Dominant Dipole Research Articles

Long-term persistency of a strong non-dipole field in the South Atlantic

Earth’s magnetic field exhibits a dominant dipole morphology. Notwithstanding, significant deviations from the dipole are evident today, particularly the South Atlantic Anomaly (SAA), characterized by anomalously low-field intensity and high directional variability, diminishing the field’s shielding effect. To assess the persistence of SAA-like features over multimillion-year scales, we combine paleomagnetic data from Trindade Island (20°30’S, 29°22’W) with an evaluation of paleosecular variation (PSV) over the past 10 Myr. We employ synthetic models to explore how the position and intensity of magnetic flux patches at the core-mantle boundary can influence the long-term field behavior. Here we present results that reveal anomalous field signatures in the South Atlantic and the Atlantic-Pacific hemispheric asymmetries are enduring features, likely linked to a bottom-up control of PSV by the inner core’s heterogeneities but with contributions from mantle anomalies in the long-time range.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7.

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb3SeI7, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb3SeI7 with C6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χzxx. Meanwhile, although odd-layered Nb3SeI7 with C3v symmetry has both OP and IP dipole components (χzxx and χyyy), the value of χzxx is 1 order of magnitude greater than that of χyyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χzxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb3SeI7 can facilitate the rapid development of chip-integrated NLO devices.

Polarization perspectives on Hercules X-1: further constraining the geometry

ABSTRACT We conduct a comprehensive analysis of the accreting X-ray pulsar, Hercules X-1, utilizing data from Imaging X-ray Polarimetry Explorer (IXPE) and Nuclear Spectroscopic Telescope Array. IXPE performed five observations of Her X-1, consisting of three in the Main-on state and two in the Short-on state. Our time-resolved analysis uncovers the linear correlations between the flux and polarization degree as well as the pulse fraction and polarization degree. Geometry parameters are rigorously constrained by fitting the phase-resolved modulations of Cyclotron Resonance Scattering Feature and polarization angle with a simple dipole model and Rotating Vector Model, respectively, yielding roughly consistent results. The changes of χp (the position angle of the pulsar’s spin axis on the plane of the sky) between different Main-on observations suggest the possible forced precession of the neutron star crust. Furthermore, a linear association between the energy of Cyclotron Resonance Scattering Feature and polarization angle implies the prevalence of a dominant dipole magnetic field, and their phase-resolved modulations likely arise from viewing angle effects.

Electrical Properties of Vertical Dominant Charge Structures Observed in Corsican Thunderstorms With a LMA

AbstractLightning characteristics of Corsican storms with different charge structures are investigated in this study. Observations of an LMA network are used to document the total lightning activity. Complementary lightning observations of the lightning detection network Météorage are also used. A clustering algorithm is used to build a database of electrical cells from June to October 2018. A method is also applied to infer the vertical charge structure, as dominant dipoles, per 10‐min period for each electrical cell. As an example, one cell recorded in July 2018 is discussed. The cell database is then presented as well as the main electrical properties according to the dominant charge structures. For instance, the higher in altitude the dominant dipole, the higher the flash rate. Overall, dominant negative dipole are observed for 25% of the 10‐min periods and can be separated into two categories: (a) low altitude negative dipole class dominated by negative cloud‐to‐ground (CG) flashes with a main positive layer located between 2 and 4 km height and (b) high altitude negative dipole class, dominated by negative intracloud (IC) with a main positive layer at 5 km height. Dominant positive dipole can also be separated into two categories with (a) a dominant positive dipole located between 4.5 and 10 km high, −CG dominance, weak flash rate and (b) higher altitude dominant positive dipole, +IC dominance and a larger +CG fraction. The synergistic use of LMA and Météorage observations independently gives a rational type and polarity classification with regard to the vertical charge structure.

Sound absorption by a soft medium embedded with complex-shaped hard inclusions

Acoustic coatings for marine applications are commonly designed using a soft elastic matrix embedded with an array of resonant inclusions. In this work, a simple analytical approach to predict the acoustic performance of a soft elastic matrix embedded with hard inclusions of complex shape is presented. Utilizing analogies from fluid dynamics and electrostatics, a small number of well-known lumped parameters such as the added mass and capacitance are employed to relate the dominant dipole component of scattering of a complex-shaped hard inclusion to that of an equivalent sphere. Effective acoustic properties are then derived using a homogenization. The absorption coefficient of the coating for a range of complex shapes is compared with numerical simulations.

