In this paper, we developed and fabricated a microcircuit based on “superconductor-insulator-superconductor” (SIS) junction as the detector of terahertz (THz) emission. Using this circuit, a signal in a range of 1-1.1 THz from backward wave oscillator (BWO) was detected. Pumping of the SIS detector by a signal with frequency higher than 1 THz lead to significant suppression of the gap voltage $V_{g}$ of the junction due to heating, which is not observed for frequencies lower than ~750 GHz. We proposed a model of the photon-assisted heating which could be useful for numerical calculations of parameters of the SIS junction affected by external emission. According to this model, a significant impact on the electron distribution function arises from a term responsible for Cooper pair breaking in the junction electrodes, which becomes relevant only at frequencies higher than twice gap frequency of the electrode material.

Suitably patterned single crystals made of the cuprate superconductor Bi2Sr2CaCu2O8+x (BSCCO), intrinsically forming a stack of Josephson junctions, can generate electromagnetic radiation in the lower terahertz regime. Because of Joule heating, the emission power of single stacks seems to be limited to values below 100 mu W. To increase the radiation power, mutually synchronized arrays situated on the same BSCCO base crystal have been studied. A maximum power of almost 1 mW has been achieved by synchronization of three stacks. Mutual electromagnetic interactions via a connecting BSCCO base crystal have been considered essential for synchronization, but the approach still suffers from Joule heating, preventing the synchronization of more than three stacks. In the present paper we show, on the basis of two emitting stacks, that mutual synchronization can also be achieved by standalone stacks contacted by gold layers and sharing only a common gold layer. Compared with BSCCO base crystals, the gold layers have a much higher thermal conductivity and their patterning is not very problematic. We analyze our results in detail, showing that the two oscillators exhibit phase correlations over a range of +/- 0.4 GHz relative to their center frequencies, which we studied mainly between 745 and 765 GHz. However, we also find that strong phase gradients in the beams radiated from both the mutually locked stacks and the unlocked stacks play an important role and, presumably, diminish the detected emission power due to destructive interference. We speculate that the effect arises from higher-order cavity modes that are excited in the individual stacks. Our main message is that the mutual interaction provided by a common gold layer may open new possibilities for relaxing the Joule-heating problem, allowing the synchronization of a higher number of stacks. The approach may also allow one to synchronize several stacks that are comparatively small in size and less prone to the strong phase gradients we observed. Our findings may boost attempts to substantially increase the output power levels of BSCCO terahertz oscillators.

In this article, we present the results of the research aimed at improving the fabrication process of a SIS-mixer for operation at frequencies close to 1 THz. We study the impact of buffer aluminum oxide layer and anodization effects on the properties of superconducting NbTiN films which form the electrodes of the transmission lines in THz-range devices. These layers are traditionally used in technological processes. The measurements of THz response are performed using terahertz time-domain spectrometer at frequencies from 0.3 to 2.0 THz. It was found that the critical temperature, normal-state conductivity just above the transition temperature, superconducting gap value and London penetration depth of the NbTiN film sputtered on aluminum oxide buffer layer are almost the same as of the film sputtered directly onto the substrate. The difference between the parameters is comparable to the measurement uncertainty for NbTiN films with and without additional surface layers of aluminum and anodization.

We performed the spectral measurements of a niobium based Josephson junction array in order to estimate the linewidth of Josephson emission. The array is formed by 9996 serially connected Nb/NbSi/Nb Josephson junctions occupying the area of 5 × 7 mm 2 on a silicon substrate and divided into 7 distinct subarrays. The array was immersed in a liquid helium dewar from where the emission was brought to open space via an oversize waveguide. Firstly, the measurements on a Fourier-transform spectrometer were carried out in a wide frequency range 139–343 GHz at almost all self-induced steps of current-voltage curve. As in our previous work, the observed linewidth of Josephson emission corresponded to the resolution of the spectrometer. Then, we carried out more precise and sensitive measurements using the 211–275 GHz heterodyne receiver based on a Nb/AlO x /Nb mixer with a spectral resolution better than 0.1 MHz. Dependencies of the linewidth on the step number and on the number of connected subarrays are studied. The peaks corresponding to the 2nd harmonic of the Josephson generation are also observed in the spectra. The linewidth of the main harmonic down to 1.5 MHz was observed in these measurements.

