Component Of Wave Vector Research Articles

Ferromagnetic Resonance Modes in the Exchange-Dominated Limit in Cylinders of Finite Length

We analyze the magnetic mode structure of axially magnetized finite-length nanoscopic cylinders in a regime where the exchange interaction dominates, along with simulations of the mode frequencies of the ferrimagnet yttrium iron garnet. For the bulk modes, we find that the frequencies can be represented by an expression given by Herring and Kittel by using wavevector components obtained by fitting the mode patterns emerging from these simulations. In addition to the axial, radial, and azimuthal modes that are present in an infinite cylinder, we find localized ``cap modes'' that are ``trapped'' at the top and bottom cylinder faces by the inhomogeneous dipole field emerging from the ends. Semiquantitative explanations are given for some of the modes, in terms of a one-dimensional Schrodinger equation, which is valid in the exchange-dominant case. The assignment of the azimuthal-mode number is carefully discussed, and the frequency splitting of a few pairs of nearly degenerate modes is determined through the beat pattern emerging from them.

Experimental observation of transverse spin of plasmon polaritons in a single crystalline silver nanowire

We report an experimental observation of the transverse spin and associated spin-momentum locking of surface plasmon polaritons (SPPs) excited in a plasmonic single crystalline silver nanowire (AgNW). In contrast to the SPPs excited in metal films, the electromagnetic field components of the evanescent SPP mode propagating along the long axis (x axis) of the NW can decay along two longitudinal planes (x–y and x–z planes), resulting in two orthogonal transverse spin components (sz and sy). Analysis of the opposite circular polarization components of the decaying SPP mode signal in the longitudinal plane (x–y) reveals spin dependent biasing of the signal and, hence, the existence of transverse spin component (sz). The corresponding transverse spin density (s3) in the Fourier plane reveals spin-momentum locking, where the helicity of the spin is dictated by the wave-vector components of the SPP evanescent wave. Furthermore, the results are corroborated with three-dimensional numerical calculations. The presented results showcase that how a chemically prepared plasmonic AgNW can be harnessed to study optical spins in evanescent waves and can be extrapolated to explore sub-wavelength effects, including directional spin coupling and optical nano-manipulation.

Anomalous Reflection with Omnidirectional Active Metasurfaces Operating in Free Space

Metasurfaces that manipulate the reflection of impinging sound have recently received significant attention, but virtually all past designs are passive structures with high temporal and spatial dispersion and thus have narrow bandwidths and are unidirectional. In this work, we introduce a method to design omnidirectional and broadband anomalous acoustic reflectors composed of nonresonant active scatterers arranged in a periodic lattice of subwavelength periodicity. The method relies on the manipulation of the wave-vector component parallel to the reflector's surface (i.e., transverse component) in a broadband manner and for arbitrary impinging sound directions. To illustrate our method experimentally, we design and measure an anomalous reflecting metasurface operating in free space for which the transverse wave-vector components of the incident and reflected waves differ by a prescribed constant. We show that the effect does not depend on the direction of incidence and has a bandwidth an order of magnitude larger than passive anomalous reflectors. This work shows that the active scatterers behave similarly to the atoms of a natural material rather than the unit cells of gratings or phononic crystals in that rearranging the active scatterers changes the metasurface geometry while preserving the device's desired functionality.

Modeling the spectral modification of lower hybrid wave in the presence of drift-wave type density fluctuation in the scrape-off-layer of the EAST tokamak

The spectrum change of the lower hybrid (LH) waves caused by a low-frequency density fluctuation in the scrape-off-layer is studied by applying the wave scattering model developed by Bonoli and Ott [Phys. Fluids 25, 359 (1982)] via a Monte Carlo method. Due to the influence of the density fluctuation, the perpendicular component of the LH wave-vector can be rotated in the 2D perpendicular space, which will further change the ray trajectory of the LH wave. A ray-tracing model specific to this purpose is developed to evaluate the probability distribution of both the poloidal refractive index (Nθ) and the parallel refractive index ( N∥) of the LH wave at the last closed flux surface (LCFS). The scattering probability and the rotation scattering angle are determined by the combined effect from the geometric optics approximation term and the E×B drift term in the LH tensor elements. The probability distributions of N∥ and Nθ at the LCFS are studied using the EAST parameters as a function of wave frequency, the initial N∥, and the polar injection position, which may influence the lower hybrid current drive efficiency.