Third Harmonic Generation Enhanced by Generalized Kerker Condition in All‐Dielectric Metasurfaces

AbstractThe abundant multipolar resonances in all‐dielectric metasurfaces provide a new paradigm to simultaneously induce strong near‐field confinement in the interior of a nanocavity as well as to manipulate the far‐field scattering property, which is beneficial for the enhancement of nonlinear effects. Here, third‐harmonic generation (THG) of all‐dielectric silicon metasurfaces that sustain dominant electric dipole (ED), toroidal dipole (TD), and magnetic dipole (MD) moments in near‐infrared is numerically and experimentally studied. The effect of the interplay of these resonant modes on THG is investigated, and a pronounced THG enhancement is observed when these modes become spectrally overlapped, corresponding to the generalized Kerker condition. The constructive interference of the total electric dipole (refers to the summation of the ED and TD scattered fields) and MD modes results in the suppression of the backward scattering along with a strong local‐field enhancement inside the dielectric resonators. The simulation (experimental) results show a 214‐fold (17‐fold) THG enhancement in the vicinity of the generalized Kerker condition compared with the signals of the spectrally separated TD and MD resonances. The silicon‐based metasurfaces with their simple geometry are facile for large‐area fabrication and open new possibilities for the optimization of upconversion processes to achieve efficient nonlinear devices.

НОВЫЕ ДАННЫЕ ОБ ОСОБЕННОСТЯХ ЛОКАЛЬНОЙ СТРУКТУРЫ ТВЕРДОГО РАСТВОРА Ca9La(PO4)7(1-x)(VO4)7x:Eu3+(1 %)

Solid solutions of phosphate-vanadates Ca9La(PO4)7(1–x)(VO4)7x:Eu3+(1 %) with the structure -Ca3(PO4)2 activated by europium were synthesized by the standard solid-phase method. It is shown that a continuous series of solid solutions is formed upon substitution P5+ → V5+. At the same time, the study by the second harmonic generation method showed that a symmetry inhomogeneity is observed in the series. When studying the luminescent properties, standard transitions of the europium cation with the dominant electric dipole transition 5D0 → 7F2 were found. The most intense emission was demonstrated by compositions with a centrosymmetric structure.

Finite-Element Method Simulations of the Tunable Magnetic Anapole State in an All-Graphene Metasurface: Implications for Near-Field Sensing, Nanoscale Optical Trapping, and Cloaking

In this article, we have proposed an all-graphene metasurface where the magnetic anapole state can be dynamically tuned. The structure is composed of two layers of graphene separated by a dielectric spacer on an oxide substrate, forming a gap surface plasmons resonator structure in the long-wavelength infrared region. The top graphene layer is patterned into interconnected disks, while the interconnection is for electrical tuning purposes only. By multipolar analysis using the finite-element method (FEM) simulation, we have shown that at the particular dimension, the dominant electric dipole scattering can be minimized. This happens at the resonant point where along with the electric dipole suppression, the magnetic and magnetic toroidal dipoles destructively interfere, which gives rise to a magnetic anapole state. This resonant point can be tuned in two ways, either passively, which is the lithographical variation of structure parameters, or actively, by harnessing the chemical potential of the graphene. We have shown here both mechanisms whereby actively tuning we can shift the magnetic anapole to the far-infrared region. Through the interplay of multipole analysis, we have shown that by varying the angle of incidence, we can control the radiating channels, which can be termed an anapole switch. For the active tuning cases, by voltage mapping of the chemical potential, we have shown the effect of chemical potential on the components of the multipolar families. Our device concept and such implementations hold promise for the application in near-field sensing, nanoscale optical trapping, cloaking, etc.

20 s Cadence TESS Photometry of HR 8799

We analyzed 20 s cadence Transiting Exoplanet Survey Satellite time-series photometry of the exoplanet host star HR 8799 collected in Sector 56. The amplitude spectrum shows Gamma Doradus (γ Dor) pulsations consistent with previous space-based photometry from MOST. Assuming that HR 8799 is a representative of γ Dor stars in the Kepler sample, the dominant dipole mode at 1.98 cycles day−1 implies a core rotation period of ∼0.7 day, which combined with and stellar radius measurements would result in a preliminary stellar inclination of ∼28° assuming rigid rotation. We find no significant residual photometric variation after removing the pulsation signal aside from a ∼9 days trend that is likely a systematic effect or an artifact from performing aggressive frequency subtraction in the presence of red noise.

Narrow-band dazzling red-emitting (LiCaLa(MoO4)3:Eu3+) phosphor with scheelite structure for hybrid white LEDs and LiCaLa(MoO4)3:Sm3+,Eu3+-based deep-red LEDs for plant growth applications.

