Values Of Wave Vector Research Articles

Determining pair distribution functions of thin films using laboratory-based X-ray sources.

This article demonstrates the feasibility of obtaining accurate pair distribution functions of thin amorphous films down to 80 nm, using modern laboratory-based X-ray sources. The pair distribution functions are obtained using a single diffraction scan without the requirement of additional scans of the substrate or of the air. By using a crystalline substrate combined with an oblique scattering geometry, most of the Bragg scattering of the substrate is avoided, rendering the substrate Compton scattering the primary contribution. By utilizing a discriminating energy filter, available in the latest generation of modern detectors, it is demonstrated that the Compton intensity can further be reduced to negligible levels at higher wavevector values. Scattering from the sample holder and the air is minimized by the systematic selection of pixels in the detector image based on the projected detection footprint of the sample and the use of a 3D-printed sample holder. Finally, X-ray optical effects in the absorption factors and the ratios between the Compton intensity of the substrate and film are taken into account by using a theoretical tool that simulates the electric field inside the film and the substrate, which aids in planning both the sample design and the measurement protocol.

Optical intensity figures of merit of insulator-metal-insulator and metal-insulator-metal thin-film stacks

This paper discusses the quality factors Q and the intensity figures of merit (IFoM) evaluating the intensity and leakage of modes of the reflection flux and of the plane-wave and locally excited transmitted fluxes of insulator-metal-insulator (IMI) and metal-insulator-metal (MIM) 2D planar thin-film stacks, here air-Au-glass and air-Au-SiO2-Au-glass stacks respectively. These thin film stacks sustain a single surface plasmon polariton (SPP) and multiple planar waveguide (PWG) modes. The Q and IFoM of the 3D dispersion graph (in-plane wave vector k ρ /k 0 ∈ [0, 1.52]/frequency ω ∈ [0.5, 2.7] eV/observable dispersion) are calculated and analyzed along 2D cuts where either the in-plane wave vector k ρ /k 0 or the frequency ω are varied the other independent variable being kept fixed. Here these two cuts are called spatial (ω fixed) and frequency (k ρ /k 0 fixed) domains. Due to a lower leakage, the Q and IFoM of the IMI and MIM thin film stack modes are significantly larger in the spatial domain than in the frequency domain. In the spatial domain the IMI and MIM stack modes dominate at low and high frequencies respectively. In the frequency domain, the Q and IFoM of a MIM stack mode is always larger than that of an IMI stack. Our results span a large domain of frequencies in the SPP and RPP region and of the in-plane wave vector whereas the results in the literature presented above concern only particular laser frequencies and limited in-plane wave vector values. Our Q and IFoM of the 2D planar thin film stack modes, obtained with optimized independent variables, are larger than those of other planar thin film stacks but smaller than some 2D/3D nano scale samples with an involved geometry. The simplicity of producing these simple IMI and MIM stacks permit their use in the applications.

Non-reciprocity in photonic crystal due to parity time symmetric defect state

In this article, defect state of photonic crystal regarding parity time known as PT-symmetric is explored thoroughly. For this purpose, transfer matrix technique in setting Bloch theorem is employed to characterize the dispersion relation regarding PT-symmetric defect state. The study uncovers the norteworthy wave phenomenon entitle “non-reciprocity” is exhibited. Moreover, an increasing the in-plane component of wavevector contributes two or more branches in the short wavelength boundary of the stop-band which is not observed in conventional structures. The reflectance of structure is conducted across the optical regime leads to minimum point that aligns with the wavelength of the eigen mode for wavevector value (kx = 0). These findings of research may have significant applications in various areas of photonics and optics likes filters, modern optical photonic cavity resonators and thermo-photovoltaic devices etc.

Surface recoil force on dielectric nanoparticle enhancement via graphene acoustic surface plasmon excitation: non-local effect consideration.

Controlling optomechanical interactions at sub-wavelength levels is of great importance in academic science and nanoparticle manipulation technologies. This Letter focuses on the improvement of the recoil force on nanoparticles placed close to a graphene-dielectric-metal structure. The momentum conservation involving the non-symmetric excitation of acoustic surface plasmons (ASPs), via near-field circularly polarized dipolar scattering, implies the occurrence of a huge momentum kick on the nanoparticle. Owing to the high wave vector values entailed in the near-field scattering process, it has been necessary to consider the non-locality of the graphene electrical conductivity to explore the influence of the scattering loss on this large wave vector region, which is neglected by the semiclassical model. Surprisingly, the contribution of ASPs to the recoil force is negligibly modified when the non-local effects are incorporated through the graphene conductivity. On the contrary, our results show that the contribution of the non-local scattering loss to this force becomes dominant when the particle is placed very close to the graphene sheet and that it is mostly independent of the dielectric thickness layer. Our work can be helpful for designing new and better performing large plasmon momentum optomechanical structures using scattering highly dependent on the polarization for moving dielectric nanoparticles.

