Transverse Dipole Research Articles

Electrified Nanogaps under an AC Field: A Molecular Dynamics Study.

The organization and dynamics of ions and water molecules at electrified solid-liquid interfaces are generally well understood under static fields, especially for macroscopic electrochemical systems. In contrast, studies involving alternating (AC) fields tend to be more challenging. In nanoscale systems, added complexity can arise from interfacial interactions and the need to consider ions and molecules explicitly. Here we use molecular dynamics (MD) simulations to investigate the behavior of NaCl aqueous solutions at different concentrations confined in nanogaps under AC fields ranging from 10 MHz to 10 GHz. We explore the impact of the gap size (2-60 nm) and of the solid material composing the electrode (silica, charged silica, or gold). Analysis of the transient and stable responses of the system shows that the total transverse dipole M z,total formed by the water molecules and the ions across the gap is always able to counter the applied field regardless of AC frequency, NaCl concentration, or electrode material. As expected, the ions lag at higher frequencies, leading to a capacitive behavior. This effect is fully compensated by water dipoles that lead the field, reaching a maximum lead at a specific frequency which depends on salt concentration and gap size. Changing the gap size affects the magnitude of M z,total. Finally, the electrode material is shown to affect the electrolyte behavior in the gap region. We anticipate these results to be useful for nanoscale dielectric spectroscopy, including scanning probes.

Evolution of the color dipole cross section

Using Laplace transform techniques, we describe the evolution of the color dipole cross section at the leading-order and next-to-leading-order approximations from the Bartels–Golec-Biernat–Kowalski model in a kinematic region of low values of the Bjorken variable x and a wide range of transverse dipole size r. This evolution method shows that the saturation scale and geometric scaling are retained. We derived analytical results for the integrated and unintegrated color dipole gluon distribution functions and compared them with the CJ15 parameterization group and the unintegrated color dipole gluon distribution models, respectively. The Sudakov form factor in the evolution of the unintegrated color dipole gluon distribution is incorporated, and the results are considered at small and large values of kt2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$k_{t}^{2}$$\\end{document}.

Switchable multiple quasibound states in the continuum based on the phase transition of vanadium dioxide

Resonant dielectric nanostructures have achieved significant advancements in the manipulation of light at the nanoscale. Particularly, bound states in the continuum (BICs) based on dielectric metasurfaces have greatly enhanced the intensity of light–matter interaction. However, most BICs in dielectric metasurfaces are fixed in their functionality once they are made. In this study, we present the development of switchable multiple quasi-BICs by combining dielectric nanostructures with vanadium dioxide. The resulting hybrid dielectric metasurface can support three types of BICs with different multipole origins for vanadium dioxide in the insulating phase. By introducing structural asymmetry through width adjustment, one quasi-BIC with a longitudinal toroidal dipole characteristic is excited under x-polarized incidence. Further, tuning the width allows for the generation of two additional quasi-BICs with distinct electromagnetic sources under y-polarized incidence. Additionally, the hybrid dielectric metasurface also supports a high-Q transverse toroidal dipole mode. Moreover, all quasi-BICs and toroidal dipole modes can be turned off when vanadium dioxide transitions into the metallic phase. The switchable multiple quasi-BICs hold promise for applications in optical modulators, tunable harmonic generation, and biosensors.

Unidirectional Subsystem Symmetry in a Hole-Doped Honeycomb-Lattice Ising Magnet.

We study a model of a hole-doped collinear Ising antiferromagnet on the honeycomb lattice as a route toward realization of subsystem symmetry. We find nearly exact conservation of dipole symmetry verified both numerically with exact diagonalization on finite clusters and analytically with perturbation theory. The emergent symmetry forbids the motion of single holes-or fractons-but allows hole pairs-or dipoles-to move freely along a one-dimensional line, the antiferromagnetic direction, of the system; in the transverse direction both fractons and dipoles are completely localized. This presents a realization of a "unidirectional" subsystem symmetry. By studying interactions between dipoles, we argue that the subsystem symmetry is likely to continue to persist up to finite (but probably small) hole concentrations.