Presently, the preparation of dazzling narrow-band red-emitting phosphors for solid-state lighting is still a challenge. In this context, herein, a series of pure narrow-band red-emitting LiCaLa1-xEux(MoO4)3 phosphors was synthesized and characterized, and their spectroscopic properties were systematically studied. In addition, a series of orange-red-emitting LiCaLa1-ySmy(MoO4)3 phosphors with the simultaneous doping of Eu3+ was synthesized for plant growth applications. The optical studies revealed that the phosphors showed pure red emission with a full width at half maximum of ∼5 nm and 97% color purity. Alternatively, their absorption spectrum showed good absorption strength in the near UV to blue region. Non-concentration quenching behavior was observed even when the concentration of Eu3+ in the lattice was 100%. The dominant electric dipole transition in the emission spectrum indicated that the Eu3+ ion occupies a non-centrosymmetric site in the lattice. At 150 °C, the phosphor retained 88.83% of its emission intensity calculated at room temperature. Thus, it can be useful for the fabrication of LEDs. Subsequently, Eu-rich red and white LEDs (integrated with yellow phosphor) were fabricated with near-UV and blue LED chips, respectively. The fabricated hybrid white LED showed pure white emission with a CCT of 4762 K, CRI of 81%, and close CIE coordinates of (0.34, 0.33). The absolute quantum yield for the fully substituted LiCaEu(MoO4)3 composition was calculated to be 44.50% upon excitation at 395 nm. To utilize LED light for plant growth applications, efforts were made to synthesize orange-red (Sm3+) and deep-red (Sm3+, Eu3+) phosphors and utilize the simultaneously doped phosphor for the fabrication of deep-red LEDs. The spectral lines well-matched the spectrum of phytochrome (Pr). Thus, the phosphor in the present study is a potential candidate as a red and deep-red phosphor for the fabrication of hybrid white LEDs and deep-red LEDs (for plant growth purposes), respectively.

Terahertz Emission Spectroscopy of Ultrafast Coupled Spin and Charge Dynamics in Nanometer Ferromagnetic Heterostructures.

Due to its high sensitivity and because it does not rely on the magneto-optical response, terahertz (THz) emission spectroscopy has been used as a powerful time-resolved tool for investigating ultrafast demagnetization and spin current dynamics in nanometer-thick ferromagnetic (FM)/heavy metal (HM) heterostructures. Here, by changing the order of the conductive HM coating on the FM nanometer film, the dominant electric dipole contribution to the laser-induced THz radiation can be unraveled from the ultrafast magnetic dipole. Furthermore, to take charge equilibration into account, we separate the femtosecond laser-induced spin-to-charge converted current and the instantaneous discharging current within the illuminated area. The THz emission spectroscopy gives us direct information into the coupled spin and charge dynamics during the first moments of the light-matter interaction. Our results also open up new perspectives to manipulate and optimize the ultrafast charge current for promising high-performance and broadband THz radiation.

A novel red-emitting Na5W3O9F5:Eu3+ phosphor with high color purity for blue-based WLEDs

Red phosphor plays a key role in improving the lighting and display quality of phosphor-converted white light-emitting diodes (pc-WLEDs). Meanwhile, developing new luminescent matrix materials can positively contribute to the acquisition of ideal and efficient phosphors. In this work, we propose a novel red-emitting Na5W3O9F5:Eu3+ (NWOF:Eu3+) phosphor. The phase composition, morphology, electronic structure and photoluminescence properties of the NWOF:Eu3+ phosphor were systematically investigated. The EXAFS results prove that the Eu3+ dopants occupy the Na2 and Na3 sites in the NWOF host. Under 466 nm blue light excitation, NWOF:xEu3+ (0.05 ≤ x ≤ 0.25) phosphors display a dominant red emission at 607 nm and achieves a high color purity (97.44%) due to the dominant electric dipole transition (5D0→7F2) of Eu3+ ions. Impressively, this red-emitting NWOF:0.25Eu3+ phosphor exhibits relatively superior thermal stability (450 K, >50%) and excellent chromaticity stability (2.32 × 10−4 ≤ ΔE ≤ 6.23 × 10−3) from 298 K to 498 K. The activation energy for thermal quenching effect is determined to be 0.22 eV. Moreover, the pc-WLED was fabricated by coupling a 460 nm blue chip with the as-synthesized NWOF:0.25Eu3+ red phosphor and commercial YAG:Ce3+ phosphor. The optical parameters of the as-fabricated pc-WLED are also measured, and the CIE coordinates remain almost constant as the drive current increases from 20 mA to 120 mA. These results indicate that the NWOF:0.25Eu3+ red phosphors should be a suitable candidate as a red component for the preparation of pc-WLEDs.

