Metal-dielectric Composites Research Articles

Electrostatic resonances and optical responses of cylindrical clusters

We developed a Green function formalism (GFF) for computing the electrostatic resonance in clusters of cylindrical particles. In the GFF, we take advantage of a surface integral equation to avoid matching the complicated boundary conditions on the surfaces of the particles. Numerical solutions of the eigenvalue equation yield a pole spectrum in the spectral representation. The pole spectrum can in turn be used to compute the optical response of these particles. For two cylindrical particles, the results are in excellent agreement with the exact results from the multiple image method and the normal mode expansion method. The results of this work can be extended to investigate the enhanced nonlinear optical responses of metal-dielectric composites, as well as optical switching in plasmonic waveguides.

Subwavelength imaging from a multilayered structure containing interleaved nonspherical metal-dielectric composites

A thin multilayered structure with interleaved nonspherical metal-dielectric composites slices and dielectric slices may be modeled as the metamaterial with anisotropic permittivity. The signs of diagonal elements of the permittivity tensor can be controlled by the particles' shape, the volume fraction of metal particles, and the incidental wavelengths. To one's interest, when the spheroidal nanoparticles are oblate in shape, the wavelength range in which components of the permittivity have different signs is widened, and the magnitude of optical absorption band becomes weak. Since both physical anisotropy and low absorption are helpful for improving the subwavelength image resolution, the multilayered structure containing metal-dielectric composite layer of nonspherical particles may be designed as a superlens device. In addition, the incident wavelength and the number of nanolayers are found to play crucial roles in enhancing the evanescent field performance too.

Optical and structural properties of pulsed laser deposited Ti:Al 2O 3 thin films

Spectrally selective solar absorber coatings using metal–dielectric composites offer a high degree of flexibility since their solar selectivity can be optimized by the proper choices of constituents, coating thickness, metal particle concentration, size, shape, and orientation. In this article, Ti:Al 2O 3 composite films of various Ti contents were prepared by the pulsed laser deposition technique, and their structural and optical properties were investigated by scanning electron microscope, X-ray photoelectron spectroscope, and X-ray diffraction methods. The results demonstrate promising solar spectrally selective behavior indicating Ti:Al 2O 3 composite as an excellent choice for solar thermal applications.

Superlens from metal-dielectric composites of nonspherical particles

A tunable near-field superlens (NFSL) from metal-dielectric composites has advantages over that one from bulk metal. In this paper, we realize the tunable NFSL by tuning the shape of granular inclusions. It is found that the tunability of NFSL can be enhanced by adjusting both the volume fraction and the particles' shape. Moreover, we also find that when the shape of particles is not spherical, the percolation threshold is low, resulting in a wider wavelength range of superlens. The imaging quality and resolution can be enhanced not only by changing the particles' shape but also by using a multilayered structure with alternate metal-dielectric composites and host medium.

Stochastic optimization of low-loss optical negative-index metamaterial

Optical metamaterial consisting of metal-dielectric composites creates a complicated system that is not amenable to analytical solutions. This presents a challenge in optimizing these intricate systems. We present the application of three nature-inspired stochastic optimization techniques in conjunction with fast numerical electromagnetic solvers to yield a metamaterial that satisfies a required design criterion. In particular, three stochastic optimization tools (genetic algorithm, particle swarm optimization, and simulated annealing) are used for designing a low-loss optical negative index metamaterial. A negative refractive index around −0.8+0.2i is obtained at a wavelength of 770 nm. The particle swarm optimization algorithm is found to be the most efficient in this case.

Negative refractive index in optics of metal-dielectric composites

Specially designed metal-dielectric composites can have a negative refractive index in the optical range. Specifically, it is shown that arrays of single and paired nanorods can provide such negative refraction. For pairs of metal rods, a negative refractive index has been observed at 1.5 micrometer. The inverted structure of paired voids in metal films may also exhibit a negative refractive index. A similar effect can be accomplished with metal strips in which the refractive index can reach -2. The refractive index retrieval procedure and the critical role of light phases in determining the refractive index is discussed.

