Bruggeman Effective Medium Approximation Theory Research Articles

Wideband Endfire 3-D-Printed Dielectric Antenna With Designable Permittivity

This letter presents a three-dimensionally (3-D)-printed low-profile endfire dielectric antenna with wideband performance. The proposed device comprises four dielectric sections that are designed with tapered permittivities and sizes to achieve impedance-matched surface-wave launching over a wide bandwidth. The effective permittivity of dielectric blocks is precisely controlled within the range of 2.0 to 8.3 through varying the infill factor of the 3-D printing process. The relation between permittivity and infill factor has been experimentally validated and compared against Bruggeman effective medium approximation theory. Experimental results show that the proposed antenna exhibits a wide impedance bandwidth of 57.7 $\%$ from 6.9 to 12.5 GHz, a low tilt angle of around 20 $^\circ$ over the endfire direction, and a high realized gain above 10 dBi from 7.2 to 12.5 GHz. All these findings demonstrate that 3-D printing technology can be leveraged to conveniently fabricate sophisticated antennas with controllable material properties.

Determination of optical constants including surface characteristics of optically thick nanostructured Ti films: analyzed by spectroscopic ellipsometry.

In the present work, optically thick nanostructured titanium (Ti) films of thickness ranging from ∼100 to 900 nm were deposited on a glass substrate by DC magnetron sputtering at room temperature. Microstructural and surface properties of the samples were studied by x-ray diffraction and x-ray photoelectron spectroscopy (XPS). The morphological results revealed a systematic normal grain growth mechanism with increasing thickness analyzed by a scanning electron microscope. The influence of thickness on film surface roughness has been investigated by atomic force microscopy (AFM). The optical dispersion behavior was examined by spectroscopic ellipsometry (SE) over the long wavelength range of 246-1688 nm. The experimentally observed SE parameters were theoretically fitted with Drude-Lorentz and Bruggeman effective medium approximation theory. The surface properties of the Ti film measured by XPS and AFM were further accounted for in the optical model to determine optical constants (n and k) and the obtained results are expected to be the best available for bulk Ti metal.

Optical properties of L10 FePt nanoparticles dispersed in a C matrix

Optical properties of L10 FePt nanoparticles dispersed in a C matrix have been studied by using an approach combining spectroscopic ellipsometry and intensity transmission. Dielectric functions of FePt nanoparticles over a wavelength range of 380–1700 nm have been determined based on the Bruggeman effective medium approximation theory. Via analysis using the Drude–Lorentz model, the extracted optical properties provide electronic information of FePt nanoparticles. As compared with the thin film composed of larger grains, decrease of free electron absorption is observed in FePt nanoparticles, and this absorption decrease causes dielectric suppression in the near-IR photon energy range. And it can be due to the increased surface scattering and reduced unscreened plasma frequency in nanoparticles. On the other hand, two interband electronic transitions located at 0.7 and 1.0 eV are derived, which are in good agreement with the reported density of states of L10 FePt. In the present work, they show only a small dependence on FePt particle size. Such a study of optical properties of FePt nanoparticles is important for technological applications and fundamental physics.

Optical Characterization and Modeling of Coatings Intended as High Temperature Solar Selective Absorbers

SiCNH thin coatings synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD) are considered as high temperature ceramic materials for solar selective multilayer absorbers. As indicated by ellipsometry and spectrophotometry measurements, their optical characteristics make them suitable as solar absorbing and antireflective coatings in multilayer stacks and/or ceramic- metal composites (cermets). These measurements gave access to spectral values of complex refractive index N = n + ik, reflectance R and transmittance T of the coatings. R and T were also modeled from N, based on Fresnel equations, and a good agreement was found between simulated and measured reflectance in the visible region. A transfer matrix method was then applied to predict the optical response (R, T) of virtual multilayer stacks including SiCNH materials. W-SiCNH cermets were also considered as absorptive layers in these stacks and their refractive indices were calculated using Bruggeman effective medium approximation theory. Solar absorptance αS and thermal emittance ɛ(T) were estimated from simulated reflectance. Layer thicknesses and cermet composition were optimized using CODE software and a MATLAB in-house code, so as to maximize solar-heat conversion efficiency which relied on high αS (up to 95%) and low ɛ (down to 6% at 500°C). The corresponding global yield which could be attained in typical Fresnel and central tower solar plants using these selective coatings was also calculated and compared to current technology.

Effect of High-temperature Annealing on Evaporated Silicon Oxide Films: A Spectroscopic Ellipsometry Study

Silicon oxide films, vacuum evaporated and annealed in Ar atmosphere at a temperature of 1000 and 1100ºC have been studied by multiple-angle spectroscopic ellipsometry (SE) in the spectral range of 280-820 nm. The thickness, complex refractive index and composition of the films have been determined from the SE data analysis, performed with multiple-layer optical models and applying the Bruggeman effective medium approximation theory. The SE results have revealed that during annealing at 1000oC the Si clusters with volume fraction of ~ 14 % crystallize in the substoichiometric SiO1.61 matrix, while annealing at 1100oC transforms the oxide structure to a fully stoichiometric SiO2 with embedded nc-Si clusters with enhanced volume fraction of 22%. The TEM micrographs visualized the Si crystallites, which have random distribution in the amorphous oxide matrix and an average size of 3 nm.

Study on a vapor sensor based on the optical properties of porous silicon microcavities

In this paper, we set up a sensing model of PSMs (porous silicon microcavities) by applying the Bruggeman effective medium approximation theory and the transfer matrix method. In addition, we explain in detail the adsorption characteristics of porous silicon. Finally, using an experimental setup to measure the reflectivity spectrum of PSMs when the sensor is exposed to different organic vapors, the experimental results prove that it is a feasible optical sensor for the detection of organic species. Resolution of the PSMs sensor is high, response time and resume time is short and repetition is good.