Frequency Shifts Of Modes Research Articles

Review, analysis, and performance evaluation of the most common four active methods for islanding detection in grid-connected photovoltaic systems

• The AFD, SFS, SMS, and SVS active islanding detection methods. • The studied methods provide good detection times within the new grid standards. • The methods have good detection times under different parameters and scenarios. • The active methods are studied and compared for the first time in a detailed study. This paper systematically analyzes the islanding performance under different case studies and scenarios of the most well-known active islanding detection methods (IDMs) for single-phase grid-connected photovoltaic (PV) systems. They are named as follows: Active Frequency Drift (AFD), Sandia Frequency Shift (SFS), Slip Mode frequency Shift (SMS), and Sandia Voltage Shift (SVS) anti-islanding detection strategies. The performance of these four active anti-islanding methods has been examined in detail using Matlab/Simulink. Moreover, the quality factor and non-detection zone (NDZ) influence on islanding detection is also analyzed. According to the new grid codes and standards, the studied active IDMs provide good detection times. Furthermore, the case studies illustrate that these active detection techniques can successfully detect the islanding operation mode under different quality factors, types of loads, solar irradiance changes, and fault-ride through (FRT) operation mode.

An optical atomic clock based on a highly charged ion.

Optical atomic clocks are the most accurate measurement devices ever constructed and have found many applications in fundamental science and technology1-3. The use of highly charged ions (HCI) as a new class of references for highest-accuracy clocks and precision tests of fundamental physics4-11 has long been motivated by their extreme atomic properties and reduced sensitivity to perturbations from external electric and magnetic fields compared with singly charged ions or neutral atoms. Here we present the realization of this new class of clocks, based on an optical magnetic-dipole transition in Ar13+. Its comprehensively evaluated systematic frequency uncertainty of 2.2 × 10-17 is comparable with that of many optical clocks in operation. From clock comparisons, we improve by eight and nine orders of magnitude on the uncertainties for the absolute transition frequency12 and isotope shift (40Ar versus 36Ar)(ref.13), respectively. These measurements allow us to investigate the largely unexplored quantum electrodynamic (QED) nuclear recoil, presented as part of improved calculations of the isotope shift, which reduce the uncertainty of previous theory14 by a factor of three. This work establishes forbidden optical transitions in HCI as references for cutting-edge optical clocks and future high-sensitivity searches for physics beyond the standard model.

Significant Anharmonicity of Thermal Transport in Amorphous Silica at High Temperature

Amorphous Silica The significant anharmonicity, including temperature-dependent linewidths and frequency shifts of the vibrational modes in amorphous silica, is discussed in article number 2200217 by Bing-Yang Cao and Lei Yang. The anharmonic frequency shifts notably affect the strength of coupling between pairs of vibrational modes and dictate the positive temperature dependence at high temperatures, whereas the increasing linewidths with the rising temperature are of minor importance.

Significant Anharmonicity of Thermal Transport in Amorphous Silica at High Temperature

The anharmonicity of thermal transport in amorphous solids is underappreciated and inadequately understood, although considerable attention is paid to that of crystals. Herein, the anharmonic effects on the heat conduction of amorphous silica are investigated by combining the normal mode decomposition method with the Wigner transport equation. The existence of significant anharmonicity in amorphous silica, including temperature‐dependent linewidths and frequency shifts of the vibrational modes, is demonstrated. By considering the anharmonic effects, the predicted temperature dependence of thermal conductivities at high temperatures agrees well with the increasing trend observed in experiments. The underlying mechanisms are further revealed. The anharmonic frequency shifts notably affect the strength of coupling between pairs of vibrational modes and dictate the positive temperature dependence, whereas the increasing linewidths with increasing temperature are of minor importance. This work contributes to the understanding of the anharmonicity of thermal vibrations and heat conduction in amorphous materials.

Enhancement of Dissipative Sensing in a Microresonator Using Multimode Input.