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.

Superconducting integrated circuits based on NbTiN/Al transmission lines at frequencies of up to 1.1 THz have been developed and experimentally investigated. The numerical simulation has been carried out for two topologies of a microcircuit with an operating frequency range of 0.9‒1.2 THz, which contains a slot antenna formed in the NbTiN thin film and output-matched with a microstrip transmission line and a superconductor‒insulator‒superconductor (SIS) tunnel junction with an area of 1 µm2 acting as a terahertz detector. Experimental samples of the microcircuit have been fabricated and tested in an experimental setup utilizing a backward-wave oscillator with an output frequency of up to 1.1 THz used as a source. The powerful pumping of the SIS detector has been obtained to demonstrate the applicability of the fabricated NbTiN/Al transmission lines for operation in superconducting circuits at frequencies above 750 GHz, where the conventional Nb/Nb transmission lines cannot operate due to high losses.

We present the results of experimental study of the harmonics of Josephson radiation from a terahertz (THz) source based on a long Josephson junction (LJJ). The source consists of an integrated circuit including the LJJ, a matched transmitting antenna, and a harmonic mixer used for phase locking of the output signal. The LJJ and the harmonic mixer are both made of three-layer Nb-AlOx-Nb superconductor–insulator–superconductor (SIS) structures and operate at a temperature of 4.2 K. The antenna is located in the base superconducting Nb electrode of the microcircuit. To study the spectral composition and characteristics of the output radiation, two different methods were employed, using a THz Fourier transform spectrometer based on a broadband semiconductor detector and using a high-resolution THz spectrometer based on a heterodyne SIS receiver. The spectral composition of the radiation demonstrates the presence of both the fundamental harmonic of the Josephson effect and the higher harmonics. The spectral characteristics of the fundamental (at frequencies of 300–335 GHz) and the second (at frequencies of 600–670 GHz) harmonics, respectively, with a spectral resolution of about 0.1 MHz were studied using a SIS detector.

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/AlO x /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 2 nd 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.

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 μW was obtained with a maximum of about 3 μW at 330 GHz.

The application of a Josephson generator of the terahertz range based on a long superconductor–insulator–superconductor tunnel junction matched with a transmitting antenna and emitting a signal into open space is demonstrated for gas spectroscopy. The generator is used as an active source, the signal of which is absorbed by a sample of a gas mixture in a cell with a length of 60 cm and then recorded by a spectrometer based on a superconductor–insulator–superconductor receiver with a spectral resolution better than 100 kHz. In the experiment, the absorption lines of ammonia and water in the terahertz range were recorded, and the dependence of the spectral characteristics of the absorption lines on the pressure of the gas mixture in a wide range (from 0.005 to 10 mbar) was demonstrated.

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.

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.

Superconducting integrated circuits based on high-quality Nb–AlO x –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.

In this paper, we demonstrate the application of a terahertz (THz) Josephson oscillator based on a tunnel superconductor-insulator-superconductor (SIS) long junction coupled to a transmitting antenna and emitting a signal to open space, for gas spectroscopy. The oscillator is utilized as an active source, the signal of which is absorbed by a gas mixture in a cell 60 cm long and then detected by a spectrometer based on a SIS receiver with a spectral resolution of better than 100 kHz. In the experiment, the absorption lines of ammonia and water in the THz range were recorded, and the dependence of the absorption spectral characteristics on the pressure of the gas mixture was shown in a wide range (from 0.005 to 10 mbar).

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.

A source of terahertz (THz) frequency range based on a long Josephson junction is proposed and studied experimentally. The emission is transmitted to open space using a slot antenna located on a single chip with the oscillator and matched to collecting lens. We fabricated and studied several designs of the emitter developed for frequency ranges of 250 – 410 GHz, 330 – 530 GHz and 390 – 700 GHz with continuous frequency tuning. The spectral characteristics of output emission are studied using a THz spectrometer based on the superconducting receiver with spectral resolution of about 0.1 MHz. The antenna characteristics in full range of operation are studied using a cooled silicon bolometer with high sensitivity. Experimental results match to numerical simulations. The research was supported by the Russian Science Foundation (Project No. 17-79-20343). The fabrication of samples was carried out using the Unique Science Unit (USU #352529), developed within the framework of the state task.

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.