Gas sensing near exceptional points

Here we investigate the possibility of gas sensing with ultrahigh sensitivity by exploiting the exceptional point (EP) in a non-Hermitian system. The underlying mechanism is that the transverse displacement induced by the photonic spin Hall effect (PSHE), which is significantly enhanced near EPs, is ultra-sensitive to the variation of refractive index in gas media. With the application of the weak measurement technique, we find that the sensitivity of the gas sensing can reach . Moreover, if we further reduce the in-plane wavevector component of probe Gaussian light, the sensitivity can be readily improved, up to , which is much better than that of previous PSHE-based gas sensors operating away from the EP. Therefore, our proposed scheme may be favorable for the development of high-performance sensors.

Engineering acoustic reflections with programmable metasurfaces

Acoustic anomalous reflectors have recently drawn intense attention due to their ability to reflect impinging waves in non-specular directions. However, virtually all previously analyzed anomalous reflectors are passive structures with high spatial and/or temporal dispersion. Therefore, they are typically narrowband and unidirectional, namely they operate in the desired manner for only one direction of incidence. We discuss here a method to design quasi-omnidirectional and broadband anomalous metasurface reflectors composed of programmable non-resonant scatterers capable to manipulate the in-plane wave vector component in prescribed ways. We illustrate the method in 3D experiments in which the incident waves undergo negative reflections and show that the desired effect does not depend on the direction of incidence and has a bandwidth an order of magnitude larger than passive designs. Furthermore, we show experimentally that anomalous reflectors of various geometries can be realized by simply rearranging the active scatterers to fill-in the desired geometries as long as the periodicity of the scatterer lattice remains subwavelength. This is direct evidence that the presented metasurfaces behave like continuous materials rather than gratings or phonic crystal. The implications of this finding on the design of future sound manipulation devices will be discussed.

Finite-Difference Time-Domain Modeling of Periodic Structures: A review of constant wave vector techniques

This article reviews state-of-the-art periodic boundary conditions (PBCs) in finite difference time-domain (FDTD) simulations that operate by enforcing constant wave vector components. The mathematical principles and 3D FDTD implementation details are systematically outlined. Techniques for extracting scattering parameters, Brillouin diagrams, and attenuation constants are presented along with the array scanning method (ASM) used to model the interaction of nonperiodic sources with periodic structures. Through these techniques, the robustness, utility, and efficiency of PBCs are demonstrated and a unified view of the various approaches to the FDTD implementation of PBCs is presented.

Vector solitons in nonlocal optical media with pseudo spin-orbit-coupling.

We numerically investigate the existence and stability of nonlocal vector solitons with pseudo spin-orbit-coupling (SOC). The pseudo SOC is realized by a framework based on the spatial-domain copropagation of two beams with mutually orthogonal polarizations and opposite transverse components of the carrier wave vectors in nonlocal optical media. The numerical results show that there are two kinds of solutions for vector solitons, one is central symmetric, and the other is noncentral symmetric. The solitons may exist below a certain threshold value of the effective SOC strength in the system.

Mathematical modeling of elastic wave radiation during infinite screw dislocation bending vibrations

In this paper, we consider elastic waves radiation (acoustic emission) by infinite screw dislocation performing small bending vibrations under action of external influences. Braking of the dislocation vibrations by medium was not taken into account. To solve this problem, inverse generalized susceptibility of an infinite screw dislocation is written down. It is found that condition for the emission of elastic waves by a screw dislocation is satisfied for two ranges of wave vector component values along the dislocation line. For average value of energy emitted per unit time per unit length of the screw dislocation, corresponding expressions are obtained. Internal friction caused by radiation losses during bending vibrations of the screw dislocation is found.

