwave astroclimate research, and the development of equipment for sub-terahertz radio astronomy studies justify the need and feasibility of a major infrastructure project in Russia to create a sub-terahertz telescope, as well as to enhance the implementation of the ongoing Millimetron and Suffa projects. The following results are discussed: i) superconducting coherent receivers and broadband subterahertz detectors for space, balloon, and ground-based radio telescopes have been developed and tested; ii) ultrasensitive receiving systems based on tunnel structures such as superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal metal-insulator-superconductor (SINIS) have been created, fabricated, and examined; iii) a receiving array based on SINIS detectors and microwave readout system for such structures has been implemented; iv) methods for manufacturing high-quality tunnel structures Nb/AlOx/Nb and Nb/AlN/NbN based on niobium films with a current density of up to 30 kA/cm(2) have been developed. Receivers operated at 200 to 950 GHz and having a noise temperature only a factor of 2 to 5 higher than the quantum limit have been created and tested.

We describe the performance of a superconductor–insulator–superconductor (SIS) mixer operating in the frequency range of 780–950 GHz. Unlike most SIS mixers, the tunnel junction employs two different superconductors, a niobium nitride top and a niobium bottom electrode sandwiching an aluminum nitride barrier layer, fabricated on a niobium titanium nitride ground plane. The mixer was tested in a pulse tube cryostat, with all the optical components, in the signal path, mounted inside the vacuum environment to avoid attenuation of the RF signal as it propagates from the hot/cold loads to the mixer. With this setup, we have measured an RF-corrected noise temperature of $\sim$ 220 K. In this article, we focus on investigating the influence of local oscillator (LO) power heating on the performance of the terahertz mixer. The increase in the junction's physical temperature can be observed experimentally by noting the suppression of the gap voltage in the pumped current–voltage ( $I\text{--}V$ ) curve as the LO pumping level is increased. Similar observation has already been reported, and attempts were made to estimate the effective temperature of the device using equations of heat transfer between the mixer chip layers. Here, we present an experimental method of quantifying this effect by recovering the effective temperature of the junction through comparing the pumped $I\text{--}V$ curves at different pumping levels and fixed bath temperature, with the unpumped $I\text{--}V$ curves obtained at varying bath temperatures. We also estimate, for the first time, the effect of heating on the noise temperature as a function of bath temperature and frequency. We show that for typical experimental parameters, the LO heating can increase the double-sideband receiver noise temperature by as much as 20%, and that in the frequency range of the measurements, the effective temperature of the junction at fixed LO power increases linearly with frequency at a rate of 0.5 K/100 GHz.

The main parameters of the tunnel barrier of Josephson Nb/AlOx/Nb and Nb/AlN/Nb junctions are estimated in a wide range of the current density using the Simmons method. The dependences of the tunnel barrier height and width of the resistivity are determined experimentally for each type of the junctions. A decrease in the tunnel barrier height of the junction with the AlN interlayer by 0.3 eV as compared to an oxide junction enables us to obtain the junctions with a current density higher than 15 kA/cm2 at an insulating layer thickness of 10 A allowable technologically, which gives the possibility to obtain quality parameter Rj/Rn no lower than 25.

An electromagnetic source of terahertz radiation on the basis of a tunnel long Josephson junction was proposed and experimentally studied. Output radiation was transmitted to open space by means of a slot antenna, which was located on the same microcircuit with the junction and matched with the collecting lens. Several Oscillator structures designed for operation within the frequency ranges of 250–410, 330–530, and 390–700 GHz with the possibility of continuous frequency tuning were manufactured and tested. The amplitude-frequency characteristics of output radiation with a spectral resolution of nearly 0.1 MHz were studied with the use of a terahertz-spectrometer based on a superconducting receiver. The study of transmitting antenna characteristics throughout the entire bandwidth was performed with the use of a cooled highly sensitive silicon based bolometer. The experimental results correspond to numerical calculations.

Samples of superconductor–insulator–superconductor (SIS) and normal metal–insulator–superconductor (NIS) junctions with superconducting aluminum of different thickness were fabricated and experimentally studied, starting from conventional shadow evaporation with a suspended resist bridge. We also developed alternative fabrication by magnetron sputtering with two-step direct e-beam patterning. We compared Al film grain size, surface roughness, resistivity deposited by thermal evaporation and magnetron sputtering. The best-quality NIS junctions with large superconducting electrodes approached a resistance R(0)/R(V2Δ) factor ratio of 1000 at 0.3 K and over 10,000 at 0.1 K. At 0.1 K, R(0) was determined completely by the Andreev current. The contribution of the single-electron current dominated at V > VΔ/2. The single-electron resistance extrapolated to V = 0 exceeded the resistance R(V2Δ) by 3 × 109. We measured the influence of the magnetic field on NIS junctions and described the mechanism of additional conductivity due to induced Abrikosov vortices. The modified shape of the SINIS bolometer IV curve was explained by Joule overheating via NIN (normal metal–insulator–normal metal) channels.