Superlens based on metal-dielectric composites

Pure noble metals are typically considered to be the materials of choice for a near-field superlens that allows subwavelength resolution by recovering both propagating and evanescent waves. However, a superlens based on bulk metal can operate only at a single frequency for a given dielectric host. In this Letter, it is shown that a composite metal-dielectric film, with an appropriate metal filling factor, can operate at practically any desired wavelength in the visible and near-infrared ranges. Theoretical analysis and simulations verify the feasibility of the proposed lens.

Synthesis and characterization of metal-dielectric composites with copper nanoparticles embedded in a glass matrix: A multitechnique approach

The precipitation and growth of copper nanoparticles in an optically transparent aluminosilicate glass matrix was investigated. The size of particles in this heterophase glass-based composite was modified in a controlled manner by isothermal heat treatments. A multitechnique approach, consisting of Raman scattering spectroscopy, high-resolution transmission electron microscopy, x-ray diffraction technique, and optical absorption spectroscopy, has been used to study the nucleation and crystallization processes. Optical absorption spectroscopy revealed the presence of intense absorption bands attributed to oscillations of free electrons, known as the surface-plasmon resonance band of copper particles, and confirmed a gradual increase of the particles’ mean size and density with annealing time. The Raman scattering on acoustical phonons from Cu quantum dots in the glass matrix measured for off-resonance conditions demonstrated the presence of intense, inhomogeneously broadened peaks that have been assigned to the confined acoustic eigenmodes of copper nanoparticles. The particle-size dependence of the acoustic peak energies and the relation between the size distribution and bandwidths of these peaks were derived. High-resolution transmission electron microscopy was used to monitor the nucleation of the nanoparticles and to estimate their mean size.

Highly absorbing black Mg and rare-earth-Mg switchable mirrors

Apart from a reflecting and a transparent state, rare-earth-Mg alloys (RE-Mg) exhibit also a highly absorbing, black state during loading with hydrogen. The occurrence of such a black state is due to the disproportionation into subwavelength size ${\mathrm{REH}}_{2+\ensuremath{\epsilon}}$ and Mg grains during the first hydrogen loading. While the optical properties of ${\mathrm{REH}}_{x}$ change continuously with a further increase in hydrogen concentration x, Mg changes abruptly from a good reflector to a transparent insulator $({\mathrm{MgH}}_{2}).$ Thin pure Mg films also show this black state when (un)loaded carefully at elevated temperatures. By using the Bruggeman effective medium approximation in combination with the transfer matrix method it is shown that the coexistence of Mg and ${\mathrm{MgH}}_{2}$ grains is the cause of this high absorption. Furthermore, we compare this phenomenon to the high absorption of light observed in metal-dielectric composites.

Interaction of high-power microwave beams with metal-dielectric media

Results of experimental investigation of powerful microwave beams action on the metal-dielectric compositions are presented. Dielectric surfaces with introduced metallic grains as well as dielectric powder containing small admixtures of a metallic one have been explored as an objects of irradiation. At a relatively small microwave power ( mW) all investigated targets were practically completely transparent for incident electromagnetic wave. At a relatively high power (microwave generators based on the gyratrons and powerful magnetrons) the irreversible changes in the electric and radiophysical properties of metal-dielectric composites exposed to microwave radiation whose intensity is below the threshold intensity for plasma production have been observed (sharp increase of conductivity and microwave absorption coefficient).

Effective nonlinear optical properties of composite media of graded spherical particles

We have developed a nonlinear differential effective dipole approximation (NDEDA), in an attempt to investigate the effective linear and third-order nonlinear susceptibility of composite media in which graded spherical inclusions with weak nonlinearity are randomly embedded in a linear host medium. Alternatively, based on a first-principles approach, we derived exactly the linear local field inside the graded particles having power-law dielectric gradation profiles. As a result, we obtain also the effective linear dielectric constant and third-order nonlinear susceptibility. Excellent agreement between the two methods is numerically demonstrated. As an application, we apply the NDEDA to investigate the surface plasma resonant effect on the optical absorption, optical nonlinearity enhancement, and figure of merit of metal-dielectric composites. It is found that the presence of gradation in metal particles yields a broad resonant band in the optical region, and further enhances the figure of merit.