Terahertz high-Q magnetic dipole resonance induced by coherent Fano interactions

High Q-factor resonance holds great promise for bio-chemical sensing and enhanced light–matter interaction. However, terahertz (THz) magnetic resonances usually demonstrate low Q-factors, resulting in huge energy radiation loss particularly in high frequency bands. Here, we show that high Q-factor magnetic dipole resonance at THz frequencies can be achieved by exploiting the coherent Fano interactions with strong field enhancements in an array composed of single metallic split-ring resonators, working at Wood–Rayleigh anomalies. It can give rise to ultrahigh Q-factor beyond 104 in the THz regime. Experimentally, the measured Q-factor of dominant magnetic dipole resonance can achieve no less than a level of ∼261 by Lorentzian fitting to the experimental data. In addition, a high Q-factor of the fundamental-order magnetic dipole resonance is demonstrated beyond 30. High-Q magnetic dipole resonance is closely associated with ultralow-damping and negative permeability in the THz band. The measurements of magnetic dipole resonances are in good agreement with the theoretical analyses. Our scheme suggests a feasible route to suppress radiative loss for enhanced THz field-matter interaction.

Ultrasensitive Refractive Index Sensing Based on the Quasi‐Bound States in the Continuum of All‐Dielectric Metasurfaces (Advanced Optical Materials 19/2022)

In article number 2200812, Hui-Hsin Hsiao and co-workers demonstrate a narrowband and ultrahigh sensitivity refractive-index sensor based on all-dielectric metasurfaces. A quasi-bound state in the continuum (quasi-BIC) is excited in tilted silicon nanobar pairs with a dominant toroidal dipole (TD) and electric quadrupole (EQ) resonant feature. For high aspect-ratio nanopillars, the quasi-BIC TD-EQ mode exhibits an enhanced refractive-index sensitivity.

Ultrasensitive Refractive Index Sensing Based on the Quasi‐Bound States in the Continuum of All‐Dielectric Metasurfaces

AbstractSymmetry‐protected quasi‐bound states in the continuum (BIC) controlled by metasurfaces with broken in‐plane symmetry are widely exploited to achieve highly surface‐sensitive and spectrally sharp resonances for nanophotonic biosensors. Through the engineering of silicon‐based asymmetric nanobar pairs, a quasi‐BIC mode is excited showing a dominant toroidal dipole (TD) and electric quadrupole (EQ) resonant feature in the near‐infrared and performs ultrahigh sensitivity in the refractometric monitoring of local environment changes. Contrary to the typical electric and magnetic Mie‐type resonances of dielectric resonators with the enhanced field mostly inside the resonator volume, the TD‐EQ quasi‐BIC mode is found to exhibit strong and tightly confined optical fields at the surface of tilted nanobar pairs, and its refractive‐index (RI) sensitivity can be dramatically increased for nanopillars with larger aspect‐ratio. The measured (simulated) sensitivity and figure of merit for nanobar pairs with a height of 450 nm reach 608 nm/RIU and 46 (612 nm/RIU and 85), respectively. Such ultrahigh sensitive all‐dielectric platform can be fabricated through complementary metal‐oxide‐semiconductor compatible process and is promising for on‐chip integration and sensor miniaturization to a wide range of diagnostic applications.

Ulysses Flyby in the Heliosphere: Comparison of the Solar Wind Model with Observational Data

A model capable of reproducing a set of solar wind parameters along the virtual spacecraft orbit out of an ecliptic plane has been developed. In the framework of a quasi-stationary axisymmetric self-consistent MHD model the spatial distributions of magnetic field and plasma characteristics at distances from 20 to 1200 Solar radii at almost all solar latitudes could be obtained and analyzed. This model takes into account the Sun’s magnetic field evolution during the solar cycle, when the dominant dipole magnetic field is replaced by the quadrupole one. Self-consistent solutions for solar wind characteristics were obtained, depending on the phase of the solar cycle. To verify the model, its results are compared with the observed characteristics of solar wind along the Ulysses trajectory during its flyby around the Sun from 1990 to 2009. It is shown that the results of numerical simulation are generally consistent with the observational data obtained by the Ulysses spacecraft. A comparison of the model and experimental data confirms that the model can adequately describe the solar wind parameters and can be used for heliospheric studies at different phases of the solar activity cycle, as well as in a wide range of latitudinal angles and distances to the Sun.