Mixers based on superconductor–insulator–superconductor (SIS) tunnel junctions are the best input devices at frequencies from 0.1 to 1.2 THz. This is explained by both the extremely high nonlinearity of such elements and their extremely low intrinsic noise. Submicron tunnel junctions are necessary to realize the ultimate parameters of SIS receivers, which are used as standard devices on both ground and space radio telescopes around the world. The technology for manufacturing submicron Nb–AlN–NbN tunnel junctions using electron-beam lithography was developed and optimized. This article presents the results on the selection of the exposure dose, development time, and plasma chemical etching parameters to obtain high-quality junctions (the ratio of the resistances below and above the gap Rj/Rn). The use of a negative-resist ma-N 2400 with lower sensitivity and better contrast in comparison with a negative-resist UVN 2300-0.5 improved the reproducibility of the structure fabrication process. Submicron (area from 2.0 to 0.2 µm2) Nb–AlN–NbN tunnel junctions with high current densities and quality parameters Rj/Rn > 15 were fabricated. The spread of parameters of submicron tunnel structures across the substrate and the reproducibility of the cycle-to-cycle process of tunnel structure fabrication were measured.

We have built a Terahertz Dispersive Fourier Transform Spectrometer \cite{Birch1987} to study frequency properties of superconducting films used for fabrication of THz detectors. The signal reflected from the tested film is measured in time domain, which allows to separate it from the other reflections. The complex conductivity of the film depends on frequency and determines the reflection coefficient. By comparing the film reflection in the superconducting state (temperature is below $T_c$) with the reflection of the normal state, we characterise the film quality at terahertz frequencies. The method was applied to 70 and 200nm thick Nb films on a silicon wafer and to 360nm thick NbTiN films on silicon and quartz wafers. The strong-coupling coefficient, $\alpha$, was found to be 3.52 for Nb, and 3.71-4.02 for the NbTiN films. The experimental results were fitted using extended Mattis-Bardeen theory \cite{Noguchi2012} and show a good agreement., Comment: The following article has been accepted by Applied Physics Letters. After it is published, it will be found at https://aip.scitation.org/journal/apl

We present the design and results of characterization of a new sideband separating (2SB) mixer for 800-950GHz, based on superconductor-insulator-superconductor (SIS) junctions. This is the first waveguide 2SB SIS mixer demonstrated at such a high frequency. The design is following the classical quadrature hybrid architecture, meanwhile additional attention was put on the reduction of reflections in the RF structure in order to minimize the RF imbalance, to achieve a high image rejection ratio (IRR). The RF waveguide block was manufactured by micromilling and populated by single-ended SIS mixers developed earlier for upgrade of the CHAMP+ high-band array on the APEX telescope. These SIS mixers have double-sideband (DSB) noise temperatures from 210 to 400K. The assembled 2SB mixer yields a SSB noise temperature from 450 to 900K, with an IRR above 15dB in 95 of the band. Comparing the DSB and the SSB sensitivities, we find that the waveguide losses are as low as expected and do not exceed 0.6dB. The presented mixer is a prototype for use in a 2SB dual polarization receiver planned for deployment on the APEX telescope.

We have elaborated, fabricated, and tested a terahertz source based on the Josephson flux-flow oscillator (FFO) integrated with a transmitting lens antenna. The oscillator was coupled to the on-chip double-slot antenna via microstrip lines, and the chip was mounted on the silicon lens providing the continuous terahertz emission output. The oscillator samples were made of superconductor–insulator–superconductor (SIS) trilayers based on Nb/AlN/NbN, with a gap voltage of about 3.6 mV. The output emission was studied using two independent techniques: a THz spectrometer based on the SIS receiver with a high spectral resolution (better than 0.1 MHz) and an Si bolometer. An operating range of the oscillator of 400–580 GHz and a ratio of detected signal to background signal at the receiver output of up to 55 dB are obtained. In addition, a design for the oscillator with an integrated harmonic mixer for FFO locking is developed and fabricated using Nb/AlO x /Nb trilayers, which is better for FFO operation than Nb/AlN/NbN trilayers at some frequencies due to lower surface losses and hence better spectral properties. The pumping of the mixer by the FFO output power was measured and found to be sufficient for phase locking.

