Dielectric Constant Of Metal Research Articles

The Localized Enhancement of Surface Plasmon Standing Waves Interacting with Single Nanoparticles

Real-time, high-sensitivity, and label-free detection to single nanoparticles has been achieved via visualizing the interaction between surface plasmon polaritons (SPPs) and nanoparticles, which is widely applied to chemistry and biology. In this work, aiming to enhance the detection sensitivity to nanoparticles, we explore the interaction of SPP standing waves with single nanoparticles. Compared with SPPs, the inhomogeneous fields of SPP standing waves modulate charge distributions around the particle and excite different electric dipole modes that tailor localized enhancements. For nanoparticles situating at electric antinodes of SPP standing waves, a vertical electric dipole is excited and high-density charges are stimulated around nanoparticle-film nanocavities, leading to further increased localized enhancement. The localized enhancement experiences more increase with smaller particle size, lower particle refractive index, lower dielectric constant of surrounding medium, and lower real part of the metal dielectric constant. Via tailoring the localized enhancement by SPP standing waves, the sensitivity of SPP microscopy can be improved, which would broaden its applications on nanotechnology, biomedicine, and environmental monitoring.

Dual channel optical fiber refractive index sensor based on surface plasmon resonance

In this paper, an optical fiber refractive index sensor based on surface plasmon resonance (SPR) is presented. This sensor is able to measure the refractive index of two analytes simultaneously. The sensor structure is based on removing the cladding of an optical fiber and replacing it with thin films of silver and gold in the first and second channels respectively. The incident light inside the fiber excites surface plasmons which results in absorption at SPR wavelengths. For analyzing the proposed structure, finite difference time domain (FDTD) method is used. The absorption behavior depends on the dielectric constants of metal and sensing medium. Simulation results show that the proposed sensor has sensitivities about 1200 nm/RIU and 1650 nm/RIU and average figures of merits equal to 46 and 55 for the first and second channels respectively. The obtained transmission spectra prove that channels of the presented sensor work independently. According to the simulation results, this structure is suitable for multi-sensing applications.

Intricacies of designing metallic patch antennas in the optical band: Enhancements of the effects of the secondary physical parameters

This study considers the modifying effects of the patch antenna thickness and complex dielectric constant of the (real metal) patch on its radiation characteristics in the optical band, in comparison to the conventional microwave patch antenna. Patch thickness aware full wave simulation is used, for the first time, to find resonant length of the back cavity of the optical patch considering its Surface Plasmon Polariton modes and cavity height expansion due to wave penetration into the bottom and top (patch) metal layers. Verification of the results are based on a Fabry Perot cavity model based semi-analytic method, without earlier precedent. It is seen that in the analysis of a patch in the optical band, in contrast to the microwave case, patch thickness and dielectric constant of metal, which was secondary parameters, should be considered. Results of this study is a good base to design optical patch radiators and related flat optical devices.

Enhanced upconversion emission of Dy3+-doped tellurite glass by heat-treated silver nanoparticles

Dy3+-doped lead sodium tellurite glasses containing silver nanoparticles (NPs) were synthesized by a melt-quenching technique and treated for different annealing time intervals above the glass transition temperature. The glass samples were characterized by UV–vis–IR absorption spectroscopy, photoluminescence spectroscopy and transmission electron microscope imaging. Eight absorption peaks of Dy3+ ions were observed indicating the transitions from ground state to different excited states. The surface plasmon resonance (SPR) peak of silver NPs was probed at 522nm. Blue, yellow and red luminescence emissions were recorded at 485, 578 and 665nm. The intensity of emission peaks for heat-treated samples containing silver NPs found to be 1.7–4 times larger than their singly-doped counterpart. Such enhancements are attributed to the modification of the local field due to difference between dielectric constants of metal and surrounding medium as the major factor, and energy transfer from nano-metal particles to Dy3+ ions as the minor aspect. The size distribution of silver NPs has a Gaussian shape with a maximum around 18nm. The glasses can be nominated as promising materials for solid state lasers.

