Flux-flow Oscillator Research Articles

Superluminal geometrical resonances observed inBi2Sr2CaCu2O8+xintrinsic Josephson junctions

We study Fiske steps in small Bi2Sr2CaCu2O8+x mesa structures, containing only few stacked intrinsic Josephson junctions. Careful alignment of magnetic field prevents penetration of Abrikosov vortices and facilitates observation of a large variety of high quality geometrical resonances, including superluminal with velocities larger than the slowest velocity of electromagnetic waves. A small number of junctions limits the number of resonant modes and allows accurate identification of modes and velocities. It is shown that superluminal geometrical resonances can be excited by subluminal fluxon motion and that flux-flow itself becomes superluminal at high magnetic fields. We argue that observation of high-quality superluminal geometrical resonances is crucial for realization of the coherent flux-flow oscillator in the THz frequency range.

Flux-flow oscillator with anticorrelated noise on the bias current and the magnetic field

A Flux-flow oscillator based on a long Josephson junction is a local oscillator in fully superconducting integrated receivers (SIRs). The low frequency voltage fluctuations define the free-running FFO radiation linewidth, an important parameter for employing the SIR as a submm spectrometer. The DC voltage of the oscillator is independently controlled by both the dissipative DC bias current IB and the loss-free external magnetic field at the two FFO ends. It is mainly the natural wideband current noise of the Josephson barrier, which is converted into the fluctuations of voltage by the differential resistance Rd on the current IB. We have developed and experimentally demonstrated the method for suppression of the voltage fluctuations by convey of the bias current noise into the anticorrelated noise of the magnetic field. The experiments were made with Nb/AlN/Nb FFOs, having a specially designed shunting structure at the end, where flux quanta enter the junction.

Research on flux-flow oscillators induced different types of pinning potentials

We have studied dynamics of Josephson vortices in strongly coupled long Josephson junctions stack, such as an intrinsic Josephson junction, by numerical simulations based on coupled sine–Gordon equations considering a periodic pinning potential. In this report, we investigate flux-flow oscillators induced two types of pinning potentials. One is magnetic periodic pinning potential, the other is periodic bias currents. Our results demonstrate that the periodic pinning potential can develop the generated power of flux-flow oscillator in certain condition.

Development and characterization of the superconducting integrated receiver channel of theTELIS atmospheric sounder

The balloon-borne instrument TELIS (TErahertz and submillimetre LImb Sounder) is athree-channel superconducting heterodyne spectrometer for atmospheric researchuse. It detects spectral emission lines of stratospheric trace gases that have theirrotational transitions at THz frequencies. One of the channels is based on thesuperconducting integrated receiver (SIR) technology. We demonstrate for thefirst time the capabilities of the SIR technology for heterodyne spectroscopy ingeneral, and atmospheric limb sounding in particular. We also show that theapplication of SIR technology is not limited to laboratory environments, butthat it is well suited for remote operation under harsh environmental conditions.Within a SIR the main components needed for a superconducting heterodynereceiver such as a superconductor–insulator–superconductor (SIS) mixer with aquasi-optical antenna, a flux-flow oscillator (FFO) as the local oscillator, and aharmonic mixer to phase lock the FFO are integrated on a single chip. Lightweight and low power consumption combined with broadband operation andnearly quantum limited sensitivity make the SIR a perfect candidate for use infuture airborne and space-borne missions. The noise temperature of the SIR wasmeasured to be as low as 120 K, with an intermediate frequency band of 4–8 GHz indouble-sideband operation. The spectral resolution is well below 1 MHz, confirmed byour measurements. Remote control of the SIR under flight conditions has beendemonstrated in a successful balloon flight in Kiruna, Sweden. The sensor and instrumentdesign are presented, as well as the preliminary science results from the first flight.

A Superconductor THz Modulator Based on Vortex Flux Flow

A terahertz modulator based on the Type-II superconductor flux flow oscillator has been proposed. Analytical calculations are presented and the effects of intrinsic and extrinsic parameters such as disorder strength of crystal, penetration depth, frequency, and amplitude of the modulated current on the radiation power spectrum have been studied. The proposed structure also exhibits a mixer-like behavior, in the sense that its output harmonics range from the washboard frequency up to the superconductor gap frequency, so the input signal is practically mixed with the washboard frequency and its harmonics. The modulation index for each harmonic of this modulator has also been investigated. This well-featured modulator has a potential to be used in next-generation terahertz integrated transceivers.