Optical whispering-gallery microresonators have proven to be especially useful as chemical sensors. Most applications involve dispersive sensing, such as the frequency shift of resonator modes in response to a change in the ambient index of refraction. However, the response to dissipative interaction can be even more sensitive than the dispersive response. Dissipative sensing is most often conducted via a change in the mode linewidth owing to absorption in the analyte, but the change in the throughput dip depth of a mode can provide better sensitivity. Dispersive sensing can be enhanced when the input to the microresonator consists of multiple fiber or waveguide modes. Here, we show that multimode input can enhance dip-depth dissipative sensing by an even greater factor. We demonstrate that the multimode-input response relative to single-mode-input response using the same fiber or waveguide can be enhanced by a factor of more than one thousand, independent of the mode linewidth, or quality factor (Q), of the mode. We also show that multimode input makes the dip-depth response nearly one hundred times more sensitive than the linewidth-change response. These enhancement factors are predicted by making only two measurements of dip depth in the absence of an analyte: one with the two input modes in phase with each other, and one with them out of phase.

Measurements of Majorana transition frequency shift in caesium atomic fountain clocks

The caesium atomic fountain clock is a primary frequency standard. During its operation, a Majorana transition frequency shift will occur once a magnetic field at some special locations along the atomic trajectory is singular. In this study, by developing a physical model, we analyzed the magnetic field requirements for atomic adiabatic transition and calculated the influence of the Majorana atomic transition on the atomic state via a quantum method. Based on the simulation results for the magnetic field in the fountain clock, we applied the Monte Carlo method to simulate the relationship between the Majorana transition frequency shift and the magnetic field at the entrance of the magnetic shielding, as well as the initial atomic population. Measurement of the Majorana transition frequency shift was realized by state-selecting asymmetrically populated atoms. The relationship between the Majorana transition frequency shift and the axial magnetic field at the entrance of the magnetic shielding was obtained. The measured results were essentially consistent with the calculated results. Thus, the magnetic field at the entrance of the magnetic shielding was configured, and the Majorana transition frequency shift of the fountain clock was calculated to be 4.57 × 10−18.

Refractive index sensitivity of Brillouin acoustic modes in single-mode subwavelength-diameter fibers.

The acousto-optic interaction is strongly modified and different in subwavelength confinement. Here, the optical propagation and acoustic propagation in a subwavelength-diameter fiber (SDF) have been investigated through adopting a two-layer fiber model of air-coated silica rod. Theoretical investigation indicates that SDF with a diameter below 1.2 µm supports the single mode of light propagation, and various Brillouin acoustic modes with well-spaced spectral distribution can be also excited. Due to the light propagation with the outer environment as the cladding layer, the surrounding medium will greatly affect Brillouin scattering of SDFs. Both the simulation and experiment results indicate a relatively good linear relationship between the Brillouin frequency shift of the lower acoustic modes and surrounding environmental refractive index (RI), and the higher RI sensitivity in finer SDFs can be obtained. In addition, hybrid acoustic waves have shown higher sensitivity and stability than surface acoustic modes. A RI sensitivity of about 5.1 GHz/RIU has been achieved in a 1.1 µm SDF, demonstrating its potential application in RI sensing.

Enhanced optical mode coherence in exchange coupled soft magnetic multilayers

We report on an all-optical investigation of coupled spin excitation modes in a series of magnetic trilayer structures. Using time-resolved magneto-optic Kerr effect (tr-MOKE) magnetometry, we observe multi-mode coherent spin excitations in Ni81Fe19/Ru/Co49Fe49V2 multilayers even though the tr-MOKE optical detection is sensitive only to the Co49Fe49V2 magnetization dynamics. Frequency shifts of the different modes indicate that the coupling between the Ni81Fe19 and Co49Fe49V2 layers varies from anti-ferromagnetic to ferromagnetic to uncoupled as the Ru spacer layer thickness is increased from 8 Å to 200 Å. The lifetime of the high frequency coherent oscillations in the Co49Fe49V2 layer increases by over 200%–300% even in the case of uncoupled Ni81Fe19 and Co49Fe49V2 layers with a 200 Å thick Ru spacer. The results suggest an additional method to decrease the damping of high-moment alloys in layered magnetic nanostructures.

Virtual Sensor Array Based on Piezoelectric Cantilever Resonator for Identification of Volatile Organic Compounds.