Exact surface plasmon dispersion relation with ponderomotive nonlinearity in a metal/dielectric structure

The nonlinear effects in a metal due to the ponderomotive potential, which is induced by a steep spatial gradient in the electric field intensity, on the surface plasmon dispersion relation at a planar metal/dielectric interface are studied exactly. Previous studies invoked an approximation that amounts to keeping only the lowest order Kerr-like term in the metal dielectric constant. Comparing the exact result with the Kerr approximation reveals that the Kerr approximation often overestimates a cutoff frequency for the existence of surface waves and gives erroneous results in the range of frequencies below the overestimated cutoff and the real cutoff. Our result thus sheds light on the applicability of the Kerr approximation. Properly expanding the exact results recovers the standard linear results and the results starting with a Kerr-like approximation.

Refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers

We propose refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers. The surface plasmon resonance modes and the sensing properties are theoretically analyzed using finite element method (FEM). In the calculation, Drude–Lorentz model is used to describe the Metal Dielectric constant. The calculation results show that the sensitivity of Ag-metalized SPR sensor can reach 1500nm/RIU corresponding to a resolution of 6.67×10−5RIU. Comparing with conventional detecting material-Au under the same structure, the sensitivity and 3dB bandwidth of our device are better.

All-optical thermo-plasmonic device

We demonstrate an all-optical thermo-plasmonic effect to switch/modulate the surface plasmon resonance signal intensity excited at the metal-air interface. This optically addressed thermo-plasmonic measurement scheme is suitable to amplify very small changes in the complex dielectric constant (ε(m)(T)) of thin gold (Au) film, induced by the Ar(+) laser. The predominant contributions due to small but highly repeatable transient photo-thermal effects in the complex metal dielectric constant is confirmed to be the reason behind the highly reproducible all-optical thermo-plasmonic device performance presented here.

Surface plasmon resonance sensors based on Ag-metalized nanolayer in microstructured optical fibers

Surface plasmon resonance sensors based on Ag-metalized nanolayer in microstructured optical fibers are theoretically analyzed by using finite element method (FEM). In our simulations we use Drude–Lorentz model to describe the metal dielectric constant. The numerical results show that the sensitivity of Ag-metalized SPR sensor could reach 1167 nm/RIU and corresponding resolution is 8.57×10 −5 RIU. Compared to conventional Au-metalized SPR sensors the performance of our device is obviously better.

Theoretical study of terahertz quantum well photodetectors: Effect of metallic diffraction coating

The possibility of enhancement of sensitivity of quantum well photodetectors by adding metallic diffraction coating on top of the dielectric layer of photodetectors is studied. With the grating the spatial distribution of the intensity of electromagnetic wave within the active region of the photodetector is highly non-uniform with the intensity variation over a few orders of magnitude within a period of the grating. This effect is due to the coupling of surface plasmon with incident electromagnetic wave. At terahertz frequencies the average intensity of the transmitted radiation wave through the grating strongly depends on the dielectric constant of metal.

Local field calculations of the anisotropic nonlinear absorption coefficient of aligned gold nanorods embedded in silica

The anisotropic nonlinear third-order susceptibility of nanocomposites consisting of aligned ellipsoidal metallic nanoparticles embedded in a dielectric matrix is modeled from the generalized Maxwell-Garnett equation involving depolarization factors. Depolarization factors take into account different anisotropic particle geometries such as flat disks, rods, or ellipsoids. The equations traditionally used to model third-order susceptibility of nanocomposites are valid only for very low metal volume fractions. Modified equations that allow metal volume fractions up to the limit of validity of the Maxwell-Garnett equation are used. The effect of the different model parameters, namely, the metal volume fraction, the real and imaginary parts of the metal dielectric constant, the matrix dielectric constant, and, finally, the ratio of the real and imaginary parts of the metal third-order susceptibility were investigated using the model gold/silica nanocomposite system. As previously reported in the literature for the isotropic particle case, counterintuitive effects such as sign reversal between the bulk metal and composite nonlinear susceptibilities have been observed. The calculations were applied to the case of gold nanorods embedded in silica that were experimentally found to exhibit anisotropic saturable absorption.

