Values Of Wave Vector Research Articles

Derivation of Hamaker Dispersion Energy of Amorphous Carbon Surfaces in Contact with Liquids Using Photoelectron Energy-Loss Spectra

Hamaker interaction energies and cutoff distances have been calculated for disordered carbon films, in contact with purely dispersive (diiodomethane) or polar (water) liquids, using their experimental dielectric functions e (q, ω) obtained over a broad energy range. In contrast with previous works, a q-averaged q is derived from photoelectron energy-loss spectroscopy (XPS-PEELS) where the energy loss function (ELF) q is a weighted average over allowed transferred wave vector values, q, given by the physics of bulk plasmon excitation. For microcrystalline diamond and amorphous carbon films with a wide range of (sp3/sp2 + sp3) hybridization, non-retarded Hamaker energies, A 132 (L < 1 nm), were calculated in several configurations, and distance and wavenumber cutoff values were then calculated based on A 132 and the dispersive work of adhesion obtained from contact angles. A geometric average approximation, H 0 CVL = (H 0 CVC H 0 LVL )1/2, holds for the cutoff separation distances obtained for carbon-vacuum-liquid (CVL), carbon-vacuum-carbon (CVC) and liquid-vacuum-liquid (LVL) equilibrium configurations. The linear dependence found for A CVL, A CLC and A CLV values as a function of A CVC, for each liquid, allows predictive relationships for Hamaker energies (in any configuration) using experimental determination of the dispersive component of the surface tension, $$ {\gamma}_{CV}^d $$ , and a guess value of the cutoff distance H 0 CVC of the solid.

Thermally controlled mid-IR band-gap engineering in all-glass chalcogenide microstructured fibers: a numerical study

Presence of photonic band-gap (PBG) in an all-glass low refractive index (RI) contrast chalcogenide (Ch) microstructured optical fibers (MOFs) is investigated numerically. The effect of external temperature on the position of band-gap is explored to realize potential fiber-based wavelength filters/sensors at functional mid-IR spectral range. The cross-sectional geometry of the MOF is formed by considering a Ch glass to form the overall background cross-section as well as the central fiber core. The core region is surrounded by periodically arranged (hexagonal pattern) smaller holes, which are assumed to be filled up with another Ch glass. Thermally compatible and fabrication suitable, two Ch glasses are chosen, one (higher RI) as background material and the other (of lower RI) to fill up the holes. Two sets of such pairs of thermally compatible Ch-glasses are considered as fiber structural materials with relative RI contrast of ∼12% and ∼24%. For both such low RI contrast hexagonal structures, PBG appears only for suitable finite values of longitudinal wave vector. The structures are suitable to realize band-gap at mid-IR wavelengths and specifically optimized for operation around the ∼2 μm region. Then the temperature sensitivity of band-gaps is investigated to design fiber-based mid-IR wavelength filters/sensors.

Unified homogenization of photonic/phononic crystals with first-band negative refraction

It is shown, for the first time, that negative refraction with positive phase velocity refraction can be realized (and controlled) on a wide frequency-range of the first (lowest) pass band of simple photonic and phononic crystals.First a unified approach is presented to accurately, efficiently, and uniquely produce the homogenized effective material properties of doubly periodic phononic crystals for anti-plane shear (SH) and photonic crystals for transverse electric (TE) and transverse magnetic (TM) electromagnetic Bloch-form waves in such a manner that they exactly reproduce the band structure of the composite over any desired frequency band. Then the correspondence between phononic and photonic field equations is established and their effective homogenized material parameters are calculated. Finally the actual calculation procedure is detailed and illustrative examples are worked out for each case, revealing a rich body of refractive characteristics of these crystals, and showing by means of illustrative examples that the homogenized effective parameters do in fact yield the exact results. Thus the resulting homogenized solid embodies exactly the actual band structure and dispersive properties of the considered phononic/photonic crystal, which clearly distinguishes it from any anisotropic normal material.When in contact with a normal homogeneous half-space, the homogenized medium would display positive, negative, or even no energy refraction depending on the frequency and wave vector of plane waves incident from the normal homogeneous solid to the interface. Remarkably, using the same volume fraction of the same constituents, it is possible to design a (homogenized) material that would display negative energy refraction with positive phase velocity refraction even on the first (acoustic) pass band. The window of the frequency and the range of wave-vector values at which, say, the negative energy refraction would occur can be controlled by judicious selection and design of the unit cell’s constituent geometry and properties.The unit cell of the crystal may consist of anisotropic constituents of any constant or variable properties that may also admit large discontinuities. For elliptical and rectangular inclusion geometries, analytical expressions are given for the calculation of the key quantities.