Piezoelectric cantilever resonator is one of the most promising platforms for real-time sensing of volatile organic compounds (VOCs). However, it has been a great challenge to eliminate the cross-sensitivity of various VOCs for these cantilever-based VOC sensors. Herein, a virtual sensor array (VSA) is proposed on the basis of a sensing layer of GO film deposited onto an AlN piezoelectric cantilever with five groups of top electrodes for identification of various VOCs. Different groups of top electrodes are applied to obtain high amplitudes of multiple resonance peaks for the cantilever, thus achieving low limits of detection (LODs) to VOCs. Frequency shifts of multiple resonant modes and changes of impedance values are taken as the responses of the proposed VSA to VOCs, and these multidimensional responses generate a unique fingerprint for each VOC. On the basis of machine learning algorithms, the proposed VSA can accurately identify different types of VOCs and mixtures with accuracies of 95.8 and 87.5%, respectively. Furthermore, the VSA has successfully been applied to identify the emissions from healthy plants and "plants with late blight" with an accuracy of 89%. The high levels of identifications show great potentials of the VSA for diagnosis of infectious plant diseases by detecting VOC biomarkers.

Temperature-Dependent Terahertz Spectra of Isonicotinamide in the Form I Studied Using the Quasi-Harmonic Approximation.

Anharmonicity of molecular vibrational motions is closely associated with the thermal property of crystals. However, the origin of anharmonicity is still not fully understood. Low-frequency vibrations, which are usually defined in the terahertz (THz) range, show excellent sensitivity to anharmonicity. In this work, anharmonicity of isonicotinamide in the form I was investigated by using temperature-dependent terahertz time-domain spectroscopy and the quasi-harmonic approximation (QHA) approach at PBE-D3 and PBE-MBD levels. Both DFT calculations suggest the variation of π-π stacking conformation dominates in the thermal expansion of the unit cell. Frequency shifts of the modes in THz range obtained by QHA approach are found to be qualitatively consistent with experimental observations, demonstrating QHA approach is a useful tool for the interpretation of frequency shifts of modes induced by temperature.

Tune shift due to the quadrupolar resistive-wall impedance of an elliptical beam pipe

For a beam pipe with an elliptical cross section, the quadrupolar impedance will be produced. During multibunch operation in a circular accelerator, the complex mode frequency shift is sensitive to the long range wake, which will be further enhanced by the quadrupolar component. In order to get more accurate estimation of the quadrupolar impedance and its induced tune shift, materials outside the beam pipe should be considered and impedance model for multilayer elliptical beam pipe is required. In this paper, the horizontal and vertical quadrupolar impedances, as well as the form factors, are derived for an elliptical beam pipe of infinite thickness. The impedances of a multilayer elliptical beam pipe are expressed as the product of the form factors and the impedances of the multilayer cylindrical beam pipe, in which both the conductivity and the permeability of different layers with finite thickness are considered. On the other hand, the complex incoherent frequency shift due to the quadrupolar impedance is derived from beam oscillation equations, which shows that the direct current (DC) value of the quadrupolar impedance dominates the frequency shift. Numerical studies indicate that the incoherent tune shift due to the quadrupolar resistive-wall impedance is sensitive to the permeability of the magnetic material surrounding the beam pipe. In addition, the occupation of the magnets around the accelerator should be considered when evaluating the incoherent tune shifts. Meanwhile, the metallic coating with constant conductivity on the inner surface of the beam pipe tends to have negligible contribution to the tune shift, in contrary to the distinct contribution found to the coherent tune shift.

Influence of anharmonicity and interlayer interaction on Raman spectra in mono- and few-layer MoS2: A computational study

Two-dimensional (2D) MoS2 possesses unique optical and electrical properties and has many practical applications in nano-optoelectronics. Raman spectroscopy allows readily identify monolayer of MoS2, but the unambiguous determination of the number of layers in a few-layer structure remains a challenging task. In this work, the modification of frequencies of the most intense bands arising from in-plane and out-of-plane vibrations in the spectra of mono- and few-layer MoS2 crystals is studied experimentally, using the Raman scattering technique, and systematically analyzed theoretically, considering the influence of the interlayer interaction as well as the anharmonic interactions within the layer. A good agreement between experimental spectra and theoretical simulations is demonstrated, which allows to estimate the thickness of few-layer MoS2 and also determine parameters of the interlayer and anharmonic interactions. Possible effects of Fermi resonance in 2D MoS2 and the corresponding frequency shift of fundamental modes are considered. The proposed theoretical approach can be used to analyze Raman spectra, estimate the interlayer interaction, anharmonism, and the number of layers for other 2D layered materials.