—The parameters of the tunneling barrier of Nb/Al–AlOx/Nb Josephson junctions were estimated in a wide range of current densities using the Simmons method. It is shown that with an increase in resistivity RnS from 60 to 2100 Ω μm2 (a reduction in the density of the tunneling current of junction J from 3.5 to 0.1 kA/cm2), the height of the tunneling barrier increases from 0.85 to 1.18 eV and its width increases from 10.7 to 13.3 A. The experimentally observed linear dependence of the tunneling barrier parameters on RnS made it possible to estimate the capacitance of junctions in the range of RnS 10–30 Ω μm2 required to create low-noise submillimeter-range receivers.

A principal layout of a Josephson terahertz radiation oscillator integrated with a transmitting antenna-lens system and a harmonic mixer (HM) for phase locking of radiation has been proposed and was successfully implemented. Two antenna-feeder systems designed for the central frequencies of 0.3 and 0.6 THz and located on the same chip with the oscillator are numerically simulated and fabricated. A microstrip transmission line between the oscillator and the antenna is used as the feeder. A study was carried out on matching the oscillator power and HM for two designs; the frequency range of pumping HM was 0.25-0.45 and 0.5-0.68 THz for the designs at 0.3 and 0.6 THz, respectively. Good agreement was obtained between experimental results and numerical simulations. A study of the spectral characteristics of the radiation of the oscillator into the external space for the 0.6 THz design using a superconducting integrated spectrometer was carried out. The linewidth of an emission line in free-running regime was of the order of several megahertz; in the phase locking regime down to tens of kilohertz with a signal-to-noise ratio of more than 20 dB was obtained.

In this paper, we calculate power of electromagnetic wave emitted to open space by a terahertz (THz) source based on a Josephson flux-flow oscillator (FFO). The FFO is integrated with a transmitting slot antenna and a superconductor-insulator-superconductor (SIS) tunnel junction on a single chip, providing a frequency-tunable signal in a wide range of 200 - 700 GHz to open space. For non-calibrated power measurements, we used a highly sensitive silicon bolometer at 4.2 K. A small part of output FFO power is branched to the on-chip SIS-mixer and was estimated by absolute value by means of the Tien and Gordon photon-assisted tunneling theory. Using the absolute value, a calibrated power emitted to open space was estimated in the whole operating range. A power of from fractions to a few $\mu \mathrm{W}$ was obtained with a maximum of about $3 \mu \mathrm{W}$ at 330 GHz.

The sideband separating SIS receivers are known as the most sensitive instruments for millimeter and submillimeter ground based radio astronomy. Sideband Ratio or Sideband Rejection Ration (SRR), is one of the key parameters of heterodyne receivers, because it is strongly influencing the system sensitivity. This effect takes place due to sufficient signal losses in the atmosphere or in the instrument optics, which is reducing the signal to noise ratio. To develop high performance receiver SIS sideband separating receiver with have performed a comprehensive analysis of the signal transformation in both high frequency (RF) and low frequency (IF) parts of the receiver. As result, the entire IRR pattern was simulated. It was found that SRR performance is very much restricted by reflections in RF and IF parts of the receiver. Minimization of these reflections is curtail for achieving SRR levels of 15 dB or higher. This knowledge is used in development of sideband separating SIS receiver for ground based telescope APEX and for Millimetron space mission.

A Terahertz Dispersive Fourier Transform Spectrometer was used to study the RF properties of superconducting films used for THz detectors. The experimental results were fitted using an extended Mattis-Bardeen theory and show a good agreement.

The main task of the RFBR_BRICS project 'Superconducting terahertz receivers for space and ground radio astronomy' is the development of terahertz technologies and the creation of superconductor-insulator-superconductor (SIS) receivers with quantum sensitivity for novel radio telescopes, including ultra-long baseline radio interferometers. This research program brings together scientific groups from four BRICS countries (Brazil, Russia, China and South Africa), which have a high international reputation and successful experience in the development of ultrasensitive receivers. The report discusses the working plan and the first results of the project; in particular development of the SIS-receivers for Russian space program 'Millimetron'

A study of the output terahertz (THz) emission to open space from Nb/AlO x /Nb-based long Josephson junction (LJJ) was carried out. Both bolometric technique with a frequency resolution of about1 GHz and heterodyne technique with spectral resolution of about 0.1 MHz were used for detection of the THz emission. A comparative analysis with other types of THz sources was made, such as backward wave oscillator (BWO) and frequency multiplier based on a semiconductor superlattice GaAs/AlAs with large harmonic numbers. A qualitative and quantitative comparison with respect to weight and size characteristics, the spectral properties, the emission power, as well as a general usability in laboratory conditions, was made. The highest detected power was achieved with an LJJ-based oscillator, and the most convenience was demonstrated using a superlattice-based oscillator with large harmonic numbers.