Functional crossover in the dispersion relations of magnons and phonons

Experimental data are presented showing that the dispersion relations of magnons and acoustic phonons can consist of two sections with different functions of wave vector. In the low wave vector range a power function of wave vector often holds over a finite q-range while dispersions for larger wave vector values better approach the atomistic model predictions. In the magnon spectra ∼⃒qx power functions with exponents x=1.25, 1.5 and 2 are identified. The dispersion of the acoustic phonons can be a linear function of wave vector over a surprisingly large range of energy. Since the slope of the linear section agrees with the known sound velocities it can be concluded that the dispersion of the acoustic phonons has got attracted by the linear dispersion of the mass less Debye bosons (sound waves). Due to the different (translational) symmetries of bosons and atomistic excitations (magnons, phonons) the associated dispersions can attract each other. In the same way the different ∼⃒qx power functions in the magnon dispersions indicate that magnon dispersions are attracted by the dispersion of the bosons of the magnetic continuum (Goldstone bosons). This allows evaluation of the otherwise difficult to obtain dispersions of the Goldstone bosons from the known magnon dispersions. Interestingly, the dispersions of Goldstone bosons (Debye bosons) attract magnon dispersions (phonon dispersions) and not vice versa.

Design of a Polymer-Based Hollow-Core Bandgap Fiber for Low-Loss Terahertz Transmission

We use numerical simulations to design a hollow-core microstructured polymer optical fiber (HC-mPOF) suitable for broadband, terahertz (THz) pulse transmission with relatively low losses and small dispersion. The HC-mPOF consists of a central large air-core surrounded by periodically arranged wavelength-scale circular air holes in a hexagonal pattern, embedded in a uniform Teflon matrix. The THz guidance in this fiber is achieved by exploiting the photonic bandgap (PBG) effect. In our low index contrast Teflon-air (1.44:1) hexagonal periodic lattice, the PBG appears only for a certain range of non-zero values of the longitudinal wavevector. We have achieved PBG over a broad spectral range (bandwidth $\sim 400$ GHz) ranging from 1.65 to 2.05 THz in the proposed HC-mPOF. The achievable loss coefficient in our designed HC-mPOF is $ and the group velocity dispersion parameter is $\cdot $ cm over a 300-GHz bandwidth ( $1.65\sim 1.95$ THz).

Donor wave functions in Si gauged by STM images

The triumph of effective mass theory in describing the energy spectrum of dopants does not guarantee that the model wavefunctions will withstand an experimental test. Such wavefunctions have recently been probed by scanning tunneling spectroscopy, revealing localized patterns of resonantly enhanced tunneling currents. We show that the shape of the conducting splotches resemble a cut through Kohn-Luttinger (KL) hydrogenic envelopes, which modulate the interfering Bloch states of conduction electrons. All the non-monotonic features of the current profile are consistent with the charge density fluctuations observed between successive {001} atomic planes, including a counterintuitive reduction of the symmetry - a heritage of the lowered point group symmetry at these planes. A model-independent analysis of the diffraction figure constrains the value of the electron wavevector to $k_0 = (0.82 \pm 0.03)(2\pi/a_{Si})$. Unlike prior measurements, averaged over a sizeable density of electrons, this estimate is obtained directly from isolated electrons. We further investigate the model-specific anisotropy of the wave function envelope, related to the effective mass anisotropy. This anisotropy appears in the KL variational wave function envelope as the ratio between Bohr radii b=a. We demonstrate that the central cell corrected estimates for this ratio are encouragingly accurate, leading to the conclusion that the KL theory is a valid model not only for energies but for wavefunctions as well.