Topological hyperbolic metamaterials

Abstract Hyperbolic metamaterial (HMM) is a unique type of anisotropic material that can exhibit metal and dielectric properties at the same time. This unique characteristic results in it having unbounded isofrequency surface contours, leading to exotic phenomena such as spontaneous emission enhancement and applications such as super-resolution imaging. However, at optical frequencies, HMM must be artificially engineered and always requires a metal constituent, whose intrinsic loss significantly limits the experimentally accessible wave vector values, thus negatively impacting the performance of these applications. The need to reduce loss in HMM stimulated the development of the second-generation HMM, termed active HMM, where gain materials are utilized to compensate for metal’s intrinsic loss. With the advent of topological photonics that allows robust light transportation immune to disorders and defects, research on HMM also entered the topological regime. Tremendous efforts have been dedicated to exploring the topological transition from elliptical to hyperbolic dispersion and topologically protected edge states in HMM, which also prompted the invention of lossless HMM formed by all-dielectric material. Furthermore, emerging twistronics can also provide a route to manipulate topological transitions in HMMs. In this review, we survey recent progress in topological effects in HMMs and provide prospects on possible future research directions.

Trivelpiece–Gould modes and low-frequency electron–ion instability of non-neutral plasma

The frequency spectra of the Trivelpiece–Gould modes of a waveguide partially filled with non-neutral plasma are determined numerically by solving the dispersion equation. The modes having azimuthal number $m = 1$ are considered. The results are presented for the entire acceptable range of electron densities, magnetic field strengths, for different values of the charge neutralization coefficient. The Cherenkov resonance condition of an ion with a diocotron mode having a finite value of the longitudinal wave vector was studied. The characteristics of resonant low-frequency electron–ion instability caused by relative azimuth motion of electrons and ions in crossed fields and by the anisotropy of the distribution function of ions are discussed. Ions are created by ionization of residual gas in the plasma volume. Due to the anisotropy, instability occurs not only in the vicinity of the resonance, but also outside it. For typical values of plasma parameters in experiments, estimations of the frequency growth rate are given. A conclusion is drawn that this instability can be the cause of the low-frequency oscillations observed in linear devices with non-neutral plasma produced in an electron beam channel.

Fourier-space generalized magneto-optical ellipsometry

The magneto-optical Kerr effect (MOKE) is widely exploited in laboratory-based setups for the study of thin films and nanostructures, providing magnetic characterization with good spatial and temporal resolutions. Due to the complex coupling of light with a magnetic sample, conventional MOKE magnetometers normally work by selecting a small range of incident wave-vector values, focusing the incident light beam to a small spot, and recording the reflected intensity at that angular range by means of photodetectors. Using this approach, additional methodologies and measurements are required for full vectorial magnetic characterization. Here, we computationally investigate a Fourier-space MOKE setup, where a focused beam ellipsometer using high numerical aperture optics and a camera detector is employed to simultaneously map the intensity distribution for a wide range of incident and reflected wave vectors. We employ circularly incident polarized light and no analyzing optics, in combination with a fitting procedure of the light intensity maps to the analytical expression of the Kerr effect under linear approximation. In this way, we are able to retrieve the three unknown components of the magnetization vector as well as the material' s optical and magneto-optical constants with high accuracy and short acquisition times, with the possibility of single-shot measurements. Fourier MOKE is thus proposed as a powerful method to perform generalized magneto-optical ellipsometry for a wide range of magnetic materials and devices.

TRIVELPIECE-GOULD MODES OF WAVEGUIDE PARTIALLY FILLED WITH NON-NEUTRAL PLASMA. PART 2

The frequency spectra of electron eigenmodes with the azimuthal number m =1 of a waveguide partially filled with non-neutral plasma are determined numerically within the entire allowable range of electron densities and magnetic field strengths, for various values of the degree of charge neutralization and finite values of the longitudinal wave vector.