This repost describes our results on development of low-noise heterodyne THz receivers for ground based and space radio astronomy. Superconductor-insulator-superconductor (SIS) mixers that employ high quality Nb-based tunnel junctions are the key element of the most sensitive heterodyne THz receivers. In particular we present here the SIS receivers for frequency range 211–275 GHz and 800–950 GHz with double sideband (DSB) noise temperature of about 25 K and 220 K, respectively. The latest results on development, fabrication techniques and experimental study of the SIS receivers will be discussed.

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.

We investigate the physics of planar annular Josephson tunnel junctions quenched through their transition temperature in the presence of an external magnetic field. Experiments carried out with long Nb/Al-AlOx/Nb annular junctions showed that the magnetic flux trapped in the high-quality doubly-connected superconducting electrodes forming the junction generates a persistent current whose associated magnetic field affects the both the static and dynamics properties of the junctions. More specifically, the field trapped in the hole of one electrode combined with a d.c. bias current induces a viscous flow of dense trains of Josephson vortices which manifests itself through the sequential appearance of displaced linear slopes, Fiske step staircases and Eck steps in the junction's current-voltage characteristic. Furthermore, a field shift is observed in the first lobe of the magnetic diffraction pattern. The effects of the persistent current can be mitigated or even canceled by an external magnetic field perpendicular to the junction plane. The radial field associated with the persistent current can be accurately modeled with the classical phenomenological sine-Gordon model for extended one-dimensional Josephson junctions. Extensive numerical simulations were carried out to disclose the basic flux-flow mechanism responsible for the appearance of the magnetically induced steps and to elucidate the role of geometrical parameters. It was found that the imprint of the field cooling is enhanced in confocal annular junctions which are the natural generalization of the well studied circular annular junctions., Comment: 26 pages, 10 figures. Supercond. Sci. Technol (2020)

—An oscillator based on the distributed tunnel superconductor-insulator-superconductor junction with an ultrawide operating bandwidth of up to 100% of the central frequency seems to be a promising type of directional source of continuous electromagnetic radiation in the terahertz frequency range. In this paper, we propose a scheme of a terahertz oscillator integrated on a single microchip with a transmitting lens antenna with the slot structure in a 200-nm Nb film to radiate the signal into the open space. We also proposed and numerically simulated several designs of a planar slot antenna matched (in the input) with a Josephson oscillator and (in the output) with a silicon elliptical lens. The obtained results of the matching of the oscillator output power with the antenna of various designs operating in four frequency ranges: 250–410, 330–570, 380–520, and 420–700 GHz are presented. The antenna beam patterns and impedances are calculated as well.

We have combined a flip-chip high-Tc dc SQUID magnetometer with a ferromagnetic flux antenna made from 250 insulated 25-μm-thick stripes of amorphous soft magnetic Vitrovac 6025 foil inserted through the pick-up loop of the superconducting flux transformer. The ferromagnetic flux antenna provides improved magnetic coupling of the high-Tc SQUID to magnetic fields of ~0.4 nT/Φ0 to ~0.18 nT/Φ0. The orientation of the stripes was chosen to be normal to the foil tape rolling direction in order to minimize contributions from Barkhausen noise. A magnetic field resolution of ~2 fT/√Hz at 1 kHz and 77 K was achieved. Details of the technology and noise spectra of the novel composite high-Tc SQUID sensor are presented and their prospective applications for biomagnetic research and nondestructive evaluation are discussed.

We report on research in the field of low-noise receiving systems in the sub-terahertz (THz) range, carried out in recent years, aimed at developing receivers with quantum sensitivity for implementation in space and ground-based radio telescopes. Superconductor-Insulator-Superconductor (SIS) mixers based on high-quality tunnel junctions are the key elements of the most sensitive sub-THz heterodyne receivers. Motivations and physical background for technology improvement and optimization, as well as fabrication details, are described. This article presents the results of the SIS receiver developments for the 211–275 GHz and 790–950 GHz frequency ranges with a noise temperature in the double sideband (DSB) mode of approximTELY 20 K and 200 K, respectively. These designs and achievements are implemented in the development of the receiving systems for the Russian Space Agency mission “Millimetron”, and for the ground-based APEX (Atacama Pathfinder EXperiment) telescope.