Gravitational ringing of rotating black holes in higher-derivative gravity

We study gravitational perturbations of slowly rotating black holes in a general effective-field-theory extension of general relativity that includes up to eight-derivative terms. We show that two Schr\"odinger-like equations with spin-dependent effective potentials govern the odd- and even-parity master variables. These equations are coupled for parity-violating corrections, and this coupling affects the quasinormal modes even at linear order in the higher-derivative corrections, due to their isospectrality in general relativity. We provide results for the shifts in the fundamental quasinormal mode frequencies at linear order in the spin, which we expect to be valuable for high-precision phenomenology through future gravitational wave observations.

Finding the Ultralight Boson from a Black Hole’s Ringdown

Solving the problem of dark matter remains one of the greatest unsolved mystery of fundamental physics. One possible dark matter candidate is the scalar ultralight boson, with mass « 1eV. If they exist, ultralight bosons will form clouds of significant total mass about rotating black holes, affecting the spacetime around the black hole. After the inspiral phase of a binary merger, the bosonic cloud can affect the perturbations to the black hole, resulting in deviations in the quasinormal mode frequencies of the ringdown signal of a binary merger. Here, we compute these shifts in the gravitational quasinormal mode frequencies for such a system, and conduct an injection campaign with supermassive black holes detected by the Laser Interferometer Space Antenna. We find that detections of the ringdown phase of supermassive black holes can rule out or confirm the existence of cloud-forming ultralight bosons of mass ∼ 10−18eV at redshift z > 1 if cloud dissipation effects during the inspiral can be neglected.

Electrical spectroscopy of the spin-wave dispersion and bistability in gallium-doped yttrium iron garnet

Yttrium iron garnet (YIG) is a magnetic insulator with record-low damping, allowing spin-wave transport over macroscopic distances. Doping YIG with gallium ions greatly reduces the demagnetizing field and introduces a perpendicular magnetic anisotropy, which leads to an isotropic spin-wave dispersion that facilitates spin-wave optics and spin-wave steering. Here, we characterize the dispersion of a gallium-doped YIG (Ga:YIG) thin film using electrical spectroscopy. We determine the magnetic anisotropy parameters and Gilbert damping from the frequency and linewidth of the ferromagnetic resonance (FMR). Next, we use propagating spin wave spectroscopy in the Damon–Eshbach configuration to detect the small spin-wave magnetic fields of this ultrathin weak magnet over a wide range of wavevectors, enabling the extraction of the exchange constant αex=1.32×10−12 J/m. We observe foldover of the FMR with increasing drive power, leading to frequency shifts of the spin-wave modes and a bistable region in the spin-wave spectra. Our results shed light on isotropic spin-wave transport in Ga:YIG and highlight the potential of electrical spectroscopy to map out the dispersion and bistability of propagating spin waves in magnets with a low saturation magnetization.

Combustion oscillation characteristics of a supersonic ethylene jet flame using high-speed planar laser-induced fluorescence and dynamic mode decomposition

Flame dynamics and combustion oscillation are complex problems in propulsion systems. In this study, the combustion oscillation characteristics of a supersonic ethylene jet flame in a hot coflow were investigated utilizing a 5 kHz high-speed hydroxyl planar laser-induced fluorescence (OH-PLIF) technique and an advanced postprocessing method, namely, dynamic mode decomposition (DMD). A PLIF system equipped with a large laser sheet was used to collect the dynamic development process of the jet flame. An ethylene jet flow was burned at a hot coflow temperature of 1900 K at different Mach numbers (Mach 0.55–1.6). The dynamic evolution of flame microstructures was clearly obtained. The local extinction events and flame area distribution in the flame were statistically analyzed to characterize the instability of the jet flame. The results indicated that flame instability was enhanced with increasing Mach number, but the jet velocity did not affect the global flame oscillation frequency. Based on DMD analysis, the spatiotemporal three-dimensional oscillation characteristics of the jet flame were quantified. The DMD results indicated that during different time periods, the jet flame is dominated by coherent structures with different frequencies. For supersonic flow, increased jet velocity might lead the dominant mode frequency shift to a higher level.

Nonlinear Tunable Vibrational Response in Hexagonal Boron Nitride.