Extraction efficiency of highly confined surface plasmon-polaritons to far-field radiation: an upper limit

We propose a unique method determining an upper limit of extraction efficiency of the surface plasmon-polaritons (SPPs), E(SP), which are highly confined on a corrugated metal surface. The method is based on measurement of the spectral bandwidth of a grating-induced absorption spectrum as a function of metal dielectric constant. After finding the fact that E(SP) exhibits an extremely linear relationship with the collision frequency Gamma of metal over a SPP band below the surface plasmon frequency, an upper limit of E(SP) can be determined by an asymptotic estimation as Gamma-->0 for total decay rates of the confined SPPs. Our method based on the bandwidth measurement is inherently free from the ambiguity and underestimation difficulties pertaining to the previous prism-coupling approaches for E(SP) estimation. It will also be quite applicable for evaluating SPP-mediated light-emitting diodes (LEDs) of which total external efficiency is dominantly restricted by the upper limit of E(SP). Especially for the case when SPP excitation probability approaches unity, the proposed method would excellently figure out the maximum realizable external efficiency of SPP-mediated LEDs.

Theory of transmission of light by sub-wavelength cylindrical holes in metallic films

This paper presents theory and finite-difference time-domain (FDTD) calculations for a single and arrays of sub-wavelength cylindrical holes in metallic films presenting large transmission. These calculations are in excellent agreement with experimental measurements. This effect has to be understood in terms of the properties exhibited by the dielectric constant of metals which cannot be treated as ideal metals for the purpose of transmission and diffraction of light. We discuss the cases of well-differentiated metals silver and tungsten. It is found that the effect of surface plasmons or other surface wave excitations due to a periodical set of holes or other roughness at the surface is marginal. The effect can enhance but also can depress the transmission of the arrays as shown by theory and experiments. The peak structure observed in experiments is a consequence of the interference of the wavefronts transmitted by each hole and is determined by the surface array period independently of the material. Without large transmission through a single hole there is no large transmission through the array. We found that in the case of Ag which at the discussed frequencies is a metal there are cylindrical plasmons at the wall of the hole, as reported by Economu et al 30 years ago, that enhanced the transmission. But it turns out, as will be explained, that for the case of W which behaves as a dielectric, there is also a large transmission when compared with that of an ideal metal waveguide at large wavelengths. To deal with this problem one has to use the measured dielectric function of the metals. We discuss thoroughly all these cases and compare with the data. We notice that to discuss these data, for a single hole's transmission, in terms of the Bethe approximation of ideal metals is misleading. Therefore, the extraordinary enhancement of the transmission for the holes arrays versus the single hole does not exist.

Theoretical and experimental studies of metal-infiltrated opals

The optical properties of metal-infiltrated synthetic opals have been studied analytically, numerically, and experimentally in the visible and infrared spectral ranges. A simple model of weakly interacting resonant cavities is proposed to qualitatively understand the dispersion of electromagnetic waves in metallic photonic crystals. In the approximation of an ideal metal infiltrated into opals this model is exact, and the frequencies of the propagating modes in metallic opals are close to the eigenfrequencies of a single spherical resonator. Photonic band structures based on the dispersion of a metal dielectric constant have been calculated numerically for several metals and show similarity with the ideal metal model. Reflectivity and transmission spectra of thin-film metallic opals are calculated. Growth and saturation of the reflectivity spectra at low frequencies are manifestations of the omnidirectional band gap. The sharpest minimum in the reflectivity and maximum in the transmission spectra correspond to the position of the cutoff frequency. Silver- and copper-infiltrated opals were fabricated and their reflectivity spectra were measured in the visible to mid-IR spectral range. These spectra are in a good agreement with the theoretical calculations.