Topology of high-symmetry thin-film SQUIDs based on the Nb/AlO x /Nb tunneling junctions is developed and optimized. The devices exhibit relatively low sensitivity to static external field and electric interference. An experimentally implemented SQUID sensor with an integrated input coil with a sensitivity of 0.26 μA/Ф0 exhibits an intrinsic noise with respect to magnetic flux of less than $$5\mu {\Phi _0}/\sqrt {Hz} $$ . A system for encapsulation of sensors is developed for applications in multichannel systems for nondestructive evaluation of materials and alternative diagnostic systems.

Radiation spectra from a 119m Sn Mossbauer source are registered using Nb-based superconducting tunnel junction detectors. Mossbauer and accompanying X-ray radiation are detected via escape peaks with high energy resolution.

We describe a practical design of an ultracompact on-chip superconductive microresonator as a potential magnetic element of metamaterial. The achieved resonator size with respect to the wavelength in our experiment is about λ /14 400. The resonator consists of two superconducting Nb spirals, sandwiched face to face, with a small gap filled with dielectric. The spirals are turning in opposite directions: one clockwise, another counterclockwise. We study the resonator spectral response and its inner modes using numerical simulation in HFSS. In experiment with a laser scanning microscope, we confirm the predictions for the resonance frequencies of Nb superconductive resonator and its inner modes structures. Small size and the ease of manufacturing make the two-spiral resonator an attractive solution for superconductive filters, coupling circuits, and as a magnetic component of a metamaterial.

We report the application of a scanning high- $T_{{\rm{c}}}$ SQUID (superconducting quantum interference device) microscope with a ferromagnetic flux guide for the nondestructive evaluation of weld seams and wear tracks and scars on austenitic stainless steel plates as well as measurement of magnetic stray fields distribution above patterned by electron lithography 30-nm-thick cobalt films. A soft magnetic amorphous Vitrovac foil was used to guide the flux from the samples, which were held at room temperature, to the liquid-nitrogen-cooled SQUID-sensor and back. The flux guide passes through a hole in the $\text{1 mm}\times \text{2.5 mm}$ pick-up loop of the high- $T_{{\rm{c}}}$ SQUID sensor, thereby providing improved coupling of the magnetic flux from the object to the SQUID. In order to avoid the influence of the SQUID biasing the magnetic field on the object under investigation, a modulation and feedback coil was coupled to the pick-up loop of the SQUID directly and beyond the ferromagnetic flux guide. Such decoupling of feedback coil from soft magnetic flux antenna ensures that the high- $T_{{\rm{c}}}$ SQUID microscope does not disturb the sample magnetization during image recording. The SQUID microscope can be used to measure the spatial distribution of the z -component of the stray field above a specimen without mechanical contact to it.

We have developed, manufactured, and tested a waveguide mixer in the range 211-275 GHz on the basis of the superconductor-insulator-superconductor (SIS) tunnel structures. The methods of manufacturing high-quality tunnel structures on quartz substrates have been worked out. To extend the receiver band, the Nb/AlOx/Nb and Nb/AlN/NbN tunnel junctions with a high current density of up to 20 kA/cm2 are employed. The dependence of the characteristics of the receiving elements on the signal frequency is simulated for the intermediate-frequency band 4-12 GHz. The measurements demonstrate a good agreement of the input band of the receiving structures with the calculated results. The uncorrected noise temperature of the receiver amounts to 24 K at a frequency of 265 GHz, which is only two times higher than the quantum limit. The receivers under development are intended for a number of newly-built ground-based radio telescopes ("Suffa" and LLAMA), as well as for the "Millimetron" space program.

Compact microwave resonantors made of superconducting rings containing Josephson junctions (SQUIDs) are attractive candidates for building frequency tunable metamaterials with low losses and pronounced nonlinear properties. We explore the nonlinearity of a SQUID metamaterial by performing a two-tone resonant spectroscopy. The small-amplitude response of the metamaterial under strong driving by a microwave pump tone is investigated experimentally and theoretically. The transmission coefficient $S_{21}$ of a weak probe signal is measured in the presence of the pump tone. Increasing the power of the pump, we observe pronounced oscillations of the SQUID's resonance frequency $f_{\textrm{res}}$. The shape of these oscillations varies significantly with the frequency of the pump tone $f_{\textrm{dr}}$. The response to the probe signal displays instabilities and sidebands. A state with strong second harmonic generation is observed. We provide a theoretical analysis of these observations, which is in good agreement with the experimental results.