Ground-state energy of spin polarized two-dimensional electron gas in the presence of impurity scattering

In the present work we study the spin polarization effects on the ground-state energy of an interacting two-dimensional electron gas in the presence of disorder scattering (impurity scattering) at arbitrarily spin polarization for various electron densities. For this purpose we employ the modified density-density correlation function at small values of wave-vector and frequency in the framework of linear response theory to calculate the exchange, correlation and ground-state energy for a spin polarized two-dimensional disordered electron gas. Our results show that the effect of spin polarization on the exchange, correlation and ground-state energy becomes considerable at high densities. We also conclude that the two-dimensional disordered electron system becomes unstable by increasing the spin polarization degree.

Demonstration of hyperbolic metamaterials at telecommunication wavelength using Ga-doped ZnO.

Hyperbolic metamaterials (HMMs) have attracted much attention because they allow for broadband enhancement of spontaneous emission and imaging below the diffraction limit. However, HMMs with traditional metals as metallic component are not suitable for applications in the infrared spectral range. Using Ga-doped ZnO, we demonstrate monolithic HMMs operating at infrared wavelengths. We identify the material's hyperbolic character by various optical measurements in combination with theoretical calculations. In particular, negative refraction of the extraordinary wave and propagation of light with wave vector values exceeding that of free-space are demonstrated in the entire telecommunication window. These findings reveal a considerable potential for creating novel functional elements at telecommunication wavelengths.

On the Distinction between Debye Bosons and Phonons

A method is described that allows one to construct the dispersion of the Debye bosons (sound waves) from the known temperature dependence of the sound velocities. The method simply assumes that the sound velocity measured at temperature T gives the slope of the dispersion relation at excitation energy E=kB·T. The associated reduced wave vector is set equal to q/q0=a0kBT/hvL/T. In this way the dispersion of the Debye bosons can be constructed for all thermal energies for which sound velocities vL/T(T) are known. This can be up to melting temperature. Surprisingly, the dispersion of the Debye bosons continues beyond zone boundary. At melting temperature the wavelength of the Debye bosons is of the order of the atomic diameters. The sources of the Debye bosons therefore must have atomic dimensions. Spontaneous generation of Debye bosons by individual atoms is, however, a completely unexplored process. Interactions with the atomistic background of phonons or lattice defects provide damping to the Debye bosons and make vL/T(T) sample and temperature dependent. Quite generally vL/T(T) decreases as a function of increasing temperature. For high energies the dispersion relation of the Debye bosons therefore becomes visibly lower than linear. Interactions between Debye bosons and phonons can modify the dispersion of the acoustic phonons appreciably. Because of their different symmetries the dispersion relations of Debye bosons and acoustic phonons attract each other. It is observed that for low wave vector values the dispersion of the acoustic phonons can assume the linear wave vector dependence of the Debye bosons. At the end of the linear section a functional crossover to a sine-like function of wave vector occurs.

Second and third harmonic waves excited by focused Gaussian beams.

Harmonic generation by tightly-focused Gaussian beams is finding important applications, primarily in nonlinear microscopy. It is often naively assumed that the nonlinear signal is generated predominantly in the focal region. However, the intensity of Gaussian-excited electromagnetic harmonic waves is sensitive to the excitation geometry and to the phase matching condition, and may depend on quite an extended region of the material away from the focal plane. Here we solve analytically the amplitude integral for second harmonic and third harmonic waves and study the generated harmonic intensities vs. focal-plane position within the material. We find that maximum intensity for positive wave-vector mismatch values, for both second harmonic and third harmonic waves, is achieved when the fundamental Gaussian is focused few Rayleigh lengths beyond the front surface. Harmonic-generation theory predicts strong intensity oscillations with thickness if the material is very thin. We reproduced these intensity oscillations in glass slabs pumped at 1550nm. From the oscillations of the 517nm third-harmonic waves with slab thickness we estimate the wave-vector mismatch in a Soda-lime glass as Δk(H)= -0.249μm(-1).

