Flux-flow Oscillator Research Articles

Parametric amplification of electromagnetic waves produced by a flux-flow-oscillator made of YBa2Cu3O7−δ Josephson junction arrays

We observe parametric amplification of electromagnetic (EM) waves produced by a flux-flow oscillator made of YBa2Cu3O7−δ Josephson junctions arrays coupled to the resonant modes of a millimeter wave Fabry–Pérot resonator at a pump frequency fP = 45 GHz. For temperatures in the range (30–45) K, the frequency fS of the EM signal to be amplified could be tuned continuously in the range (1–25) GHz by an applied B-field induced flux Φ with a one-flux-quantum Φ0 periodicity. Consequently, we measured a significant parametric gain that is almost frequency independent, with a maximum of (8–10.4) dB reached at 40 K. For temperatures in the range (14–30) K, the magnetic field tunability of fS is gradually suppressed to a minimum of (1–5) GHz range where a parametric gain between 5 and 6 dB was measured. With an appropriate adjustment of design/fabrication parameters, our results suggest that the development of tunable MW generators/detectors as well as parametric amplifiers made of high transition temperature superconductors and operating in a wide range of temperatures (10 mK–77 K) is a reasonable and appealing possibility.

The influence of magnetic vortices motion on the inverse ac Josephson effect in asymmetric arrays

We report on the influence a preferential magnetic vortices motion has on the magnitude of the inverse ac Josephson effect (the appearance of dc current Shapiro steps) and the coherent operation of asymmetrical parallel arrays of YBa2Cu3O7−δ Josephson junctions (JJ) irradiated with microwave (MW) radiation in the presence of an applied magnetic field B. The preferential direction of motion of the Josephson vortices is due to the asymmetry-induced ratchet effect and has a dramatic impact: for a particular positive dc bias current I when the flux-flow is robust multiple pronounced Shapiro-steps are observed consistent with a coherent operation of the array. This suggests an efficient emission/detection of MW in related applications. In contrast, when we reverse the direction of I, the flux-flow is reduced and the Shapiro steps are strongly suppressed due to a highly incoherent operation that suggests an inefficient emission/detection of MW. Remarkably, by changing B slightly, the situation is reversed: Shapiro steps are now suppressed for a positive I while well pronounced for a reverse current −I. Our results suggest that a preferential vortex-flow has a very significant impact on the coherent MW operation of superconducting devices consisting of either multiple JJs or an asymmetrically biased single long JJ. This is particularly relevant in the case of flux-flow oscillators for sub-terahertz integrated-receivers, flux-driven Josephson (travelling-wave) parametric amplifiers, or on-chip superconducting MW generators, which usually operate at bias currents in the Shapiro step region.

Direct Experimental Observation of Harmonics of Josephson Generation in the Flux-Flow Oscillator

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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i> /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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> 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.

Terahertz Spectroscopy of Gas Absorption Using the Superconducting Flux-Flow Oscillator as an Active Source and the Superconducting Integrated Receiver.

We report on the first implementation of a terahertz (THz) source based on a Josephson flux-flow oscillator (FFO) that radiates to open space. The excellent performance of this source and its maturity for practical applications has been demonstrated by the spectroscopy of gas absorption. To study the radiated power, we used a bolometric detection method and additionally calibrated the power by means of pumping the superconductor–insulator–superconductor (SIS) junction, integrated on a single chip with the FFO. For calibration, we developed a program using the SIS-detected power calculations in accordance with the Tien and Gordon model. The power emitted to open space is estimated to be from fractions of µW to several µW in the wide region from 0.25 THz up to 0.75 THz for different designs, with a maximum power of 3.3 µW at 0.34 THz. Next, we used a gas cell and a heterodyne superconducting integrated receiver to trace the absorption lines of water and ammonia with a spectral resolution better than 100 kHz. Our experiment for gas absorption is the first demonstration of the applicability of the FFO as an external active source for different tasks, such as THz spectroscopy, near-field THz imaging and microscopy.

A superconducting flux-flow oscillator of terahertz range

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.

Terahertz Source Radiating to Open Space Based on the Superconducting Flux-Flow Oscillator: Development and Characterization

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.

Flux-flow Josephson oscillator as the broadband tunable terahertz source to open space

The flux-flow oscillator (FFO) based on a long Josephson junction has been implemented as a broadband tunable terahertz (THz) source to open space. For this purpose, the transmitting slot antenna has been coupled to the oscillator. Additionally, an elliptical lens with a diameter of 10 mm has been matched to the antenna, forming a narrow output beam of the THz emission. Two designs for the antenna, integrated with the oscillator and developed for operation at different frequency ranges of 0.32–0.55 THz and 0.4–0.7 THz, have been investigated. The FFO has been phase locked to an external reference oscillator by utilizing a harmonic mixer. Its linewidth in the phase-locking regime is determined by the phase noise of the reference oscillator and the number of harmonics used and has been measured to be less than 0.1 MHz. A free-running FFO linewidth from about 2 MHz to several MHz, depending on the operating point, has been obtained. Output emission to open space has been measured by a superconducting integrated spectrometer located in a separate cryostat. The FFO operation as an external source with the achieved emission power and spectral characteristics has demonstrated its applicability for different tasks and purposes where tunable THz sources are required.