TE-polarized surface plasmon polaritons in metal waveguides bounded by self-focusing and self-defocusing media

We have examined the dispersion relations for TE-polarized surface plasmon polaritons that are guided by a metal film bounded by a semi-infinite self-defocusing medium on one side and a self-focusing medium on the other. Depending on the material parameters, the TE-polarized waves exhibit a definite power threshold and a limited range of propagation constants. For a given metal dielectric constant, there exists a maximum film thickness and a minimum refractive index of the defocusing medium above and below which, respectively, the mode does not exist.

Enhanced Nonlinear Optical Response of Coated Nanoparticles

AbstractWe study coated, nanometer-size, ellipsoidal particles that have a semiconductor or polymer core surrounded by a metal coating. We predict that composite materials containing these particles will have much larger enhancement of the nonlinear optical response than had previously been found by using semiconductor colloid suspensions or semiconductor - doped glasses. The enhancement is due to the surface plasmon resonance from the metal dielectric constant that increases the local field in the core material. The frequency of the resonance and the enhancement depend upon the particle shape and the coating thickness, as well as on the specific materials.Also, we predict intrinsic optical bistability in these new materials and show that the threshold intensity for optical bistability can be greatly reduced by using the coated particles. We predict a switching intensity of silver coated GaAs particles below 100W/cm2

Copper-cadmium halide photochromic glasses: Evidence for a colloidal darkening mechanism

Optical absorption and electron paramagnetic resonance (EPR) spectra of darkened photochromic glasses containing both copper halides and cadmium halides are presented. Optical absorption spectra, calculated on a colloidal copper model with mean-free path (MFP) and quantum-size effect (QSE) corrections to the bulk metal dielectric constant, are fit to the data. Particles of the size derived from the MFP correction are found to be too large to be produced by photolysis of the phase-separated droplets, presumed to be CdCl2:CuCl decorated with CuCl specks, observed in these glasses. The QSE indicates the presence of oblate spheroidal copper particles with aspect ratios between 1.8 and 3.5 and major diameters ≊8 nm, depending on the heat treatment which the glass has received. The uv-induced EPR spectra show a peak due to Cu2+ ions with halide ions as ligands superimposed on another, broad-winged peak which is attributed to pure or nearly pure CuCl2. These data are consistent with a darkening mechanism in which colloidal copper particles are formed by disproportionation of Cu+ in the CuCl specks and the CdCl2:CuCl droplets.

Electrostatic edge modes along a parabolic wedge

We have calculated the electrostatic edge modes of a dielectric wedge whose boundary is a parabolic cylinder. The spectrum of edge modes is found to be discrete, and to depend on the one-dimensional wave vector $q$ which characterizes the propagation of these modes along the wedge. This is in agreement with the results of recent, similar calculations by Davis for a wedge whose boundary is a hyperbolic cylinder. The dispersion curves for the first four mdoes are plotted for the case of a parabolic wedge characterized by a free-electron metal dielectric constant. The relation of the present results to those of the original electrostatic edge mode calculation by Dobrzynski and Maradudin and to those of Davis is discussed.

On the effective dielectric constant of metal interfaces

On the effective dielectric constant of metal interfaces

Die optischen Konstanten des Kaliums und die Theorie von Drude

The results of new measurements of the optical constantsn andk of the alkali metal K, performed in ultrahigh vacuum on mirrorlike surfaces free of any contamination and distortion in the wavelength region from 0·365 μ to 2 μ at temperatures between −183° C and +85° C were compared with the predictions of the Drude-theory. This permits the following main conclusions: 1. The relation forn2-k2≡e (real part of the metal's dielectric constant) derived by Drude proves to be valid; therefore, the total polarisation 4πNα (α=polarizability) of the metal remains small compared with 1 and the interband absorption has no influence on it. 2. The quotientN/m, N beeing the number of conduction electrons andm the electronic mass has been determined, permitting either the calculation of the effective numberNeff of conduction electrons or of the effective optical massmeff,opt. Within the limits of experimental error both quantities prove to be independent of temperature between −183° C and +85° C. 3. The critical wavelengthλc corresponding to the plasma frequencyνc was found to be equal toλc=2909 A (hνc=4·25 eV).