Partial funding for Open Access provided by the UMD Libraries' Open Access Publishing Fund., We demonstrate a low-dissipation dielectric metamaterial with tunable properties based on the Josephson effect. Superconducting wires loaded with regularly spaced Josephson junctions (critical current Ic ≈ 0.25 μA) spanning a K-band waveguide and aligned with the microwave electric fields create a superconducting dielectric metamaterial. Applied dc current tunes the cutoff frequency and effective permittivity of this unique electric metamaterial. The results are in agreement with an analytical model for microwave transmission through the artificial dielectric medium.

Radiometry experiments, performed on human subjects, show that in the vicinity of a central frequency of 507 GHz the emission of the human skin is substantially nonequilibrium in its nature. The intensity of the radiation registered using a superconducting integrated receiver (SIR) [1], was correlated with the level of the physical and mental stress of subject under examination. This result suggests that human skin may generate sub-THz waves.

We present a new implementation for a sub-terahertz and terahertz source based on a superconducting Josephson flux-flow oscillator. The oscillator has been integrated with the transmitting antenna on a single chip providing the emission to open space, and with a harmonic mixer based on a superconductor-insulator-superconductor junction for a frequency and phase locking. We have used the double slot antenna to couple the oscillator output planar transmission line with the open space; the antenna is additionally matched to the elliptical silicon lens forming narrow output beam of subTHz radiation. The design of antenna and matching structures coupled to the oscillator has been developed and fabricated, the operating range 0.25 - 0.45 THz has been obtained experimentally.

Stacked intrinsic Josephson junctions in the cuprate superconductor Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+x}$ are eagerly investigated as a compact source of coherent terahertz radiation. The authors study the mechanisms of frequency tuning of this source at high and low bias. At high bias, in the presence of a hot spot, the emission frequency ${f}_{e}$ is continuously tunable via changing the bias current and the bath temperature, but at low bias ${f}_{e}$ remains discrete, indicating phase locking to cavity resonances, though some groups of junctions seem to remain unlocked. Though some differences appear, these phenomena can be reproduced in simulations combining electrodynamics and heat diffusion.

A new implementation for a terahertz flux-flow oscillator (FFO) based on a long Josephson junction is presented. The oscillator is integrated with the lens antenna on a single chip providing the THz emission in the open space, and with a harmonic mixer (HM) based on a SIS junction for a frequency and phase locking. We used the double slot antenna to output the emission from the planar transmission line to the open space, this antenna is coupled to the elliptical silicon lens forming narrow output beam of THz radiation. Two designs of antenna and matching structures coupled to the oscillator are developed and fabricated, the operating ranges 0.33 - 0.57 THz and 0.42 - 0.73 THz were obtained experimentally for these designs. The emission to open space was measured by an external superconducting receiver located in the separate cryogenic system.

We observe a broadband chaotic signal of Terahertz frequency emitted from a superconducting junction. The generated radiation has a wide spectrum reaching 0.7 THz and power sufficient to drive on-chip circuit elements. To our knowledge, this is the first experimental observation of a high-frequency chaotic signal emitted by a superconducting system which lies inside the Terahertz gap. Our experimental finding is fully confirmed by the numerical modeling based on the microscopic theory and reveals the unrealized potential of superconducting systems in chaos-based Terahertz communication, fast generation of true random numbers and non-invasive Terahertz spectroscopy applicable to physical, chemical and biological systems., 6 pages, 3 figures

A prototype of gradiometer for detection and analysis of magnetic signals that are generated by defects in metal structures and materials in the presence of external magnetic bias is based on dc-current superconducting quantum interference device (SQUID). A prototype of single-channel SQUID gradiometer that contains a fiberglass nonmagnetic cryostat, measurement probe with the SQUID sensor and magnetic flux transformer (second-order axial gradiometer), SQUID electronics, and software for control of SQUID gradiometer is studied. The prototype exhibits stable operation under laboratory conditions in the absence of additional magnetic shielding. Upgrade of the SQUID sensors and remaining elements of the prototype of magnetometer is planned for application in nondestructive testing.

Planar microwave devices that make it possible to determine parameters of fabricated layers are developed and produced. The parameters are needed for correct numerical simulation based on the Mattis-Bardeen theory and may differ from the tabulated parameters of materials.