Intrinsic Phase Shift Research Articles

Controllable Manipulation of Semifluxon States in Phase-Shifted Josephson Junctions.

The utilization of Josephson vortices as information carriers in superconducting digital electronics is hindered by the lack of reliable displacement and localization mechanisms. In this Letter, we experimentally investigate planar Nb junctions with an intrinsic phase shift and nonreciprocity induced by trapped Abrikosov vortices. We demonstrate that the entrance of a single Josephson vortex into such junctions triggers the switching between metastable ±π semifluxon states. We showcase controllable manipulation between these states using short current pulses and achieve a nondestructive readout by a nearby junction. Our observations pave the way toward ultrafast and energy-efficient digital Josephson electronics.

Shot-noise limited optical hybrid based on fused fiber couplers

We describe a fiber-based coherent receiver topology which utilizes intrinsic phase shifts from fiber couplers to enable instantaneous quadrature projection with shot-noise limited signal-to-noise ratio (SNR). Fused 3 × 3 fiber couplers generate three phase-shifted signals simultaneously that can be combined with quadrature projection methods to detect magnitude and phase unambiguously. We present a novel, to the best of our knowledge, differential detection topology which utilizes a combination of 3 × 3 and 2 × 2 couplers to enable quadrature projection with fully differential detection. We present a mathematical analysis of this 3 × 3 differential detection topology, extended methods for signal calibration, and SNR analysis. We characterize the SNR advantage of this approach and demonstrate a sample application illustrating simultaneous magnitude and phase imaging of a chrome-on-glass test chart.

All fiber optic current sensor based on phase-shift fiber loop ringdown structure.

An all fiber optic current sensor (AFOCS) utilizing ordinary optical fiber is proposed and demonstrated, which is implemented with a phase-shift fiber loop ringdown (PS-FLRD) structure. The current-induced rotation angle is converted into a minute change in transmittance of the fiber loop, which can be obtained by measuring the phase shift. The current sensitivity is improved by allowing optical signals to traverse the sensing fiber repeatedly. The relationship between the current sensitivity, intrinsic phase shift, and initial transmittance of the fiber loop is numerically analyzed, and the tunable sensitivity is experimentally verified by adjusting the modulation frequency. An optimal current sensitivity of 0.8158°/A is experimentally obtained for the proposed sensor, and the minimum detectable current is at least 100 mA. The proposed sensor requires fewer polarization elements compared with the common type of fiber optic current sensor (FOCS) and has the characteristics of simple structure, high sensitivity, and ease of operation; it will be a promising approach in practical applications.

Gate-tunable pairing channels in superconducting non-centrosymmetric oxides nanowires

Two-dimensional SrTiO3-based interfaces stand out among non-centrosymmetric superconductors due to their intricate interplay of gate-tunable Rashba spin-orbit coupling and multi-orbital electronic occupations, whose combination theoretically prefigures various forms of non-standard superconductivity. By employing superconducting transport measurements in nano-devices we present strong experimental indications of unconventional superconductivity in the LaAlO3/SrTiO3 interface. The central observations are the substantial anomalous enhancement of the critical current by small magnetic fields applied perpendicularly to the plane of electron motion, and the asymmetric response with respect to the magnetic field direction. These features cannot be accommodated within a scenario of canonical spin-singlet superconductivity. We demonstrate that the experimental observations can be described by a theoretical model based on the coexistence of Josephson channels with intrinsic phase shifts. Our results exclude a time-reversal symmetry breaking scenario and suggest the presence of anomalous pairing components that are compatible with inversion symmetry breaking and multi-orbital physics.

Investigation of parallel-connected MEMS electrostatic energy harvesters for enhancing output power over a wide frequency range