Switchable high-Q electromagnetically induced transparency based on the Ge2Sb2Te5 nanodisk dimers

Bound states in the continuum (BICs) based on metasurfaces have gained significant attention recently to enhance the strength of light-matter interaction. However, most BIC metasurfaces possess fixed optical properties once they have been fabricated. In this study, we introduce the concept of BICs to the phase-change metasurfaces composed of Ge2Sb2Te5 nanodisk dimers. The Ge2Sb2Te5 nanodisk dimers can support a low-Q transverse magnetic dipole resonance, a high-Q toroidal dipole resonance and a high-Q longitudinal magnetic dipole resonance. By adjusting geometric parameter, the high-Q longitudinal magnetic dipole resonance can be converted into a BIC mode. Notably, when Ge2Sb2Te5 is in the amorphous phase, high-Q electromagnetically induced transparency (EIT)-like resonances can be achieved due to the interaction between a low-Q transverse magnetic dipole mode and either a high-Q toroidal dipole mode or a high-Q longitudinal magnetic dipole mode. However, the EIT resonances are switched off when Ge2Sb2Te5 is transformed into the crystalline phase. Furthermore, the high-Q EIT resonances enable the realization of an ultrahigh group refractive index, and can be tuned through the phase transition of Ge2Sb2Te5. The switchable high-Q EIT resonances hold potential applications in slow-light devices, optical modulators, and biosensors.

Electric field effects on N–Sm-A–Sm-C phase transitions

A liquid crystalline system formed by rod-shaped molecules that have a small transverse dipole moment is analyzed in the presence of an external electric field normal to the smectic layers. The approach focuses on the phase transition diagram displaying the N–Sm–A-Sm-C phases, when both negative and positive dielectric anisotropies are considered. The analysis is carried out within the framework of an extended mean field theory, which shows the emergence of a re-entrant Sm-C phase in the strong field regime, accompanied by a suppression of the translational ordering of the Sm-A phase, in the case of a negative anisotropy. A different scenario is observed in the case of positive anisotropy, as the electric field tends to suppress the Sm-C phase because the molecular alignment is driven by the field along its direction. In this case, the emergence of a residual nematic phase is also observed, which replaces the isotropic phase and is characterized by a low-value orientational order parameter.

Negative group-delay assisted anomalous propagation in stacked Split Ring Resonator array

This paper explores a novel method to achieve negative group delay-assisted anomalous propagation without significant signal attenuation using stacked Split-Ring Resonator (SRR) array in the microwave regime. A sub-wavelength stacking thickness between the SRR layers creates transverse magnetic dipole moments due to the excitation of anti-parallel currents on these layers resulting in increased transmission amplitude. Multipole scattering theory has been utilized to extract the reason behind anomalous propagation by evaluating the scattered power from the principal electric and magnetic dipole moments excited on the structure. Time-domain analysis are performed on the samples, and negative group delay behavior is confirmed by observing the precedence of the output electromagnetic Gaussian pulse within the anomalous dispersion regime. The coupling effects are studied numerically using full-wave solvers and verified in experiments by performing transmission measurements inside an anechoic chamber.

Winning Strategy toward Acentric Crystals: Transverse Dipole Moment Molecules

Imines obtained by condensation of 4-hydroxybenzohydrazide with carbonyl compounds are a rare example of non-chiral compounds showing a remarkable tendency to crystallize in acentric and polar space groups, as we underlined in a previous study of imines obtained by condensation with aliphatic ketones. Here, we present an extensive study of imines obtained by condensation with aromatic aldehydes and demonstrate that over a representative set of 60 crystal structures, the overall occurrence of acentric phases in this class of compounds is 47%. This figure is remarkably higher than the overall 22% frequency of acentric structures in the Cambridge Structural Database and pinpoints a successful strategy toward the synthesis of acentric crystals. In the crystal structures of the acentric imines, complex chains of H-bonded molecules are generally present, with formation of 2D H-bonded networks, similar to other classes of compounds described in the literature with a propensity to form acentric crystals. Another reason for the high success in achieving acentric and polar structures in this class of compounds can be related to the presence of strong transverse bond dipole moments. This feature allows a polar packing of the long molecular axes while keeping the bond dipole moments antiparallel.

Annular and unidirectional transverse scattering with high directivity based on magnetoelectric coupling.