Nonlinear light–matter interactions in structured materials are the source of exciting properties and enable vanguard applications in photonics. However, the magnitude of nonlinear effects is generally small, thus requiring high optical intensities for their manifestation at the nanoscale. Here, we reveal a large nonlinear response of monolayer hexagonal boron nitride (hBN) in the mid-infrared phonon–polariton region, triggered by the strongly anharmonic potential associated with atomic vibrations in this material. We present robust first-principles theory predicting a threshold light field ∼24 MV/m to produce order-unity effects in Kerr nonlinearities and harmonic generation, which are made possible by a combination of the long lifetimes exhibited by optical phonons and the strongly asymmetric landscape of the configuration energy in hBN. We further foresee polariton blockade at the few-quanta level in nanometer-sized structures. In addition, by mixing static and optical fields, the strong nonlinear response of monolayer hBN gives rise to substantial frequency shifts of optical phonon modes, exceeding their spectral width for in-plane DC fields that are attainable using lateral gating technology. We therefore predict a practical scheme for electrical tunability of the vibrational modes with potential interest in mid-infrared optoelectronics. The strong nonlinear response, low damping, and robustness of hBN polaritons set the stage for the development of applications in light modulation, sensing, and metrology, while triggering the search for an intense vibrational nonlinear response in other ionic materials.

Frequency-scanned microresonator soliton comb with tracking of the frequency of all comb modes.

Rapid and large scanning of a dissipative Kerr-microresonator soliton comb with characterization of all comb modes along with the separation of the comb modes is imperative for the emerging applications of frequency-scanned soliton combs. However, the scan speed is limited by the gain of feedback systems, and measurement of the frequency shift of all comb modes has not been demonstrated. To overcome the limitation of the feedback, we incorporate feedback with feedforward. With an additional gain of >40dB by a feedforward signal, a dissipative Kerr-microresonator soliton comb is scanned by 70 GHz in 500µs, 50 GHz in 125µs, and 25 GHz in 50µs (= 500 THz/s). Furthermore, we propose and demonstrate a method to measure the frequency shift of all comb modes, in which an imbalanced Mach-Zehnder interferometer with two outputs with different wavelengths is used. Because of the two degrees of freedom of optical frequency combs, the measurement at two different wavelengths enables estimation of the frequency shift of all comb modes.

Dielectric perturbations: anomalous resonance frequency shifts in optical resonators.

Small perturbations in the dielectric environment around resonant dielectric structures usually lead to a frequency shift of the resonator modes directly proportional to the polarizability of the perturbation. Here, we report experimental observations of strong frequency shifts that can oppose and even exceed the contribution of the perturbations' polarizability. We show in particular how the mode frequencies of a lithium niobate whispering-gallery-mode resonator are shifted by planar substrates-of refractive indices ranging from 1.50 to 4.22-contacting the resonator rim. Both blue- and redshifts are observed, as well as an increase in mode linewidth, when substrates are moved into the evanescent field of the whispering gallery mode. We compare the experimental results to a theoretical model by Foreman et al. [J. Opt. Soc. Am. B33, 2177 (2016)JOBPDE0740-322410.1364/JOSAB.33.002177] and provide an additional intuitive explanation based on the Goos-Hänchen shift for the optical domain, with applications to dielectric structures ranging from meta-surfaces to photonic crystal cavities.

Bandgap engineering and sublattice distortion driven bandgap bowing in Cs2Ag1-xNaxBiCl6 double perovskites

Bandgap engineering in lead-free Cs2Ag1-xNaxBiCl6 (x = 0 to 1) double perovskite alloys synthesized through solution-based approach is investigated. The bandgap is shown to vary from 2.64 eV to 3.01 eV as Ag+ at B′ site gets replaced with Na+ cation. Despite a linear change in the lattice parameter according to Vegard's law, bandgap (Eg) changes in a nonlinear fashion for x = 0 to 1 with much lower Eg values observed than predicted by Vegard's rule. Further, we show the bandgap bowing effect in Cs2Ag1-xNaxBiCl6. Raman spectroscopic studies reveal that the changes in the vibrational mode positions arise due to the systematic variations in local distortions of [BiCl6]3– and [AgCl6]5– octahedra. The bandgap change, Raman mode frequency shift, Raman peak width, and the ratio of intensities of Raman modes all show a similar trend as a function of Na substitution concentration (x). The changes are minimal and linear for x from 0 to ∼0.6 and deviate sharply for higher Na concentration (x > 0.6). These observations strongly suggest that the sublattice distortion in the A2B′B″X6 lattice arises due to a mismatch in the octahedra. This imparts a nonlinear change in the bandgap. Thus, a strong interplay between the [Ag(Na)Cl6]5− and [BiCl6]3– octahedra is shown to have a significant influence on the deviation of bandgap from Vegard's rule and further enforces the bandgap bowing effect in Cs2Ag1-xNaxBiCl6.