Composite basis set of plane wave and Gaussian function or spline function

By combining plane waves with Gaussian or spline functions, a new composite basis set is constructed in this work. As a non local basis vector, the plane wave basis group needs a large number of plane waves to expand all parts of the physical space, including the intermediate regions that are not important to our problems. Our basis set uses the local characteristics of Gaussian function or spline function at the same time, and controls the energy interval by selecting different plane wave vectors, in order to realize the partition solution of Hamiltonian matrix. Orthogonal normalization of composite basis sets is performed by using Gram-Schmidt’s orthogonalization method or Löwdin’s orthogonalization method. Considering the completeness of plane wave vector, a certain value of positive and negative should be selected at the same time. Here, by changing the absolute value of wave vector, we can select the eigenvalue interval to be solved. The plane wave with a specific wave vector value is equivalent to a trial solution in the region with gentle potential energy. The algorithm automatically combines local Gaussian or spline functions to match the difference in wave vector value between the trial solution and the strict solution. By selecting the absolute value of the wave vector in the plane wave function, the calculation of large Hamiltonian matrices turns into the calculation of multiple small matrices, together with reducing the basis numbers in the region where the electron potential changes smoothly, therefore, we can significantly reduce the computational time. As an example, we apply this basis set to a one-dimensional finite depth potential well. It can be found that our method significantly reduce the number of basis vectors used to expand the wave function while maintaining a suitable degree of computational accuracy. We also study the influence of different parameters on calculation accuracy. Finally, the above calculation method can be directly applied to the density functional theory (DFT) calculation of plasmons in silver nanoplates or other metal nanostructures. Given a reasonable tentative initial state, the ground state electron density distribution of the system can be solved by self-consistent solution through using DFT theory, and then the electromagnetic field distribution and optical properties of the system can be solved by using time-dependent density functional theory.

Polarization eigenstates analysis of helically structured thin films.

The optical properties of thin films are generally determined by direct photometric quantities. We show that additional insight into the properties of anisotropic thin films can be obtained by computing the polarization eigenstates and eigenvalues of their Jones matrices. We consider helically structured thin films, which display intriguing optical response, such as the circular Bragg resonance. Using numerical simulations and actual measurements, we show that the eigenvectors are mutually orthogonal in most regions of the wavevector space, except near the circular Bragg and the oblique resonances. Special wavevector values, called exceptional points, are found where the Jones matrix becomes defective and its eigenvectors coalesce. Exceptional points are also found in pairs of wavevector values differing only by a sample rotation by π around the direction normal to the sample; this property is shown to arise from Saxton - de Hoop's reciprocity principle, which applies to lossy materials and contains time reversal symmetry, which only applies to lossless materials, as a special case.

Hybridized propagating spin waves in a CoFeB/IrMn bilayer

In this work, we report the propagating spin waves hybridized between first-order and quasiuniform modes in a $\mathrm{Co}_{20}\mathrm{Fe}_{60}\mathrm{B}_{20}$ thin film capped by $\mathrm{Ir}_{25}\mathrm{Mn}_{75}$. The anticrossing gaps are observed at room temperature both in the reflection and transmission spectra, where the coupling strength can be tuned by varying the value of the in-plane wave vector at which the dispersion curves cross. The key mechanism behind this feature is theoretically ascribed to the dipole-dipole interaction by a model which accounts for many features of our experimental results. The strong coupling with a cooperativity up to 2.0 is achieved with taking the dissipation rates of two coupled branches into account. A reference CoFeB sample without IrMn reveals that the interfacial pinning effect also plays a role in the magnon-magnon hybridization. Tunable hybridization of magnons may foster the field of magnonics in information processing.

Infrared plasmons in ultrahigh conductive PdCoO2 metallic oxide

PdCoO2 layered delafossite is the most conductive compound among metallic oxides, with a room-temperature resistivity of nearly 2,mu Omega ,{{{{{rm{cm}}}}}}, corresponding to a mean free path of about 600 Å. These values represent a record considering that the charge density of PdCoO2 is three times lower than copper. Although its notable electronic transport properties, PdCoO2 collective charge density modes (i.e. surface plasmons) have never been investigated, at least to our knowledge. In this paper, we study surface plasmons in high-quality PdCoO2 thin films, patterned in the form of micro-ribbon arrays. By changing their width W and period 2W, we select suitable values of the plasmon wavevector q, experimentally sampling the surface plasmon dispersion in the mid-infrared electromagnetic region. Near the ribbon edge, we observe a strong field enhancement due to the plasmon confinement, indicating PdCoO2 as a promising infrared plasmonic material.