Transverse scattering is a special directional scattering perpendicular to the propagation direction, which has attracted great interest due to its potential applications from directional antennas, optical metrology to optical sensing. Here we reveal annular transverse scattering and unidirectional transverse scattering by magnetoelectric coupling of Omega particle. The annular transverse scattering can be achieved by the longitudinal dipole mode of the Omega particle. Furthermore, we demonstrate the highly asymmetric unidirectional transverse scattering by adjusting the transverse electric dipole (ED) and longitudinal magnetic dipole (MD) modes. Meanwhile, the forward scattering and backward scattering are suppressed by the interference of transverse ED and longitudinal MD modes. In particular, the lateral force exerted on the particle is accompanied by the transverse scattering. Our results provide a useful toolset for manipulating light scattered by the particle and broaden the application range of the particle with magnetoelectric coupling.

Intrinsic Rashba effect and anomalous valley Hall effect in one-dimensional magnetic nanoribbon

Stable one-dimensional (1D) magnetic nanoribbons are superior to the two-dimensional (2D) or bulk materials due to the quantum size effect and edge effect as well as the controllable bandwidth. Through first-principles calculations, Wannier functions and Monte Carlo simulation, it is found that the GdX2 (X = Cl, Br) zigzag nanoribbons (ZNRs) are dynamically stable, with Curie temperature of ferromagnetic ground state significantly higher than that of their 2D counterpart. The results show that inherent transverse dipole moment and perpendicular magnetocrystalline anisotropy energy (PMAE) induce the coexistence of Rashba effect and valley polarization in such 1D magnetic ZNRs. GdBr2 ZNR with four zigzag chains (GdBr2-4ZNR) has larger valley splitting and PMAE than that of GdCl2-4ZNR, demonstrating room temperature ferromagnetic ground state. Through regulating the bandwidth of GdBr2 ZNR, intrinsic anomalous valley Hall (AVH) effect is observed in GdBr2-5ZNR. Importantly, Dirac-cone-like electronic states, PMAE and AVH effect are robust against the increment of nanoribbon bandwidth which is essential for utilization. These results provide important insights and pave an avenue for the potential application of magnetic nanoribbons in spin-dependent and valley-dependent miniature information storage devices.

Selective phase filtering of charged beams with laser-driven antiresonant hollow-core fibers

Emerging accelerator concepts increasingly rely on the combination of high-frequency electromagnetic radiation with electron beams, enabling longitudinal phase space manipulation which supports a variety of advanced applications. The handshake between electron beams and radiation is conventionally provided by magnetic undulators which unfortunately require a balance between the electron beam energy, undulator parameters, and laser wavelength. Here we propose a scheme using laser-driven large-core antiresonant optical fibers to manipulate electron beams. We explore two general cases using ${\mathrm{TM}}_{01}$ and ${\mathrm{HE}}_{11}$ modes. In the former, we show that large energy modulations $O$(100 keV). can be achieved while maintaining the overall electron beam quality. Further, we show that by using larger field strengths $O$(100 MV/m) the resulting transverse forces can be exploited with beam-matching conditions to filter arbitrary phases from the modulated electron bunch, leading to the production of $\ensuremath{\approx}100$ attosecond FWHM microbunches. Finally, we also investigate the application of the transverse dipole ${\mathrm{HE}}_{11}$ mode and find it suitable for supporting time-resolved electron beam measurements with sub-attosecond resolution. We expect the findings to be widely appealing to high-charge pump-probe experiments, metrology, and accelerator science.

Оптическая анизотропия пленок из поливилового спирта с металлическими наностержнями при одноосном растяжении

The possibility of obtaining macroscale anisotropic thin films from polyvinyl alcohol with included metal nanorods under uniaxial tension under conditions that do not lead to a change in the morphology of metal nanoparticles is shown. Silver and gold nanorods were obtained by directed growth from nuclei and then embedded in a polymer matrix based on polyvinyl alcohol. Initially isotropic films with absorption independent of the polarization of the probing light became anisotropic after stretching, which manifested itself in the dependence of the extinction spectra on the polarization of the probing radiation. The weakening of the longitudinal dipole plasmon resonance mode with the simultaneous enhancement of the transverse dipole plasmon resonance when the light polarization is rotated from 0 to 90 degree indicates the orientation of the nanorods in the film along the direction of its stretching. In addition to the change in absorption in the bands of dipole modes, a strong orientational dependence of absorption in the band of the quadrupole mode of the plasmon resonance of metal nanorods was found.