Effects of periodic suction-blowing excitation on the aerodynamic sound generated by a laminar flow past a square cylinder using the direct numerical simulation approach

This paper analyzes the effects of a periodic suction-blowing excitation on the aerodynamic sound generated by a laminar flow past a square cylinder using the direct numerical simulation approach. The periodic suction-blowing excitation has been prescribed on the top and bottom surfaces of the square cylinder. The proper orthogonal mode decomposition (POD) technique has been used to find information about important modes associated with disturbance pressure fields. The POD technique separated the contribution of the dominant lift dipole equivalent sources and the drag dipole equivalent sources to the disturbance pressure field for the no-excitation case. The POD technique also revealed that the periodic suction-blowing excitation introduced an additional monopole equivalent sound source and a drag dipole equivalent sound source due to periodic enhancement and reduction of the body’s effective cross-sectional area. Modifications in the sound field due to changes in excitation amplitude, forcing frequency, and the phase delay between the excitation and vortex shedding process have been studied in detail. Although no significant changes in the flow field were noticed due to a small amplitude of excitation, the directivity of the sound field was significantly altered. The sound fields have been classified into five distinct zones for different periodic suction-blowing excitation frequencies. The beats of sounds were noted when the forcing frequency of excitation and the Strouhal frequency associated with vortex shedding were sufficiently close. It is observed that the in-phase excitation in which either blowing or suction is applied on both surfaces of a cylinder at a particular instant introduces a significant bias in the sound field directivity. The interaction between the lift dipole equivalent sources due to vortex shedding and the monopole and the drag dipole equivalent sources due to excitation introduces a bias in the sound field directivity. As a result, a dominant sound field is observed either in the top-left or in the bottom-left parts of the domain.

Simultaneous collision-induced transitions in H2O+CO2 gas mixtures

A collision induced absorption (CIA) band has been measured near 6000 cm−1 in a spectrum of humidified CO2 recorded by cavity ring down spectroscopy (CRDS) at low pressure (<1 atm). This absorption adds a contribution to the H2O+CO2 continuum mostly originating from far wings of the CO2 and H2O resonance lines broadened by collisions with H2O and with CO2, respectively. The observed CIA corresponds to a simultaneous excitation of 12CO2 and H2O colliding molecules in the ν3 antisymmetric and ν1 symmetric stretching mode, respectively. CRDS spectra recorded near 5940 cm−1 with a highly enriched 13CO2 sample provide a confirmation of the assignment since the measured CIA isotopic spectral shift (of about −68 cm−1) coincides with that between the ν3 bands of 12CO2 and 13CO2. The integrated binary coefficient of the two CIA is evaluated and found to be on the order of 2.3 × 10−3 cm−2amagat−2. Classical molecular dynamics simulations (CMDS) of the considered CIA are also presented, based on the dominant dipole induction mechanism associated with the vibrational matrix elements of the dipole of CO2 (ν3) and isotropic polarizability of H2O (ν1). The results of the calculations are found in good agreement with the observations, thus further validating the attribution of the observed CIA structure to the above mentioned double transitions.

Bound states in the continuum from a symmetric mode with a dominant toroidal dipole resonance

Bound states in the continuum (BICs), both the symmetry-protected and accidental ones, can be realized in the band of antisymmetric mode in a nanoparticle chain as long as the particles radiate exactly out of phase pairwisely. However, for the symmetric mode, BICs cannot form in this way. Here, we show that accidental BICs can be generated from the symmetric mode, as exemplified by a toroidal dipole mode-induced BIC in a trimerized system. This type of BIC is fully studied by an eigenmode analysis as well as multipole decomposition. It is also demonstrated that the BICs correspond to certain phase singularity of the system, indicating the robust existence of the BICs ensured by their topological nature. The combination of BICs and toroidal dipole resonance may find applications in lasing and subwavelength waveguiding.

Incident-angle-insensitive toroidal metamaterial.

The incident-angle-insensitive toroidal dipole resonance on an asymmetric double-disk metamaterial is investigated in the near infrared band. Numerical results show that when the incident angle of excitation light varies from 0° to 90°, our metastructure not only always maintains stable toroidal dipole resonance characteristics, but also presents an excellent local field confinement. Under normal incidence, the polarization angle accessible to a dominant toroidal dipole resonance can be expanded to 70° in spite of the weakened electric field amplitude probed in the gap-layer. Moreover, the dependent relationships of toroidal dipole resonance on the radial asymmetry Δr and gap distance are also explored. The local electric field amplitude can also reach a maximum by structural optimization. The works enrich the research of toroidal moment and provide more application potentials in optical devices.