Dynamic Simulation of the Energy Spectrum of Phonons in the Magnetic BCC Iron


 
 
 We used the symplectic and scalable algorithm for spin lattice dynamics embedded in LAMMPS to model the coupled relaxation processes of the spin and lattice subsystems to investigate the phonon dispersion of bcc Fe at T = 300 K. The atomic interactions were modelled via three semi-empirical many-body potentials within the embedded atom method, while the distance-dependent spin coupling relied on the Heisenberg-type Hamiltonian. In the state of mutual equilibrium of the spin and atom ensembles, we have calculated the dynamical matrix and the phonon spectra in bcc iron. We found that for a small to moderate wavevector absolute values, the phonon dispersion curves agree well with the experimental results obtained from inelastic neutron scattering, while discrepancies between theory and experiment were observed for larger wavevector values, particularly near the zone boundaries. Moreover, the impact of magnons on the phonon spectra is pronounced for all employed potentials.
 
 

Spin-resonant tunneling in CdTe/Cd1-xMnxTe double-barrier heterostructure with zero external field: Effect of in-plane wave vector

The tunneling of light hole (LH) and heavy hole (HH) in CdTe/Cd1-xMnxTe double-barrier heterostructure with Dresselhaus spin-orbit interaction is studied. The effect of in-plane wave vector on barrier transparency, polarization efficiency and dwell time is theoretically analyzed. The in-plane wave vector increases the polarization efficiency of LH and HH. The enhanced spin-separation is achieved at high value of in-plane wave vector. The dwell time of LH and HH is also analyzed as a function of in-plane wave vectors.

Theoretical Modeling and Analysis of the Contribution of the Near-Field Absorption to the Dipole Radiation Power in Top-Emitting Organic Light-Emitting Diodes

We theoretically model the near-field (NF) absorption for a multilayer micro-cavity (MMC) structure and investigate the contribution of the NF absorption to the dipole radiation power in top-emitting organic light-emitting diodes (OLEDs). The NF absorption occurs due to the interaction between an evanescent wave with a large in-plane wave vector and a planar metal layer in the vicinity of the dipole radiation. The analytical expressions of the NF absorption in the MMC structure are derived from the plane wave expansions of the electric field amplitude, which includes the two-beam and multi-beam interference terms. The transverse magnetic polarization light emitted by both horizontally and vertically oriented dipole emitters is considered in the NF absorption while the contribution of the transverse electric polarization light is neglected. Based on the total spectral power density calculated in a top-emitting OLED, the respective spectral response functions of surface plasmon (SP) modes and NF absorption are compared, where the summation of the Lorentzian line shape functions is used to represent spectral responses of SP modes. At large values of in-plane wave vectors, the spectral response caused by the NF absorption becomes significant and approaches the total spectral power density. In addition, the relative optical powers from various dipole dissipation mechanisms are calculated with respect to the dipole emitter position in the emission layer (EML), which shows the optical power coupled to the NF absorption is predominant over other mechanisms when the distance between the dipole emitter and the EML/Ag interface is less than 10 nm in the top-emitting OLED.

SMALL-ANGLE SCATTERING OF NEUTRONS ON Sr<sub>2</sub>FeMoO<sub>6–δ</sub> SAMPLES WITH DIFFERENT-DEGREE SUPERSTRUCTURAL ORDERING OF Fe/Mo CATIONS

Single-phase Sr2FeMoO6-δ samples with different-degreesuperstructiral ordering Fe/Mo cations superstructural ordering (P, 76, 86 and 93 %) were obtained by the solid-phase technique. Based on the results of measuring the magnetic characteristics in the samples, we found that an increase in magnetization (26.41, 32.36 and 42.66 A·m2·kg–1), magnetic moment (1.33, 3.07 and 3.58 μB/f.u.) and Curie temperatures (422, 428 and 437 K) withparameter P (76, 86 and 93 %) can be explained by the presence of antistructural defects, as well as antiferromagnetic inclusions. This determines the redistribution of electron density, which is accompanied by the change in electronic configuration of a part of Fe/Mo cations. Based on the temperature dependences of the magnetic moment of the samples measured in ZFC and FC modes, and on small-angle polarized neutron scattering (SANS), we found that the samples are in a magnetically inhomogeneous state. An important result to mention is that we detected the difference between the slope of the SANS curves of samples with different oxygen content, which demonstrates a different microstructure of inhomogeneities. The main inhomogeneities are magnetic inclusions with the dimensions depending on the superstructural ordering of Fe/Mo cations. According to the Porod law, it was shown that the Sr2FeMoO6-δ samples with wave vector values 0.1 > q > 0.002 Å–1 contain polydisperse grains with a smooth surface. For q > 0.1 Å–1 a deviation from the Porod law is observed, confirming the presence of magnetic inhomogeneities with a diameter < 6 nm in the grains.