Effective medium approximation for optical bistability in nonlinear metal-dielectric composites

A general theory based on the spectral representation method and effective medium approximation is adopted to investigate the optical bistable behavior in a nonlinear two-phase composite with symmetrical microstructure, in which the metal particles of the volume fraction p and the dielectric particles of the volume fraction 1− p are randomly dispersed but oriented with respect to one another. The relation between the spatial average of local field squared and the external applied field is established through the spectral density function m( x), obtained from the modified Bruggeman effective medium approximation. We find that the optical bistability (OB) is dependent on the depolarization factor L of the components and the volume fraction p. For a given p, we predict that OB can be observed only when L is larger than the critical value L c, and bistable behavior is more pronounced at large L. Moreover, numerical results show that both the upper threshold field and the width of OB region increase monotonically as L increases. The field-dependent reflectance at normal incidence R in random composites is also investigated.

METAL-DIELECTRIC COMPOSITE FILTERS WITH CONTROLLED SPECTRAL WINDOWS OF TRANSPARENCY

In this report we show the possibility of broadband low-pass filters with windows of transparency in pre-set spectral ranges. Those filters are based on the unique optical properties of metal-dielectric composites near the percolation threshold. In such composites, metal clusters of different sizes and shapes have different plasmon resonances and resonate nearly independently at different wavelengths. All together, the resonant plasmon modes cover a very broad spectral range. Applying the effect of spectrally selective photo-modification, we develop a procedure that creates mid-infrared windows of transparency within the broadband filters. We also investigate the possibilities of making low-pass filters composed of spheroids and conductive sticks with certain distributions of aspect ratios.

Giant enhancement of optical nonlinearity in mixtures of graded particles with dielectric anisotropy

To investigate the effective linear dielectric constante and third-order nonlinear susceptibil- ity χe of composite media, in which graded inclusions with radial dielectric anisotropy are randomly embedded in a linear isotropic matrix, we develop an nonlinear anisotropic differential effective dipole approximation (NADEDA). Alternatively, based on a first-principles approach, the exact expressions for � e and χe are also derived for the linear dielectric profiles with small slopes. Then, excellent agreement between the two methods is numerically demonstrated. As an application, we further apply the NADEDA to a nonlinear metal-dielectric composite, in which the metal particles possess spatially varying radial dielectric anisotropy, in an attempt to study the nonlinearity enhancement and the figure of merit of the composite. To this end, it is shown that the presence of gradation in the radial dielectric constant plays a crucial role in enhancing the optical nonlinearity as well as the figure of merit.

Resonant absorption quenching and enhancement of optical nonlinearity in Au:BaTiO3 composite films by adding Fe nanoclusters

We present an approach to enhance the figure of merit χ(3)/α (with χ(3) being the third-order nonlinear susceptibility and α the optical absorption coefficient) in metal-dielectric composites. Nanocomposite thin films containing metal nanoclusters were obtained by embedding gold, or gold+iron in BaTiO3 matrices using the pulsed laser deposition technique. In the optical absorption spectra, the Au:BaTiO3 films showed a strong absorption peak around 562 nm due to surface plasmon resonance of Au particles. However, when a small amount of Fe was added, the resonant absorption was quenched, and χ(3) was enhanced so that χ(3)/α was significantly increased. Our study suggests a method by which to design optical nonlinear materials with large figure of merits for practical application.