A cryogenic phase locking loop system for a superconducting integrated receiver

The authors present a new cryogenic device, an ultrawideband cryogenic phase locking loopsystem (CPLL). The CPLL was developed for phase locking of a flux-flow oscillator(FFO) in a superconducting integrated receiver (SIR) but can be used for anycryogenic terahertz oscillator. The key element of the CPLL is the cryogenic phasedetector (CPD), a recently proposed new superconducting element. The CPDis an innovative implementation of a superconductor–insulator–superconductortunnel junction. All components of the CPLL reside inside a cryostat at 4.2 K, withthe loop length of cables 50 cm and the total loop delay 4.5 ns. So small a delayresults in a CPLL synchronization bandwidth as wide as 40 MHz and allowsphase locking of more than 60% of the power emitted by the FFO, even for FFOlinewidths of about 11 MHz. This percentage of phase locked power is three timesthat achieved with conventional room temperature PLLs. Such an improvementenables reducing the FFO phase noise and extending the SIR operation range.

RF impedance of intrinsic Josephson junction in flux-flow state with a periodic pinning potential and its optimum condition for RF radiation

We reported dynamics of Josephson vortices interacting with electromagnetic waves in strongly coupled long Josephson junctions stack, such as an intrinsic Josephson junction (IJJ), by numerical simulations based on coupled sine-Gordon equations considering a periodic pinning potential of sinusoidal form. The numerical simulation results for the influence of the electromagnetic waves on flux-flow properties show that the periodic pinning potential induces an in-phase motion of Josephson vortices over the junction stacks, which achieve high performances of IJJ flux-flow oscillator. In order to prove it from another viewpoint, we calculate RF impedance of long Josephson junction stacks in flux-flow state. A remarkable negative real part region of RF impedance appears at 1st harmonic step, it means that the long Josephson junction stacks in flux-flow state acts as an oscillator at the negative real part region. In this study, we evaluate the optimum condition for RF radiation with the periodic pinning potential.

New ideas for long Josephson junctions realistic devices simulation

In the paper, we consider the problem of numerical simulation of long Josephson Junction (JJ) devices. Our interest to the problem is motivated by need of erective parameters calculation of Flux Flow Oscillator (FFO). Practical FFOs have complicated structures and traditional model in form of simple perturbed sine-Gordon equation is inadequate because it doesn't capture important physical details. In this work, we evaluate variable coeficient 1D sine-Gordon equation and boundary conditions for this 1D equation starting from full 2D model of junction and 3D magnetic field problem for bias and control line currents. Then this 1D problem is solved numerically using implicit time stepping numerical procedure. As result program FFOSIM was developed. It is a general tool that allow calculations of critical current, I-V curves and other values. We present results of calculations of junctions inside microstrips where boundary conditions were evaluated using 3D magnetic field calculation program 3DMLSI.

Flux-flow dynamics in a long Josephson junction with nonuniform bias current

The flux-flow dynamics in a long Josephson junction is investigated for a spatiallynonuniform bias current density. Starting from the linearization about a rotatingbackground, a simple approximate solution of the modified sine–Gordon equation ispresented, making possible to derive the current–voltage characteristic of the junction. Asan example of nonuniform driving, an overlap structure with an unbiased tail is discussed.In particular, it is shown how a nonuniform distribution of the bias current density mayaffect the dynamical resistance of a flux-flow oscillator. Analytical results are comparedwith direct numerical simulations and good agreement is found for a wide range of junctiondimensions.

RF responses and possible applications of intrinsic Josephson junctions in different vortex states

We report various RF induced effects on Josephson vortex flow of Bi2Sr2CaCu2O8+y intrinsic Josephson junctions in vortex states being different with existence of pancake vortices. For pancake free state, RF induced zero-crossing steps, which had been found in ordinary Josephson flux flow oscillators, were first observed at voltage V=NΦ0f for N-stacked intrinsic Josephson junctions. In contrast the RF irradiations to the junctions in mixed state with pancakes those step responses changed features similar to Shapiro steps at Vn=nNΦ0f with order n. The former evidently demonstrates that electromagnetic waves propagate in the intrinsic Josephson junctions interacting with moving vortices. The later indicates that Josephson vortices move at the same speed coherently.

Minimizing the linewidth of the flux-flow oscillator

The linewidth of the flux-flow oscillator has been calculated by direct computer simulation of the sine-Gordon equation with noise. Good agreement of the numerical results with the formula derived in Phys. Rev. B 65, 054504 (2002) has been achieved. Depending on the length of the unbiased tail, the power may be maximized and the linewidth may be minimized in a broad range of bias currents. The linewidth can be decreased further by 1.5 times by proper load matching.