Microelectromechanical-systems (MEMS) electrostatic energy harvesters, converting vibration energy into electricity, is an active area of research and is envisioned to supply power to the myriad of sensors for Internet of Things applications. However, although a variety of approaches have been investigated to improve either power output or frequency range, a solution capable simultaneously addressing both is yet to emerge. This paper explores an energy harvesting system comprised of two parallel connected devices of different resonant frequencies, which is demonstrated to exhibit a broadened operating frequency range and enhanced power output as compared with an individual device. While it is straightforward that the frequency operating band is to be broadened due to the difference in natural frequencies, an increase in system power output is not guaranteed due to intrinsic phase shift exhibited by the two dissimilar devices undergoing oscillation. However, it is shown that the power output by the two parallel connected devices may be significantly enhanced above that of a single device, an effect referred to as power-combining or constructive interference, which appears under operating conditions that enable partial synchronization of the oscillatory motion of each device. Such an approach is especially promising since it is possible to employ multiple parallel connected devices to increase the output power to levels matching those seen in applications. However, this technique is not yet explored theoretically or experimentally, a task undertaken here, where a case study consisting of a system with two parallel coupled devices is presented. System dynamics is investigated theoretically and experimentally to understand conditions enabling power-combining. A modular theoretical model, which can be readily extended to any number of devices connected in parallel, is developed with co-dependent, simultaneous mechanical and electrical simulations carried out in VerilogA and Cadence Spectra.

Simple-structured, subfemtosecond-resolution optical-microwave phase detector.

We demonstrate a simple all-fiber photonic phase detector that can measure the phase (timing) difference between an optical pulse train and a microwave signal with subfemtosecond resolution and -60 dB-level amplitude-to-phase conversion coefficient. It is based on passive phase biasing of a Sagnac loop by the intrinsic phase shift of a symmetric 3×3 fiber coupler. By eliminating the necessity of magneto-optic components or complex radio frequency (RF) electronics for phase biasing of the Sagnac loop, this phase detector has potential to be implemented as an integrated photonic device as well. When using this device for synchronization between a 250MHz mode-locked Er-fiber laser and an 8GHz microwave oscillator, the minimum residual phase noise floor reaches <-154 dBc/Hz (at 8GHz carrier) with integrated root mean square (rms) timing jitter of 0.97fs [1Hz-1MHz]. The long-term rms timing drift and frequency instability are 0.92fs (over 5000s) and 4×10-19 (at 10,000s averaging time), respectively.

Light shifts in atomic Bragg diffraction

Bragg diffraction of an atomic wave packet in a retroreflective geometry with two counterpropagating optical lattices exhibits a light shift induced phase. We show that the temporal shape of the light pulse determines the behavior of this phase shift: In contrast to Raman diffraction, Bragg diffraction with Gaussian pulses leads to a significant suppression of the intrinsic phase shift due to a scaling with the third power of the inverse Doppler frequency. However, for box-shaped laser pulses, the corresponding shift is twice as large as for Raman diffraction. Our results are based on approximate, but analytical expressions as well as a numerical integration of the corresponding Schr\"odinger equation.

Controllable 0–π Josephson junctions containing a ferromagnetic spin valve

Superconductivity and ferromagnetism are antagonistic forms of order, and rarely coexist. Many interesting new phenomena occur, however, in hybrid superconducting/ferromagnetic systems. For example, a Josephson junction containing a ferromagnetic material can exhibit an intrinsic phase shift of pi in its ground state for certain thicknesses of the material. Such "pi-junctions" were first realized experimentally in 2001, and have been proposed as circuit elements for both high-speed classical superconducting computing and for quantum computing. Here we demonstrate experimentally that the phase state of a Josephson junction containing two ferromagnetic layers can be toggled between 0 and pi by changing the relative orientation of the two magnetizations. These controllable 0-pi junctions have immediate applications in cryogenic memory where they serve as a necessary component to an ultra-low power superconducting computer. Such a fully superconducting computer is estimated to be orders of magnitude more energy-efficient than current semiconductor-based supercomputers. Phase controllable junctions also open up new possibilities for superconducting circuit elements such as superconducting "programmable logic," where they could function in superconducting analogs to field-programmable gate arrays.

Underwater navigation using an acoustic spiral wave front beacon

A spiral wave front beacon consists of an array of transducers which produce a signal whose phase depends on the azimuthal angle at which it is received and a reference signal with constant phase [J. Acoust. Soc. Am. 131, 3748 (2012)]. A vehicle can determine aspect to the beacon by comparing the phase of the two signals. Progress in the development of this navigation technique will be discussed including results from experiments at Dodge Pond in Connecticut where an unmanned surface vehicle determined its aspect to within 10° by receiving signals from the beacon. Also, tests of a new spiral beacon design [J. Acoust. Soc. Am. 130, 2506 (2011)] in laboratory and underwater environments will be presented. Underwater experiments are performed at the Navy’s Seneca Lake facility in upstate New York using an UUV to record signals. Overall, these results demonstrate that intrinsic phase shifts in the beacon can be handled by signal processing at the receiving vehicle and that the spiral navigation technique is robust in reverberant environments. [Work supported by the Office of Naval Research.]