Current-induced nonreciprocity and refraction-suppressed propagation in a multilayered graphene-dielectric crystal

Nonreciprocal photonic devices play a significant role in regulating the propagation of electromagnetic waves. Here we theoretically investigate the nonreciprocal properties of transverse magnetic modes in a multilayered graphene-dielectric crystal under an applied DC bias. We find that drifting electrons driven by the external DC electric field can give rise to extremely asymmetric dispersion diagrams. Furthermore, when the drifting electrons travel antiparallel to the normal component of the incident wave vector, negative refraction can be strongly suppressed, causing the energy of light to flow along the direction of the electric current. Our theoretical findings can be used to design nonreciprocal optoelectronic devices and enable light to propagate without refraction.

Comparative analysis of information measures of the Dirichlet and Neumann two‐dimensional quantum dots

AbstractAnalytic representation of both position and momentum waveforms of the two‐dimensional (2D) circular quantum dots with the Dirichlet and Neumann boundary conditions (BCs) allowed an efficient computation in either space of Shannon S, Rényi, and Tsallis T(α) entropies; Onicescu energies O; and Fisher information I. It is shown that a transition to the 2D geometry lifts the 1D degeneracy of the R(α) position components Sρ, Oρ, and Rρ(α). Among many other findings, it is established that the lower limit αTH of the semi‐infinite range of the dimensionless Rényi/Tsallis coefficient, where one‐parameter momentum entropies exist, is equal to 2/5 for the Dirichlet requirement and 2/3 for the Neumann one. As their 1D counterparts are 1/4 and 1/2, respectively, this simultaneously reveals that this critical value crucially depends not only on the position BC but the dimensionality of the structure too. As the 2D Neumann threshold is greater than one half, its Rényi uncertainty relation for the sum of the position and wave vector components is valid in the range [1/2, 2) only with its logarithmic divergence at the right edge, whereas for all other systems, it is defined at any coefficient α not smaller than one half. For both configurations, the lowest‐energy level at α = 1/2 does saturate Rényi and Tsallis entropic inequalities. Other properties are discussed and analyzed from mathematical and physical points of view.

Study of tunable locally resonant metamaterials: Effects of spider-web and snowflake hierarchies

The present research is an effort to enhance and control locally resonant bandgaps in periodic metamaterials of hexagonal and triangular topologies by introducing spider-web and snowflake-inspired hierarchies to the conventional geometries. Due to their low connectivity number, hexagonal lattice structures are unable to exhibit locally resonant bandgaps. By introducing the spider-web hierarchy, the connectivity number of the structure is increased, and the constituting beams act as local resonators. Thus, the hexagonal lattice structure can attenuate propagating waves in desirable frequency ranges by localizing the wave energy in the constituent beams. As a result, better wave-filtering performance is achieved by introducing hierarchy to hexagonal lattices. Furthermore, the snowflake hierarchical triangular lattice structures exhibit new tunable attenuation zones for which the locations depend on the mechanical and geometrical parameters. The finite element method and Bloch’s theorem are applied to analyze the wave propagation in the considered architected structures. Moreover, theoretical formulations are presented to predict the locations of each locally resonant bandgaps. It is shown that by changing the mechanical and geometrical parameters of the added hierarchical structures, the locally resonant bandgaps can be perfectly tuned to the desired frequencies in the long-wavelength region. Finally, two theoretical diagrams are proposed to represent initial design concepts of tunable acoustic/elastic metamaterials of hexagonal and triangular unit cells with maximum bandgap widths in low frequencies. To present a more comprehensive analysis of the wave propagation in hierarchical structures, the effects of imaginary wave-vector components on the dispersion curves and attenuation behavior of each topology are also investigated. The results of the current study enable the engineers to design architected periodic structures with the ability to present bandgaps in desired frequencies, which has been the goal of structural engineers for years.