Influence of magnetic field orientation on laser-induced magnetization precession in Ni-Mn-Sn Heusler alloy film

Laser-induced magnetization precession is investigated in Ni-Mn-Sn Heulser alloy film using pump-probe all-optical technique. The studies are performed as a function of external magnetic field H applied at various angles θH with respect to the sample normal. Strong dependence of precession parameters, frequency f, amplitude A and effective damping αeff on H and θH was found. For H>3 kOe and θH<25°, two precession modes, identified as Damon-Eshbach (DE) and first-order standing spin wave (PSSW) modes were observed. The values of wave vector k=0.37 μm−1 for DE mode and exchange constant Aex=0.22 μerg cm−1 for PSSW mode have been determined. The A(H) increases up to six times as θH decreases from 55° to 5° and exhibits a maximum at around H=1.8 kOe for each θH. From experimentally obtained exponentially decayed αeffH dependencies, intrinsic damping α0θH is estimated to ≈0.025. The dependencies of the magnetization precession parameters have been well simulated in the frame of phenomenological approach, based on Landau-Lifshitz-Gilbert equation.

Light-matter interaction of a quantum emitter near a half-space graphene nanostructure

The Purcell factor and the spontaneous emission spectrum of a quantum emitter (QE) placed close to the edge of a graphene half-space nanostructure is investigated, using semi-analytical methods at the electrostatic regime. The half-space geometry supports an edge and a bulk surface plasmon (SP) mode. The Purcell factor of the QE is enhanced over eight orders of magnitude when its emission energy matches the resonance energy modes, for a specific value of the in-plane wave vector, at a separation distance of $5\,$nm. The different transition dipole moment orientations influence differently the enhancement factor of a QE, leading to large anisotropic behavior when positioned at different places above the half-space geometry. The field distribution is presented, showing clearly the excitation of the SP modes at the edge of the nanostructures. Also, we present the spontaneous emission spectrum of the QE near the half-space graphene nanostructure and show that strong light-matter coupling may emerge. When a QE with a free-space lifetime of $1\,$ns is placed at a distance of $10\,$nm away from the edge of the graphene half-space, a Rabi splitting of $80 \,m$eV is found. Our contribution can be used for designing future quantum applications using combination of QEs and graphene nanostructures.

TM band diagram of a waveguide in a honeycomb photonic lattice composed by triangular-shaped rods

Using the plane wave expansion and the supercell technique, we calculate the TM band diagram of a waveguide in a honeycomb lattice that composed rods with triangular cross-sections embedded in air. For this study, we use GaAs rods with a dielectric function dependent on pressure and temperature. We report the existence of a defect band within the photonic band gap that presents large confinement for small wave vector values along the line defect. However, for large wave vector values, the defect band approaches the regular crystal bands, thus scattering a greater amount of energy. When the triangles are rotated, greater confinement and smaller scattering of energy is observed than that for unrotated triangles. Additionally, we report that the TM band diagram and the defect band may be tuned to higher frequencies by increasing the applied pressure.

Nonlinear optical lattices with a void impurity.

We examine a one-dimensional nonlinear (Kerr) waveguide array which contains a single "void" waveguide where the nonlinearity is identically zero. We uncover a family of nonlinear localized modes centered at or near the void, and their stability properties. Unlike a usual impurity problem, here the void acts like a repulsive impurity causing the center of the simplest mode to lie to the side of the void's position. We also compute the stability of extended nonlinear modes showing significant differences from the usual homogeneous nonlinear array. The transmission of a nonlinear pulse across the void shows three main regimes-transmission, reflection, and trapping at the void's position-and we identify a critical momentum for the pulse below (above) where the pulse gets reflected (transmitted), or trapped indefinitely at the void's position. For relatively wide pulses, we observe a steep increase from a regime of no transmission to a regime of high transmission, as the amplitude of the soliton increases beyond a critical wave-vector value. Finally, we consider the transmission of an extended nonlinear wave across the void impurity numerically, finding a rather complex structure of the transmission as a function of wave vector, with the creation of more and more transmission gaps as nonlinearity increases. The overall transmittance decreases and disappears eventually, where the boundaries separating passing from nonpassing regions are complex and fractal-like.