Linewidth and DC properties of the flux-flow oscillator with mixed inline-overlap bias

A Flux-Flow Oscillator (FFO) is a long Josephson junction in which a unidirectional flow of magnetic flux quanta is maintained by an applied DC magnetic field and a bias current. In the overlap geometry the FFO voltage and thus its oscillation frequency is controlled independently by the (overlap) bias current and the so-called "control line" (inline) current, which creates the magnetic field at the ends of the FFO. We have studied both DC properties and the linewidth of the emitted radiation for an FFO, which can be biased in different ways; as a four current terminal object. The bias current of the mixed inline-overlap type is applied through the terminals commonly used for the "control line" current. In this configuration the bias current also contributes to the magnetic field at one of the two FFO ends. This changes the steepness of the junction's I-V characteristics and therefore the bias current dynamic resistance. The experimental FFO linewidth was found to be independent of the bias configuration and determined by the dynamic resistance of the junction with pure overlap bias current. A new method has been developed to evaluate the difference in the DC magnetic field at the ends of the junction. It may be attributed to control line redistribution caused by the impedance matching RF circuit connected to the FFO at the end where the fluxon chain annihilates and radiation is emitted. We found that the opposite end, where the fluxons enter the junction, appears to be about three times more sensitive to variations of the magnetic field than the radiating end.

Model of a long Josephson tunnel junction including surface losses and self-pumping effect

We have numerically investigated the dynamics of a long linear Josephson tunnel junction with overlap geometry (Flux-Flow Oscillator, FFO). The study is performed in the frame of a modified sine-Gordon model, which includes surface losses, self-pumping effect, and in an empirical way the superconducting gap. The electromagnetic coupling to the environment is modeled by a simple resistor-capacitor load (RC-load) placed at both ends of the FFO. In our model the damping parameter depends both on the spatial coordinate and on the amplitude of the AC voltage. In order to find the DC current-voltage curves the damping parameter has to be calculated self-consistently by successive approximations and time integration of the perturbed sine-Gordon equation. The modified model gives better qualitative agreement with experimental results than the conventional perturbed sine-Gordon model.

Spectral properties of phase-locked flux flow oscillator

In the present paper, the results of theoretical modeling of flux flow oscillator (FFO) included into phase-locked loop (PLL) system and its comparison with experiment are outlined. To describe theoretically the dynamics of phase-locked FFO, we have considered two models: first order PLL system and the PLL system with integral-proportional filter. While the first order PLL system may be treated analytically even in the frame of nonlinear PLL model and gives satisfactory agreement with experimental results, it does not describe all peculiarities of spectral density, which may be described well by a more sophisticated model with the integral-proportional filter. The quantitative prediction of spectral ratio improvement due to increase of PLL regulation bandwidth is given.

Superconducting Integrated Submillimeter Receiver for TELIS

In this report an overview of the results on the development of a single-chip superconducting integrated receiver for the Terahertz Limb Sounder (TELIS) balloon project intended to measure a variety of stratosphere trace gases is presented. The Superconducting Integrated Receiver (SIR) 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 FFO phase locking. As a result of the FFO design optimization a free-running linewidth between 9 and 1.5 MHz has been measured in the frequency range 500-710 GHz resulting in phase-locking of 35 to 95% of the FFO power correspondingly. 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. As a result all required TELIS parameters were demonstrated. Phase-locked FFO operation over entire SIR channel frequency range has been realized, spectral resolution below 1 MHz has been confirmed by gas cell and CW signal measurements. An uncorrected double side band (DSB) noise temperature below 250 K has been measured with the phase-locked FFO. The intermediate frequency bandwidth 4-8 GHz has been realized. To ensure remote operation of the phase-locked SIR several procedures for its automatic computer control have been developed and tested.

Superconducting Integrated Receiver Based on Nb-AlN-NbN-Nb Circuits

The superconducting integrated receiver (SIR) comprising in one chip a superconductor-insulator-superconductor (SIS) mixer and a phase-locked superconducting flux flow oscillator (FFO) is under development for the international project TELIS. To overcome temperature constraints and extend operation frequency of the SIR we have developed and studied Nb-AlN-NbN-Nb circuits with a gap voltage Vg up to 3.7 mV and extremely low leakage currents . Based on these junctions integrated microcircuits comprising FFO and harmonic mixer have been designed, fabricated and tested; the radiation from such circuits has been measured at frequencies up to 700 GHz. Employment of NbN electrode does not result in the appearance of additional noise. For example, FFO linewidth as low as 1 MHz was measured at 600 GHz, that allows us to phase lock up to 92% of the emitted by FFO power and realize very low phase noise about 90 dBc. Preliminary results demonstrated uncorrected DSB noise temperature of the Nb-AlN-NbN SIR below 250 K at frequencies around 600 GHz.