Unbounded spatial spectrum of propagating waves in a polaritonic wire medium

In this paper, we study a topological phase transition in a wire medium operating at infrared frequencies. This transition occurs in the reciprocal space between the indefinite (open-surface) regime of the metamaterial and its dielectric (closed-surface) regime. Due to the spatial dispersion inherent to a wire medium, a hybrid regime turns out to be possible at the transition frequency. Both such surfaces exist at the same frequency and touch one another. At this frequency, all values of the parallel wave vector correspond to propagating spatial harmonics. The implication of this regime is the overwhelming radiation enhancement. We numerically investigate the gain in radiated power for a subwavelength dipole source submerged into such medium. In contrast to previous works, this gain (called the Purcell factor) turns out to be higher for a parallel dipole than for a perpendicular one.

Jeans instability with exchange effects in quantum dusty magnetoplasmas

Jeans instability is examined in magnetized quantum dusty plasmas using the quantum hydrodynamic model. The quantum effects are considered via exchange-correlation potential, recoil effect, and Fermi degenerate pressure, in addition to thermal effects of plasma species. It is found that the electron exchange and correlation potential have significant effects over the threshold value of wave vector and Jeans instability. The presence of electron exchange and correlation effect shortens the time of dust sound that comparatively stabilizes the self gravitational collapse. The results at quantum scale are helpful in understanding the collapse of the self-gravitating dusty plasma systems.

Plasmons in spin-polarized graphene: A way to measure spin polarization

We study the collective charge excitations (plasmons) in spin polarized graphene, and derive explicit expressions for their dispersion in the undamped regime. From this, we are able to calculate the critical wave vector beyond which the plasmon enters the electron-hole continuum, its quality factor decreasing sharply. We find that the value of the critical wave vector is strongly spin polarization-dependent, in a way that has no analog in ordinary two-dimensional electron gases. The origin of this effect is in the coupling between the plasmon and the inter-band electron-hole pairs of the minority spin carriers. We show that the effect is robust with respect to the inclusion of disorder and we suggest that it can be exploited to experimentally determine the spin polarization of graphene.

Surface plasmon polaritons in a composite system of porous silicon and gold

A composite system of silicon quantum dots and gold particles with properties periodically changing along the surface (i.e., a system exhibiting the properties of a diffraction grating) is obtained by a one-step metal-assisted chemical etching. The spectral and angular dependences of the photoresponse for the composite system on single-crystal silicon are studied. The photoresponse peaks were observed, which behavior (the dependence on the parameters of the diffraction grating, wavelength and incidence angles of light) is attributed to the excitation of plasmon-polariton modes at the surface of the composite system with the diffraction grating. At the same time, the obtained values of the wave vectors for these modes are smaller than those calculated for plasmon polaritons excited at the interface between air and metal (gold) diffraction grating.

A correction for the Hartree-Fock density of states for jellium without screening.

We revisit the Hartree-Fock (HF) calculation for the uniform electron gas, or jellium model, whose predictions--divergent derivative of the energy dispersion relation and vanishing density of states (DOS) at the Fermi level--are in qualitative disagreement with experimental evidence for simple metals. Currently, this qualitative failure is attributed to the lack of screening in the HF equations. Employing Slater's hyper-Hartree-Fock (HHF) equations, derived variationally, to study the ground state and the excited states of jellium, we find that the divergent derivative of the energy dispersion relation and the zero in the DOS are still present, but shifted from the Fermi wavevector and energy of jellium to the boundary between the set of variationally optimised and unoptimised HHF orbitals. The location of this boundary is not fixed, but it can be chosen to lie at arbitrarily high values of wavevector and energy, well clear from the Fermi level of jellium. We conclude that, rather than the lack of screening in the HF equations, the well-known qualitative failure of the ground-state HF approximation is an artifact of its nonlocal exchange operator. Other similar artifacts of the HF nonlocal exchange operator, not associated with the lack of electronic correlation, are known in the literature.

