Study Of The Optical Properties In Metal-dielectric Granular Composites

The optical properties of the metal-dielectric granular composite media have attracted much attention for its plentiful physical nature, scientific and technologic significance. Metal-dielectric composite, which is a kind of artificially functional material, is able to remain the virtues of each component and possess new properties which the components haven't. Non-magnetic metal-dielectric granular composite takes on different properties in the vicinity of the percolation threshold. Optical absorptance, reflectance and transmittance are independent of the frequency. When two kinds of different materials are distributed randomly, there are stop bands. While a magnetic field is applied to inhomogenous magnetic composite and periodic structure, there are different optical and magneto-optical properties. The study of the properties of non-magnetic metal or magnetic metal granular composite is not only of theoretical importance but also valuable for the technological application in condensed matter physics and optical communication. The purpose of our work is to study optical properties of non-magnetic metal and magnetic metal-dielectric granular composites. The main results of our study are listed as follows: 1. The study of optical properties in metal-dielectric percolation composites Granular composites are usually percolation composites. The properties of a composite change abruptly while the volume concentration of the particle approaches a critical concentration. Especially, the transition from insulator to metal occurs in a metal-dielectric granular composite near the percolation threshold. Most of experimental and theoretical studies indicate that the properties in the vicinity of the percolation threshold are much different from that far from the threshold. The near-infrared absorption of the composite is maximal in the vicinity of the threshold and is a bit larger than that of single metal particles. Moreover, the optical properties of the composite such as absorptance, reflectance and transmittance are almost independent of the frequency. In other aspect, the dielectric constant of the composite changes sharply in magnitude, more importantly, the sign of the dielectric constant changes from positive to negative. This critical concentration called the optical threshold is little higher than the percolation threshold. It is possible to prepare the composite whose reflection varies from 0 to 1 in a narrow frequency range. Our theoretical model can offer detailed information over the shape effect on the threshold in metal-dielectric granular composite. It seems unreasonable to attribute the change of the threshold only to the shape effect, however, it plays a key role in the dielectric response of metal-dielectric granular composite. The further study on the effect of other factors on the optical properties of percolation composite will be carried out. 2. The study of the band gap of the metal-dielectric granular composites The photonic band-gap material (PBG) is a new kind of optical communication and optical memorizer due to its band-gap structure. During the last decade, starting from the pioneering papers of Yablonovitch and John, the technological applications and devices exploitation of PBG have boomed. Rather recently some publications have emerged in which the band-gap in inhomogenous composite and multi-gap is observed and magnetic photonic band-gap materials (MPBG) are considered. In this paper, we discuss the propagation of the electro-magnetic (EM) waves in two kinds of granular composites, metal-dielectric and magnetic metal-dielectric composite, and propose our model to explain the experimental results. (1) The researchers have dealt exclusively with the metal-dielectric photonic band-gap structures almost in the microwave, millimeter and far-infrared wavelength range in which the absorption loss of the metal component is very small. Compared with the periodic structure composed of two kinds of dielectric materials, it is found that the width of the band-gap can be broadened. Recently, the metal-dielectric core-shell granular structures can also produce the band gap and by adjusting the ratio of the radius of the core to the thickness of the shell the light propagation in the composite can be controlled. Moreover, the outer shell can stop the inner core from physical andHS model to verify the possibility of the existence of band-gap in the core-shell structure. Besides, it is a surprising phenomenon that photonic band spectra have multiple gaps when the core-shell particles are multi-coated. Using the first principle, we give an explanation of the experimental results. We bring up another tunable means, the change of the particle shape, which leads to multiple stop and pass bands. In this paper, we not only give an good explanation of the related experimental results, but draw a reasonable conclusion that there are multiple stop bands on account of multiple metal-dielectric interfaces and it is possible to observe multi-gaps by changing the geometrical shape of the particles. (2) Magnetic materials are promising as prospective components of granular composites as they allow tuning their optical properties under dc magnetic field action. The nonlinear magneto-optical Faraday and Kerr effects are enhanced strongly due to the photonic localization in the magnetic layer. For most ferrites the relative magnetic permeability is quite different from 1 in the microwave range and thus can be exploited for microwave PBG materials. Up to now only a few tens of papers were devoted to the investigations of magnetic band-gap, mainly about experimental work, but less theoretical publications were reported. In this paper, the nonlinear magneto-optical properties in magnetic granular composite are studied based on a tensor effective medium approximation (TEMA). We propose a model including TEMA and transfer matrix method to consider one-dimensional magnetic metal-polymer composite. The effects of dc magnetic field on the band-gap and ferromagnetic resonance are discussed and our results are in good agreement with experiments. We also expect our work will lead to intensive theoretical research on magnetic band-gap.