Cryogenic Phase Detector for Superconducting Integrated Receiver

New superconducting element, a cryogenic phase detector (CPD) has been proposed and preliminary tested. The CPD is based on a superconductor-insulator-superconductor junction and initially intended for phase locking of a flux-flow oscillator in a superconducting integrated receiver. First results of the CPD development and study are very encouraging; a sinusoidal response of the CPD has been measured at the variation of the phase shift between input signals. Dependences of the output signal and phase response on the CPD bias voltage have been studied; main parameters of this new device are estimated. Important that the CPD output current well above 10 has been measured at the input signal provided by the harmonic mixer of the integrated receiver and amplified by the existing HEMT-amplifier. Due to the large conversion coefficient this current being supplied to the flux-flow oscillator (FFO) control line is sufficient to directly tune FFO frequency. Obtained data demonstrate that the CPD intrinsically could operate with effective bandwidth more than 100 MHz. Preliminary results of the CPD implementation for the FFO phase locking are presented; possible advantages of such combination are discussed.

Influence of surface losses and the self-pumping effect on current-voltage characteristics of a long Josephson junction

We have numerically investigated the dynamics of a long linear Josephson tunnel junction with overlap geometry. Biased by a direct current (dc) and an applied dc magnetic field, the junction has important applications as tunable high frequency oscillator [flux-flow oscillator (FFO)] in the millimeter and submillimeter range. The study is performed in the frame of a modified sine-Gordon model, which includes surface losses, self-pumping effect, and in an empirical way the superconducting gap. The electromagnetic coupling to the environment is modeled by a simple resistor-capacitor load ($RC$ load) placed at both ends of the FFO. In our model, the damping parameter depends both on the spatial coordinate and on the amplitude of the ac voltage. In order to find the dc current-voltage curves, the damping parameter has to be calculated self-consistently by successive approximations and time integration of the perturbed sine-Gordon equation. The modified model gives better qualitative agreement with experimental results than the conventional perturbed sine-Gordon model.

Superconducting Submm Integrated Receiver for TELIS

In this report we present design and first experimental results for development of the submm superconducting integrated receiver spectrometer for Terahertz Limb Sounder (TELIS). TELIS is a collaborative European project to build up a three-channel heterodyne balloon-based spectrometer for measuring a variety of atmospheric constituents of the stratosphere. The 550 - 650 GHz channel of TELIS is based on a phase-locked Superconducting Integrated Receiver (SIR). SIR is an on-chip combination of a low-noise Superconductor-Insulator-Superconductor (SIS) mixer with quasioptical antenna, a superconducting Flux Flow Oscillator (FFO) acting as Local Oscillator (LO), and SIS harmonic mixer (HM) for FFO phase locking. A number of new solutions were implemented in the new generation of SIR chips. To achieve the wide-band performance of the spectrometer, a side-feed twin-SIS mixer and balanced SIS mixer with 0.8 µm2 junctions integrated with a double-dipole (or double-slot) antenna is used. An improved design of the FFO for TELIS has been developed and optimized providing a free-running linewidth between 10 and 2 MHz in the frequency range 500 - 700 GHz. It is important to ensure that tuning of a phase-locked (PL) SIR can be performed remotely by telecommand. For this purpose a number of approaches for the PL SIR automatic computer control have been developed. All receiver components (including input optical elements and Martin-Puplett polarization rotating interferometer for single side band operation) will be mounted on a single 4.2 K plate inside a 40 × 180 × 80 mm3 box. First measurements give an uncorrected double side band (DSB) noise temperature below 250 K measured with the phase-locked FFO; more detailed results are presented at the conference.

Radiation linewidth of the flux-flow oscillator with integrated self-field coil

The FFO is a long Josephson junction in which the flow of magnetic flux quanta, and thus the oscillator frequency is controlled by the DC bias current IB, and the DC current ICL producing the magnetic field at the junction ends. We have experimentally studied the properties of the Nb/AlOx/Nb and Nb/AlN/Nb trilayer FFOs when IB also contributes to the magnetic field. A number of samples were fabricated, each containing an FFO integrated with a bias current loop designed so that the bias current can generate approximately the same positive or negative contribution to the magnetic fields at both ends of the FFO. The magnetic field is proportional to ICL + Lp*IB, where Lp is given by the loop design. We have made a study of both the DC parameters and the radiation linewidth for FFOs with different integrated loops and compared our results to that for the FFO without any loop. It was found that loops with positive contribution (Lp > 0) make the FFO linewidth smaller (although the differential resistance of the FFO on the bias current becomes larger), while loops with negative contribution (Lp < 0) result in broadening of the FFO radiation linewidth. The magnitude of this effect strongly depends on the critical current density JC ∼ (Rn*S)−1. The Rn*S product was varied in our experiments from 13 to 35 Ohm*µm2. We propose a possible explanation of this effect.