Study of optical properties of Ag ellipsoid nanostructures by discrete dipole approximation method

In this paper, we investigate optical properties of silver ellipsoid nanostructures (SENs) by means of discrete dipole approximation (DDA), when these nanoparticles are embedded into the water. Absorption, scattering and extinction cross-sections of the SENs were calculated by change of incident wavelength in visible and near infrared region. Moreover, height, wavelength and full width at half maximum (FWHM) of extinction cross-section peaks (due to plasmon resonances) were studied by change of nanostructure's size and dielectric constant of medium. Our results show that, there are only two peaks of transverse dipole and longitudinal dipole modes in this spectrum.

Spin evolution of massive fermion in QED plasma

The dynamical evolution of spin of a massive probe fermion in a massless hot QED plasma at local equilibrium is investigated through the quantum kinetic theory. We consider the massive probe fermion undergoing 2-by-2 Coulomb scattering with the massless fermions in the medium. The axial kinetic equation is derived including the collision terms to the first order of gradients and leading logarithmic order of the coupling. The collision terms are vanishing at global equilibrium, around which the relaxation time can be extracted as an operator. We further decompose the axial kinetic equation into kinetic equations of axial-charge density as well as the transverse magnetic dipole moment, and illustrate the diffusion and polarization effect through preliminary numerical analysis.

Plasmonic transverse dipole moment in chiral fermion nanowires

Plasmons are elementary quantum excitations of conducting materials with Fermi surfaces. In two dimensions they may carry a static dipole moment that is transverse to their momentum which is quantum geometric in nature, the quantum geometric dipole (QGD). We show that this property is also realized for such materials confined in nanowire geometries. Focusing on the gapless, intrasubband plasmon excitations, we compute the transverse dipole moment ${\mathcal{D}}_{x}$ of the modes for a variety of situations. We find that single chiral fermions generically host nonvanishing ${\mathcal{D}}_{x}$, even when there is no intrinsic gap in the two-dimensional spectrum, for which the corresponding two-dimensional QGD vanishes. In the limit of very wide wires, the transverse dipole moment of the highest velocity plasmon mode matches onto the two-dimensional QGD. Plasmons of multivalley systems that are time-reversal symmetric have a vanishing transverse dipole moment but can be made to carry nonvanishing values by breaking the valley symmetry, for example, via a magnetic field. The presence of a nonvanishing transverse dipole moment for nanowire plasmons in principle offers the possibility of continuously controlling their energies and velocities by the application of a static transverse electric field.

Temperature-sensitive hybridization of propagating and localized surface phonon polaritons in polar 4H-SiC nano-resonators

Polar 4H-SiC nano-resonators can host localized surface phonon polaritons (SPhPs) with low optical loss and fully exploit light–matter interactions for promising nanophotonic applications such as surface-enhanced spectroscopies and thermal imaging. To expand the mid-infrared and infrared application spaces, a sound knowledge of temperature effect on infrared dielectric functions and SPhPs of 4H-SiC is required, yet it remains largely unexplored. Herein, we focus on exploiting the temperature influence on dielectric functions, hybridization of propagating and localized SPhPs, and tailed spectral radiation properties of 4H-SiC nanopillar arrays through spectroscopic ellipsometry (SE) measurements as well as multiscale simulations. The 4H-SiC crystal is grown using the physical vapor transport method, and SE experiments measure infrared dielectric functions at temperatures between 300 and 800 K. Finite-element electromagnetic simulations confirm the emerged Monopole and transverse dipoles (TD1 and TD2) resonance modes in 4H-SiC nano-resonators, which agrees with the literature experiment. At high temperatures with strong lattice vibration, the amplitudes of resonant optical absorption peaks gradually decrease and the linewidths broaden, accompanied by the weakened electric resonances. First-principles calculations show that the anharmonic phonon scattering strengthens and less optical phonons are coupled to incident photons as temperature increases. Moreover, the propagating and localized SPhPs’ hybridization and spectral radiation properties of 4H-SiC nanopillar arrays can be largely tailed by modifying the morphology and incident angle of light. This work provides physical insights into the temperature-induced spectral tuning of 4H-SiC nano-resonators and helps exploit their applications in the high-temperature working conditions.