A 0.6-V Quadrature VCO With Enhanced Swing and Optimized Capacitive Coupling for Phase Noise Reduction

This paper presents a 0.6-V quadrature voltage-controlled oscillator (QVCO) with enhanced swing for low power supply applications. The QVCO comprises a novel capacitive coupling technique that is employed not only for quadrature signal coupling, but also for phase noise reduction. As a result, the proposed QVCO can even achieve 4.6 dB lower phase noise than its single-phase counterpart at 3-MHz offset. Optimized capacitive coupling combined with source inductive enhance-swing technique enables low power and low phase noise simultaneously. The QVCO achieves a measured phase noise of -132.2 dBc/Hz @ 3-MHz offset with a center frequency of 5.6 GHz and consumes 4.2 mW from a 0.6-V supply. This performance corresponds to a figure-of-merit (FoM) of 191.4 dB. Due to the intrinsic phase shift in the proposed quadrature-coupling path, the problem associated with ±90° phase ambiguity between the quadrature outputs has been avoided. The QVCO RFIC is implemented in a 0.13 μm CMOS process with core area of 0.6 × 0.8 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Hilbert reconstruction of phase-shifted second-harmonic holographic images

New techniques are presented that make phase-shifting holography viable for second-harmonic generation (SHG) holography with weak object fields. We developed an intrinsic phase shift calibration of SHG holograms, an algorithm that extracts the reference and object intensity directly from a set of phase-shifted holographic data, and a more robust phase-shifting holography reconstruction algorithm based on π-shifted hologram pairs that permits self-calibration of the phase shift and recovery of the complex field through a Hilbert transform.

Micromagnetic Study of Synchronization of Nonlinear Spin-Torque Oscillators to Microwave Current and Field

The nonautonomous dynamics of spin-torque oscillators in presence of both microwave current and field has been numerically studied in nanostructured devices. When both microwave current and field are applied at the same frequency, integer phase locking at different locking ratio is found. In the locking region, a study of the intrinsic phase shift between the locking force (current or field) and the giant magnetoresistive signal as a function of the bias current is also exploited.

Phase-locking and frustration in an array of nonlinear spin-torque nano-oscillators

We demonstrate that the cooperative dynamics of an array of coupled spin-torque nano-oscillators (STNO) can be controlled by introduction of an additional external phase shift βc between microwave current, which couples STNOs, and microwave voltage on the array. When this external phase shift βc compensates the intrinsic phase shift β0, caused by the STNO nonlinearity, a phase-locking regime with increased output power and vanishing inhomogeneous linewidth broadening is achieved. In the opposite case, when external and intrinsic phase shifts are added, the STNO array demonstrates a frustration regime with low output power and wide and noisy frequency spectrum.

Geometric $\pi$ Josephson Junction: Current-Phase Relations and Critical Current

Josephson junctions with an intrinsic phase shift of pi, so-called pi Josephson junctions, can be realized by a weak link of a d-wave superconductor with an appropriate boundary geometry. A model for the pairing potential of an according weak link is introduced which allows for the calculation of the influence of geometric parameters and temperature. From this model, current-phase relations and the critical current of the device are derived. The range of validity of the model is determined by comparison with selfconsistent solutions.

Phase shifts and interference in surface plasmon polariton waves

A pi phase shift between the incident wave and surface Plasmon polariton (SPP) waves launched from a one-dimensional (slit or groove) subwavelength structure has been found in numerical simulations and invoked to explain recent measurements of optical transmission in slit arrays. Although groove launchers exhibit an overall phase shift that depends on the groove depth, it is shown here how magnetic field induction at the incident surface, and oscillating dipoles from the accumulated charge at the slit or groove edges on the entrance facet lead to an intrinsic pi phase shift, independent of the groove or slit depth. Destructive interference between the pi-shifted surface wave and the incident wave explains the observed transmission minima when the pitch of an array of slits becomes equal to an integer multiple of the SPP wavelength.