Spin-dependent polaron formation in pristine graphene

We investigate the effects of spin-orbit couplings on the electron (hole)-E2g phonon interaction in graphene. We examine the effects of spin-orbit couplings on electron and hole polaron formation as well as DC conductivity, and spin polarizations of charge carriers. We use Frohlich type Hamiltonian to describe the electron-phonon system within the continuum limit. Our theoretical analysis shows that, the polaronic effect is decreased due to the spin-orbit couplings, for both spin and pseudospin states, but it is enhanced beyond some critical values of wavevector for spin-up states of both sublattices in the presence of spin-orbit couplings. The Rashba spin-orbit coupling in the graphene single layer splits up- and down- states, and produces perfect spin polarized conductivity. Also the phonon-restricted and phonon-assisted conduction are reported for positive and negative Fermi energies.

Effective Mass of Acoustic Polaron in Quantum Dots

The variational effective mass with respect to the e-p coupling constant for different values of cutoff wave vector is performed in quantum dot. The self-trapping transition of acoustic polaron in quantum dot is reconsidered by character of the effective mass curve varying with the e-p coupling. The holes are determined to be self-trapped in AlN quantum dot systems.

Nonlocal effective parameters of a coated sphere medium

An analytical theory has been developed to find the general effective parameters of a nonlocal medium. The medium is nonlocal due to presence of spatial dispersion. The proposed theory is based upon the dipolar scattering model of the inclusions comprising the medium. The bianisotropy stemming from the magnetoelectric coupling at the inclusion and lattice level has been discussed. The developed theory is then applied to a medium which consists of coated spheres with realistic materials. Effects of different values of wavevector upon the effective permittivity, effective permeability, and magnetoelectric coefficient stemming from lattice effects have been studied for a coated sphere medium. It is shown that a coated sphere medium with a weak spatial dispersion gives rise to a broader range of frequencies, where real parts of the effective permittivity and permeability are negative. On the other hand, this range of frequencies becomes smaller when the spatial dispersion of the medium is not weak.

Wave number determination of Pc 1–2 mantle waves considering He++ ions: A Cluster study

AbstractThe present case study concerns narrowband electromagnetic emission detected in the distant cusp region simultaneously with upgoing plasma flows. The wave properties match the usual properties of the Pc 1–2 mantle waves: small angle between the wave vector and the magnetic field line, left‐hand polarization, and propagation toward the ionosphere. We report here the first direct wave vector measurement of these waves (about 1.2 × 10− 2 rad/km) through multi spacecraft analysis using the three magnetic components and, at the same time, through single spacecraft analysis based on the refractive index analysis using the three magnetic components and two electric components. The refractive index analysis offers a simple way to estimate wave numbers in this frequency range. Numerical calculations are performed under the observed plasma conditions. The obtained results show that the ion distribution functions are unstable to ion cyclotron instability at the observed wave vector value, due to the large ion temperature anisotropy. We thus show that these electromagnetic ion cyclotron (EMIC) waves are amplified in the distant cusp region. The Poynting flux of the waves is counterstreaming with respect to the plasma flow. This sense of propagation is consistent with the time necessary to amplify the emissions to the observed level. We point out the role of the wave damping at the He++ gyrofrequency to explain that such waves cannot be observed from the ground at the cusp foot print location.

Mid-infrared intersubband polaritons in dispersive metal-insulator-metal resonators

We demonstrate room-temperature strong coupling between a mid-infrared (λ = 9.9 μm) intersubband transition and the fundamental cavity mode of a metal-insulator-metal resonator. Patterning of the resonator surface enables surface-coupling of the radiation and introduces an energy dispersion which can be probed with angle-resolved reflectivity. In particular, the polaritonic dispersion presents an accessible energy minimum at k = 0 where—potentially—polaritons can accumulate. We also show that it is possible to maximize the coupling of photons into the polaritonic states and—simultaneously—to engineer the position of the minimum Rabi splitting at a desired value of the in-plane wavevector. This can be precisely accomplished via a simple post-processing technique. The results are confirmed using the temporal coupled mode theory formalism and their significance in the context of the strong critical coupling concept is highlighted.