Optical Spectra of Metal-Dielectric Nanocomposites with a Layered Subwave Structure

The results of theoretical and experimental investigations of the optical spectra of layer-periodic nanocomposites consisting of alternating tightly packed monolayers of silver nanoparticles and separating KCl layers of subwave thickness are presented. The characteristic features of the spectral dependence of the transmission and reflection coefficients of this type of nanocomposites have been analyzed for the case where the photon forbidden bands are located near the plasmon-absorption band. Experimental samples were prepared by successive thermal evaporation of silver and KCl in vacuum. Theoretical calculations were carried out using the quasi-crystalline approximation of the theory of multiple scattering of waves. It is shown that the spectral position, shape, and intensity of absorption and reflection bands can be regulated by changing the optical thickness of the intermediate dielectric films in a layered metal-dielectric composite.

Optomagnetic composite medium with conducting nanoelements

A new type of metal-dielectric composites has been proposed that is characterised by a resonance-like behaviour of the effective permeability in the infrared and visible spectral ranges. This material can be referred to as optomagnetic medium. The analytical formalism developed is based on solving the scattering problem for considered inclusions with impedance boundary condition, which yields the current and charge distributions within the inclusions. The presence of the effective magnetic permeability and its resonant properties lead to novel optical effects and open new possible applications.

Laser annealing of metal-dielectric nanocomposites formed by ion implantation

Metal-dielectric composites have been fabricated by ion synthesis of silver nanoparticles in soda-lime silicate glass. Ion implantation has been performed with Ag + ions at 60keV to a dose of 4 10 16 ionscm -2 . The glass was annealed using pulses of a high-power KrF excimer laser (248nm) in an ambient atmosphere. This employed a single (25ns) pulse fluence of 0.25Jcm -2 for implanted samples. From 1 to 250 pulses with the same energy densities and at a frequency of 1Hz were accumulated in the same area on the glass surface. The formation and modification of metal nanoparticles were assessed via optical reflectance, combined with Rutherford back-scattering analysis. Generally, changes induced by laser pulses suggest there are both reductions in the nanoparticles and some longer-range diffusion of metal atoms into the glass. However, before the total dissolution of metal nanoparticles occurs, increases in the substrate temperature during many-pulse treatments initiate the regrowth of new metal nanoparticles, which leads to a rise in the reflectance. These results are discussed on the basis of surface substrate melting.

Chromium-based thin sputtered composite coatings for solar thermal collectors

One of the fundamental components of a solar thermal collector is the absorber surface that should be spectrally selective for solar radiation. Efficient solar photothermal conversion benefits from spectrally selective absorber surfaces. Most solar selective coatings use metal-dielectric composites, known as cermets, as the absorber of solar energy. In this study we will present a numerical model that allow us to correlate the selectivity of the produced absorbers to the collector efficiency. The cathodic magnetron sputtering being a promising method to produce thin solar selective films, a study of some cermet Cr–Cr 2O 3–CrO 3 coatings obtained by this technique in reactive atmosphere and using a DC power unit will be presented. The multilayered composites produced were based in metallic chromium in a matrix of a chromium oxide with a gradient in oxygen composition. The selective graded films were produced by a reactive DC magnetron sputtering of pure chromium target in a plasma of argon–oxygen at different sputtering pressures and substrate temperatures. The microstructure, surface roughness, crystallographic phases, composition and chemical analysis by X-ray photoelectron spectroscopy and reflectivity spectra in vis-NIR were analysed.

Effective nonlinear optical properties of metal-dielectric composite media with shape distribution.

The effective linear and nonlinear optical properties have been investigated in granular metal-dielectric composites taking the geometric shape of inclusions into account. Recently derived Maxwell-Garnett-type approximations are generalized to study the spectral function for composites in which the metal inclusions have a uniform distribution of geometric shapes. The numerical results show that the spectral density function becomes a prominent peak around small spectral value s within the non-self-consistent Maxwell-Garnett approximation but a broad continuous spectrum around large s besides the prominent peak around small s within the self-consistent Maxwell-Garnett approximation. Based on the spectral representation, we investigate the optical absorption and third-order nonlinear optical susceptibility. The results indicate that the shape distribution can lead to the separation of the nonlinearity enhancement peak from the absorption peak and thereby make the figure of merit more attractive.