Intrinsic phase shift between a spin torque oscillator and an alternating current

We report on a preferred phase shift Δϕ0 between a spin torque oscillator (STO) and an alternating current (Iac) injected at the intrinsic frequency (fSTO) of the STO. In the in-plane (IP) precession mode the STO adjusts to a state where its resistance (or voltage) lags Iac about a quarter of a wavelength [Δϕ0=(87–94)°]. In the IP mode Δϕ0 increases somewhat with the direct current. As the precession changes into the out-of-plane (OOP) mode, Δϕ0 jumps by about 180°, i.e., the STO resistance now precedes Iac about a quarter of a wavelength (Δϕ0=–86°). At the IP/OOP boundary, the alternating current mixes the two oscillation modes and both periodic and chaotic oscillations are observed. As a consequence of mixing, subharmonic terms appear in the STO signal. The intrinsic Δϕ0 will impact any circuit design based on STO technology and will, e.g., have direct consequences for phase locking in networks of serially connected STOs.

Half-Integer Flux Quantization in a Superconducting Loop with a Ferromagnetic π-Junction

Superconducting loops containing a π-junction are predicted to show a spontaneous magnetic moment in zero external magnetic field. In order to confirm this longstanding prediction experimentally, we performed magnetization measurements on individual mesoscopic superconducting niobium loops with a ferromagnetic (PdNi) π-junction. The loops are prepared on top of the active area of a micro Hall-sensor based on high mobility GaAs/AlGaAs heterostructures. We observe switching of the loop between different magnetization states at very low-magnetic fields, which is asymmetric for positive and negative sweep direction. This is evidence for a spontaneous current induced by the intrinsic phase shift of the π-junction. In addition, the presence of the spontaneous current at zero applied field is directly revealed by an increase of the magnetic moment with decreasing temperature, which results in a half integer flux quantization in the loop at low temperatures.

Intrinsic Phase Shift and Novel Dynamic Magnetization States of a Spin Torque Oscillator under ac Current Injection

ABSTRACTWe report on a preferred phase shift ΔΦ0 between a spin torque oscillator (STO) and an ac current (Iac) injected at the intrinsic frequency (fSTO) of the STO. In the in-plane precession mode (IP) the STO adjusts to a state where its resistance (or voltage) lags Iac about a quarter of a wave length (ΔΦ0 = 87°−94°). In the IP mode ΔΦ0 increases somewhat with the dc current. As the precession changes into the Out-Of-Plane (OOP) mode, ΔΦ0 jumps by about 180°, i.e. the STO resistance now precedes Iac by about a quarter of a wave length (|ΔΦ0| = 86°). At the IP/OOP boundary, the ac current mixes the two oscillation modes and both periodic and chaotic oscillations are observed. As a consequence of mixing, subharmonic terms appear in the STO signal. ΔΦ0 can furthermore be tuned by changing one or more of the anisotropy field, the demagnetizing field or the applied field. At the IP/OOP boundary, Iac mixes the two oscillation modes. The intrinsic ΔΦ0 will impact any circuit design based on STO technology and will e.g. have direct consequences for phase locking in networks of serially connected STOs.

A Novel Frequency Offset Estimation for OFDM Systems

SUMMARY In this letter, a blind frequency offset estimation algorithm is proposed for OFDM systems. The proposed method exploits the intrinsic phase shift between neighboring samples in a single OFDM symbol, incurred by a frequency offset. The proposed algorithm minimizes a novel cost function, which is the squared error of the candidate frequency offset compensated signals from two different observation windows. Also, the solution of the proposed algorithm is given by an explicit equation, which does not require any iterative calculation. It is shown that the performance of the proposed method is better than those of the conventional methods, especially in the presence of multipath channels. This is due to the fact that the proposed method is insensitive to inter-symbol interference (ISI).

Theory of Raman-Mediated Pulsed Amplification in Silicon-Wire Waveguides

We present a comprehensive theoretical study of pulsed stimulated Raman scattering in silicon wires. The pulse dynamics is described by a system of coupled equations, which describes intrinsic waveguide optical losses, phase shift and losses due to free-carriers (FCs) generated through two-photon absorption (TPA), first- and second-order frequency dispersion, self-phase and cross-phase modulation, TPA losses, and the interpulse Raman interaction. Furthermore, the influence of the FCs on the pulse dynamics is incorporated through a rate equation. The corresponding system of equations has then been numerically integrated, and phenomena such as noise-seeded Raman amplification, pulsed Raman amplification, and Raman-mediated pulse interaction have been described.