A theory for terahertz lasers based on a graphene hyperbolic metamaterial

We present a theory for terahertz (THz) wave emission in a cavity containing a graphene-based asymmetric hyperbolic metamaterial (AHMM), and the results of the calculation of the output power of the THz laser. An asymmetry appears in the spatial spectrum of the eigenmodes due to a tilt of an anisotropy axis with respect to the interfaces of the AHMM that results in a difference in the wave vector components for waves propagating in opposite directions. This AHMM is an active medium in the THz region due to an inverted population of charge carriers in graphene. The theory is based on the analysis of eigenwaves in the cavity containing the graphene-based AHMM and the use of the transfer matrix method. The components of the Poynting vector, normal to the interface of the AHMM, have been calculated for different eigenmodes, taking into account the gain saturation. The gain saturation manifests itself as a decrease of the negative imaginary part of the effective permittivity of the metamaterial. This gain saturation results from the dependence of the chemical potential in graphene on the component of the electric field transverse to the graphene sheets. Here, the balance of gain versus loss predicts the intensity of the AHMM THz laser emission.

Observing the Goos–Hänchen shift for a planar interface of dielectric and orthorhombic anisotropic medium

The Goos–Hänchen (GH) shift from a planar interface of a dielectric and anisotropic medium (orthorhombic dielectric magnetic anisotropic, in our case) is determined and investigated. The wavevector plane for an anisotropic medium can be divided into four regions, which further supports propagating, propagating and evanescent, evanescent, and non-uniform (ghost) waves. Tangential components of the incident wavevector are adjusted in order to address these regions one by one. The possibility of the existence/non-existence of the GH shift for waves related to these regions is reported.

Emulation of spin-orbit coupling for solitons in nonlinear optical media

We design a framework based on the spatial-domain copropagation of two light beams with mutually orthogonal polarizations and opposite transverse components of carrier wave vectors in a nonlinear waveguide with randomly varying birefringence, the averaging with respect to which introduces an effective Manakov nonlinearity in the system. The corresponding two-component system of nonlinear Schr\odinger equations is derived, being similar to the system of coupled one-dimensional Gross-Pitaevskii equations for a binary spin-orbit-coupled Bose-Einstein condensate. The system may also include an effective Rabi coupling (direct linear mixing of the components) and a periodic potential, representing a photonic-crystal structure in the underlying waveguide. For self-focusing and self-defocusing signs of nonlinearity, soliton solutions of several symmetry types are obtained by means of numerical methods, and their stability is investigated, including gap solitons in the case when the periodic potential is present.

Coherence of vortex Bessel-like beams in a turbulent atmosphere.

Coherent properties of vortex conical waves propagating through a turbulent atmosphere are theoretically studied with the use of the analytical solution of an equation that describes the evolution of the second-order transverse mutual coherence function of an optical radiation field. The following parameters of vortex conical waves are considered: the degree of coherence, the coherence radius, the integral scale of the degree of coherence, and the integral scale of the squared degree of coherence. The effect of atmospheric turbulence on these coherence characteristics of vortex conical waves is analyzed at different values of their parameters. It turns out that the degree of coherence of a vortex conical wave, formed from a Gaussian beam while passing through a conical lens (axicon) and a spiral phase plate, at its optical axis, is almost independent of the initial radius of the Gaussian beam and the radius of the axicon aperture. In addition, all the coherence characteristics of vortex conical waves depend on the topological charge stronger than on the wave-vector component normal to the radiation propagation direction. A meter of the integral scale of the degree of coherence of vortex Bessel-like optical beams is shown to be a preferred sensor of optical radiation distortions in a turbulent atmosphere as compared to a meter of the coherence radius of such beams. A lower degree of coherence of vortex conical waves than of fundamental (vortex-free) conical waves in a turbulent atmosphere is proven with the use of the integral scale of the degree of coherence of these optical waves as a referent criterion.