How Do Intermolecular Interactions Evolve at the Nematic to Twist-Bent Phase Transition?

Polarized beam infrared (IR) spectroscopy provides valuable information on changes in the orientation of samples in nematic phases, especially on the role of intermolecular interactions in forming the periodically modulated twist–bent phase. Infrared absorbance measurements and quantum chemistry calculations based on the density functional theory (DFT) were performed to investigate the structure and how the molecules interact in the nematic (N) and twist–bend (NTB) phases of thioether dimers. The nematic twist–bend phase observed significant changes in the mean IR absorbance. On cooling, the transition from the N phase to the NTB phase was found to be accompanied by a marked decrease in absorbance for longitudinal dipoles. Then, with further cooling, the absorbance of the transverse dipoles increased, indicating that transverse dipoles became correlated in parallel. To investigate the influence of the closest neighbors, DFT calculations were performed. As a result of the optimization of the molecular cores system, we observed changes in the square of the transition dipoles, which well corresponds to absorbance changes observed in the IR spectra. Interactions of molecules dominated by pairing were observed, as well as the axial shift of the core to each other.

Development of a fourfold dielectric-filled reentrant cavity as a beam position monitor (BPM) in a proton therapy facility

At the Paul Scherrer Institute (PSI), the superconducting cyclotron “COMET” delivers a 250 MeV proton beam for radiation therapy in pulses of 1ns at the cyclotron-RF frequency of 72.85 MHz. Accurate measurement of the beam position at proton beam currents of 0.1–10 nA in the beam transport line downstream of the degrader is of crucial importance for the treatment safety and quality, beam alignment and feedback systems. This is essential for efficient operation and beam delivery. These measurements are usually performed with intercepting monitors such as ionization chambers (ICs). In this paper, we present a novel non-intercepting position sensitive cavity resonator. The resonant monitor, tuned to the second harmonic of the cyclotron's RF, is based on the detection of the transverse magnetic dipole mode of the EM field generated by the beam. This mode is only excited for off-center beam positions and is measured with the help of four floating cavities within a common grounded cylinder. This paper discusses the BPM fundamental characteristics, design optimization and the underlying parametric investigations involving the contribution of the different modes and crosstalk. We estimate the expected signals from the prototype BPM for position offsets from simulations and compare them with test-bench measurements and beam measurements with the prototype and the improvised BPM design. We conclude by summarizing the achieved position sensitivity, precision, and measurement bandwidth.

Experimental study of transverse effects in planar dielectric wakefield accelerating structures with elliptical beams

The main obstacle to the practical implementation of the dielectric wakefield acceleration (DWA) concept is the development of the beam breakup instability due to transverse dipole wakefields generated when the beam propagates off axis in the accelerating structure. One of the methods to suppress this instability is to elliptically shape the beam and accelerate it in a planar structure. Here, we report a detailed experimental investigation of the transverse dynamics of elliptical beams in planar dielectric structure with parameters mimicking future DWA modules. Both dipole and quadrupole wakefields' effects on beam stability and projected emittance were studied. This study has highlighted the importance of counteracting quadrupole wakefields in future DWA implementations.

Spectral analysis of localized surface phonon polaritons in resonant silicon carbide hollow cylinder array

Manipulation of the amplitude and frequency of resonant optical surface waves in mid-infrared is of great interest for improvement of photonic devices and vibrational molecule sensing applications. Antennas supporting localized surface phonon polaritons (LSPhPs) fold the optical phonons into periodic pillar array to control the scattering process. Energy exchange, mode evolution and near-field coupling mechanism are investigated thoroughly, and it is demonstrated that the transverse dipole mode in the 6H-silicon carbide hollow cylinder array shows excellent absorption efficiency and tunable capability across a wide spectral range. Dependence of local field on structural parameters in the polarized sub-mode is explored to elucidate the optical properties. Near-field coupling is further evaluated by combining the values of current distribution with multipole decomposition. This study also provides a practical guide to establish a general framework for exploring the spectral tuning and coupling mechanisms of LSPhP modes.