Microscopic Tunneling Model of Nb–AlN–NbN Josephson Flux-Flow Oscillator

Since the very first experimental realization of a Josephson flux-flow oscillator (FFO), its theoretical description has been limited by the phenomenological perturbed sine-Gordon equation (PSGE). While PSGE can qualitatively describe the topological excitations in Josephson junctions that are sine-Gordon solitons or fluxons, it is unable to capture essential physical phenomena of a realistic system such as the coupling between tunnel currents and electromagnetic radiation. Furthermore, PSGE neglects any dependence on energy gaps of superconductors and makes no distinction between symmetric and asymmetric junctions: those made of two identical or two different superconducting materials. It was not until recently when it became possible to calculate properties of FFO by taking into account information about energy gaps of superconductors (Gulevich et al. in Phys Rev B 96:024215, 2017). Such approach is based on the microscopic tunneling theory and has been shown to describe essential features of symmetric Nb–AlO\(_\mathrm{x}\)–Nb junctions. Here, we extend this approach to asymmetric Nb–AlN–NbN junctions and compare the calculated current–voltage characteristics to our experimental results.

A 0.3-0.7 THz flux-flow oscillator integrated with the slot antenna and elliptical lens

We present a new implementation for a terahertz flux-flow oscillator based on a long Josephson junction, in this concept the oscillator is integrated with the lens antenna providing the THz emission in the open space. The slot planar antenna is used for radiation and matched to the oscillator (by the input) and to the elliptical silicon lens with a width of 10 mm (by output). Three designs of antenna coupled with the oscillator by a microstrip line are developed and calculated, the obtained operating ranges are 250 – 410 GHz, 330 – 570 GHz and 420 - 700 GHz. The impedance and beam pattern of antenna designs are calculated.

Wideband Josephson THz flux-flow oscillator integrated with the slot lens antenna and the harmonic mixer

Wideband Josephson THz flux-flow oscillator integrated with the slot lens antenna and the harmonic mixer

Josephson flux-flow oscillator: The microscopic tunneling approach

We elaborate a theoretical description of large Josephson junctions which is based on the Werthamer's microscopic tunneling theory. The model naturally incorporates coupling of electromagnetic radiation to the tunnel currents and, therefore, is particularly suitable for description of the self-coupling effect in Josephson junction. In our numerical calculations we treat the arising integro-differential equation, which describes temporal evolution of the superconducting phase difference coupled to the electromagnetic field, by the Odintsov-Semenov-Zorin algorithm. This allows us to avoid evaluation of the time integrals at each time step while taking into account all the memory effects. To validate the obtained microscopic model of large Josephson junction we focus our attention on the Josephson flux flow oscillator. The proposed microscopic model of flux flow oscillator does not involve the phenomenological damping parameter, rather, the damping is taken into account naturally in the tunnel current amplitudes calculated at a given temperature. The theoretically calculated current-voltage characteristics is compared to our experimental results obtained for a set of fabricated flux flow oscillators of different lengths. Our theoretical calculation agrees well with the obtained experimental results, and, to our knowledge, is the first where theoretical description of Josephson flux flow oscillator is brought beyond the perturbed sine-Gordon equation.

Sub-terahertz sound excitation and detection by a long Josephson junction

The paper reports on experimental observations of sub-terahertz sound wave generation and detection by a long Josephson junction. This effect was discovered in spectral measurements of sub-terahertz electromagnetic emission from a flux-flow oscillator (FFO) deposited on an optically polished Si substrate. The ‘back action’ of the acoustic waves generated by the FFO and reflected by the bottom surface of the Si substrate results in the appearance of resonant steps in the FFO IVCs with spacings as small as 29 nV for a 0.3 mm substrate thickness; these steps manifest themselves in a pronounced resonant structure in the emission spectra, with spacings of about 14 MHz, precisely according to the Josephson relation. The mechanism of acoustic wave generation and detection by the FFO is discussed; a possibility for employing the discovered effect for FFO frequency stabilization has been demonstrated. A simple and reliable way to suppress the superfine resonant structure has been developed and proved; this invention allows continuous frequency tuning and FFO phase locking at any desired frequency, all of which are vitally important for most applications.

Harmonic phase detector for phase locking of cryogenic terahertz oscillators

We present a simple and effective way to phase lock terahertz cryogenic oscillators. Extreme nonlinearity of a superconductor-insulator-superconductor tunnel junction allows its implementation as a cryogenic high-harmonic phase detector (HPD), which is used both for mixing a terahertz oscillator signal with a microwave reference and for generating a phase error feedback signal that is directly applied to the oscillator for its phase locking. An integration of the HPD with a cryogenic flux-flow oscillator results in synchronization bandwidth as wide as 70 MHz (significantly exceeding conventional room-temperature system bandwidth), providing phase locking of 84% emitted power for 15 MHz oscillator linewidth.

Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures

We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi2Sr2CaCu2O8+x and Bi1.75Pb0.25Sr2CaCu2O8+x single crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on λJ(Jc) and occur at about 0.4 and 1.3 fluxons per λJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

The effect of bias feed profile on the linewidth of noisy Josephson flux flow oscillator

For creation of a noisy non-stationary spectrometer operating in the sub-THz frequency range, the emitting power and spectral characteristics of a flux-flow oscillator based on a long Josephson junction are considered. The effect of bias current profiles on the spectral linewidth of a long overlap Josephson junction is investigated and compared with the inline junction geometry. It has been demonstrated that the spectral linewidth can be increased by a factor of three with a moderate reduction of the emitted power.

Spectral characteristics of noisy Josephson flux flow oscillator

The current-voltage and spectral characteristics of a flux flow oscillator (FFO) based on a long Josephson junction are studied. The investigations are performed in the range of small bias currents and magnetic fields where the FFO radiates a quasi-chaotic signal with extremely large radiation linewidth, and the displaced linear slope (DLS) is observed at the current-voltage characteristic. Using direct numerical simulation of the sine-Gordon equation, it is shown that for large lengths of the Josephson junction or in the case of partial matching of the FFO with external waveguide system, the DLS with extremely large linewidth is transformed into Fiske steps with very narrow linewidth. As for Fiske steps, the appearance of regime of chaotic oscillations can be explained by multiple reflections of the traveling waves from junction ends rather than simply by excitation of the internal oscillation modes in the “soft” fluxon chain at weak magnetic fields.

Balloon-Borne Superconducting Integrated Receiver for Atmospheric Research

A Superconducting Integrated Receiver (SIR) was proposed more than 10 years ago and has since then been developed up to the point of practical applications. We have demonstrated for the first time the capabilities of the SIR technology for heterodyne spectroscopy both in the laboratory and at remote operation under harsh environmental conditions for atmospheric research. Within a SIR the main components needed for a superconducting heterodyne receiver such as an SIS-mixer with quasi-optical antenna, a Flux-Flow oscillator (FFO) as the local oscillator, and a harmonic mixer to phase-lock the FFO are integrated on a single chip. Light weight and low power consumption combined with broadband operation and nearly quantum limited sensitivity make the SIR a perfect candidate for future airborne and space-borne missions. The noise temperature of the SIR was measured to be as low as 85 K, with an intermediate frequency band of 4-8 GHz in double sideband operation; the spectral resolution is well below 1 MHz. The SIR was implemented in the three-channel balloon-borne instrument TELIS (TErahertz and submillimeter LImb Sounder) that detects spectral emission lines of stratospheric trace gases (like ClO and BrO). These gases even in small quantities can have a significant impact on the atmosphere because they speed up certain chemical processes, such as ozone depletion.

Sheet current model for inductances extraction and Josephson junctions devices simulation

Sheet current model for microelectronic superconductor structures, based on Maxwell and London equations, is presented. In this model, magnetic field is three-dimensional but resulting integro-differential equations for current density potential are two-dimensional. These equations are solved using finite element method and matrix of self and mutual inductances is calculated as well as current distribution in superconductors. Then, current distribution can be used for calculation of boundary conditions for Josephson junctions equations. In particular we apply this approach for calculations of bias and control line currents for flux flow oscillator simulation problem. The program and results of calculations are presented.

Development of the physical principles of the design and implementation of a 500–700 GHz spectrometer with a superconducting integrated receiver

A spectrometer based on the effect of freely decaying polarization in the frequency range 500–700 GHz has been designed. Radiation sources are harmonics from a quantum semiconductor superlattice frequency multiplier. The receiving system of this spectrometer is constructed using a superconducting integrated receiver based on a superconductor-insulator-superconductor mixer and a flux-flow oscillator operating as a heterodyne oscillator. The spectrometer has been used to measure absorption lines of NH3 in a sample of expired air (572 GHz).

Coherent flux-flow emission from stacked Josephson junctions: Nonlocal radiative boundary conditions and the role of geometrical resonances

I derive simple non-local dynamic boundary conditions, suitable for modelling of radiation emission from stacked Josephson junctions, and employ them for analysis of flux-flow emission from intrinsic Josephson junctions in high-$T_c$ superconductors. It is shown that due to the lack of Lorenz contraction of fluxons in stacked junctions, high quality geometrical resonances are prerequisite for high power emission from the stack. This leads to a dual role of the radiative impedance: on the one hand, small impedance increases the efficiency of emission from the stack, on the other hand, enhanced radiative losses reduce the quality factor of geometrical resonances, which may decrease the total emission power. Therefore, the optimal conditions for the coherent flux-flow oscillator are achieved when radiative losses are comparable to resistive losses inside the stack.