FRB coherent emission from decay of Alfvén waves

ABSTRACTWe present a model for fast radio bursts (FRBs) where a large-amplitude Alfvén wave packet is launched by a disturbance near the surface of a magnetar, and a substantial fraction of the wave energy is converted to coherent radio waves at a distance of a few tens of neutron star radii. The wave amplitude at the magnetar surface should be about 1011 G in order to produce an FRB of isotropic luminosity 1044 erg s−1. An electric current along the static magnetic field is required by Alfvén waves with non-zero component of transverse wave vector. The current is supplied by counter-streaming electron–positron pairs, which have to move at nearly the speed of light at larger radii as the plasma density decreases with distance from the magnetar surface. The counter-streaming pairs are subject to two-stream instability, which leads to formation of particle bunches of size of the order of c/ωp, where ωp is the plasma frequency. A strong electric field develops along the static magnetic field when the wave packet arrives at a radius where electron–positron density is insufficient to supply the current required by the wave. The electric field accelerates particle bunches along the curved magnetic field lines, and that produces the coherent FRB radiation. We provide a number of predictions of this model.

Phase-matched frequency conversion in waveguides by means of transverse wavevector projections

We present a versatile technique for modal phase-matched second-harmonic generation in a waveguide, which relies on the incident electromagnetic field having transverse wavevector components on the input facet of the waveguide. Such a requirement is satisfied by using an angular-coupling arrangement, which allows for the excitation of modes having an odd spatial distribution that contribute to modal phase-matched second-harmonic generation. The strength of the second-harmonic signal is directly related to the angle at which the incident field is coupled into these modes. This is critical for on-chip nonlinear frequency conversion processes in the areas of optical communication, optical computing, and quantum computing.

Probing the in-plane electron spin polarization in Ge/ Si0.15Ge0.85 multiple quantum wells

We investigate spin transport in a set of $\text{Ge}/{\text{Si}}_{0.15}{\text{Ge}}_{0.85}$ multiple quantum wells (MQWs) as a function of the well thickness. We exploit optical orientation to photogenerate spin-polarized electrons in the discrete energy levels of the well conduction band at the $\mathrm{\ensuremath{\Gamma}}$ point of the Brillouin zone. After diffusion, we detect the optically oriented spins by means of the inverse spin-Hall effect (ISHE) taking place in a thin Pt layer grown on top of the heterostructure. The employed spin injection/detection scheme is sensitive to in-plane spin-polarized electrons, therefore, by detecting the ISHE signal as a function of the photon energy, we evaluate the spin polarization generated by optical transitions driven by the component of the light wave vector in the plane of the wells. In this way, we also gain insight into the electron spin-diffusion length in the MQWs. The sensitivity of the technique to in-plane spin-related properties is a powerful tool for the investigation of the in-plane component of the spin polarization in MQWs, which is otherwise commonly inaccessible.

Quantum threshold reflection of He-atom beams from rough surfaces

Quantum reflection of thermal He atoms from various surfaces (glass slide, GaAs wafer, flat and structured Cr) at grazing conditions is studied within the elastic close-coupling formalism. Comparison with the experimental results of B.S. Zhao et al, Phys. Rev. Lett. {\bf 105}, 133203 (2010) is quite reasonable but the conclusions of the present theoretical analysis are different from those discussed in the experimental work. The universal linear behavior observed in the dependence of the reflection probability on the incident wave vector component perpendicular to the surface is only valid at small values of the component whereas, at larger values, deviation from the linearity is evident, approaching a quadratic dependence at higher values. The surface roughness seems to play no important role in this scattering. Moreover, the claim that one observes a transition from quantum to classical reflection seems to be imprecise.