Optical Waves In Layered Media
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This paper addresses the subject of electromagnetic wave scattering in layered media, thus covering the recent progress achieved with different approaches. Existing theories and models are analyzed, classified, and summarized on the basis of their characteristics. Emphasis is placed on both theoretical and practical application. Finally, patterns and trends in the current literature are identified and critically discussed.
Indeed, most of the real structures of interest, both those occurring naturally and those fabricated artificially, can be reasonably assimilated to layered structures to some degree. As a matter of fact, the solution of Maxwell equations in such structures poses serious difficulties of a mathematical nature, and there is no general and uniform approach. In fact, methods of studying scattering phenomena in layered media are greatly diversified. They depend on the kind and description of structure (objects embedded in layers, inhomogeneities distributed in a continuous manner or in the form of randomly distributed discrete scattering elements, etc.), on the information at our disposal concerning the structure of the medium, and on the kind of information that is sought about the wave process in question. Depending on the application context, the investigation on wave phenomena in layered structure, in some cases, can be successfully conducted by resorting to deterministic description; however, as far as natural scenarios are concerned, a stochastic description can provide a more adequate description of reality.
Within this framework, a clear understanding of the wave scattering processes taking place in the layered structures still poses great theoretical challenges. Accordingly, modelling EM scattering phenomena in stratified media is an active area of research with many practical applications, and there exists a very extensive literature on the subject. The inherent problem formulation can be treated from several viewpoints. As a first distinction, the existing techniques can be grouped into two main classes: direct and inverse approaches. Direct scattering models for layered structures have a practical relevance in a number of contexts, such as radio-wave propagation, optics, radar imaging, and microwave remote sensing. Direct modelling methods can generally be categorized in analytical and numerical methods. Some of them turn out more appropriate, in terms of accuracy and computational cost, in a specific application context. Stratified structures play a paramount role also in the solution of electromagnetic inverse scattering problems, which are involved in imaging of hidden or buried targets in multilayer media. Nondestructive testing and evaluations represent significant example, as well as the modelling of layered structures in through-the-wall procedures for security applications and in the geophysical prospecting of buried cultural heritages.
Therefore, this paper aims at providing a concise and organized exposition of the existing methods of analysis of EM scattering in layered media. Moreover, the emphasis is placed on conceptual advancements and novel methods that have been recently established, thus outlining important new research directions. The different methodologies are classified on the basis of their characteristics, also discussing pertinent basic principles, advantages, and disadvantages.
The paper is organized as follows. Section 2 focused on existing analytical formulations, with a particular emphasis on recently developed functional forms for rough multilayer media scattering. Numerical methods of interest are addressed in Section 3, thus covering both well-established and innovative methods. Section 4 is devoted to EM inverse scattering problems in layered structures, also delineating current progress and trends.
Layered structure modelling is an important topic in in modem applied electromagnetics. A review of methodologies for the EM scattering in layered media has been presented in this paper. Different techniques in the literature have been examined and discussed, by emphasizing their basic principles, advantages, and disadvantages. Particularly, special attention has been given to direct methods for modelling scattering in layered rough structure that have been recently established. The role of stratified media in the solution of electromagnetic inverse scattering problems has been discussed, too. In particular, the current trend in the development of inverse procedures for target detection in half spaces and in multilayer materials has been delineated. We emphasize that the model selection generally represents a compromise between accuracy and computational complexity, with the best compromise that may significantly change, depending on the application contexts.
Therefore, our review has been aimed at providing a current perspective on the subject, also highlighting innovative research directions. However, we make no claim to cover all the many topics pertinent to layered media. Indeed, with the rapid growth of the field, such a task would be almost impossible in a single paper.
The basic principles of a non-contact, near-infrared technique for the mapping of layered tissues are discussed theoretically and verified experimentally. The propagation properties of diffuse photon-density waves in tissues depend on the optical properties of the tissue. When a layered medium is irradiated by amplitude modulated light, the difference in optical properties between the layers is evident in the phase and amplitude of the diffuse reflection coefficient, which is a result of the interference of the partial waves propagating in the different layers. Thus, diffuse photon-density waves are applicable to the analysis of the structure of layered tissue. The probing depth is determined by the modulation frequency of the incident light. For modulation frequencies between several hundred megahertz and a few gigahertz, this allows us to analyse the properties of muscle tissue of up to 4-8 mm below the surface. Experimental results based on chicken breast muscle are given. As an example, the technique might be of use for evaluating the depth of necrosis and the blood volume fraction in deep burns.
The method of frustrated total reflection is used to optically excite electromagnetic surface plasma waves (SPW) in two different layered structures. The first system studied has two films (MgF2-Ag) between glass and air, and the second has three films (Ag-MgF2-Ag) between glass and air. Surface roughness of the evaporated MgF2 films is found to increase with film thickness and this roughness has a pronounced effect on the SPW resonances. A cermet is imagined to form at the MgF2-Ag interface, and its effective optical constants can be evaluated using the Maxwell Garnett theory. If this cermet is treated as a separate layer in the structure, then good agreement is found with the experimentally observed resonances. A calculation of the Poynting vector field, current distributions, and surface charge densities, resulting from an incident monochromatic plane wave, shows the different modes of oscillation corresponding to each resonance.
We study numerically the linear properties of periodic and quasiperiodic anisotropic layered media. Each anisotropic slab can have arbitrary orientation of optic axis. We apply the general numerical code to recover the known results for solc filters. We propose novel periodic structures where the location and width of the gaps can be controlled easily. We also study the transmission properties of a Fibonacci sequence of anisotropic layers and show the interesting features like self-similarity and scaling.
We review some of the recent concepts and their realization exploiting the perfect destructive interference of light in micro and nano structures. One refers to optical structures where the effective absorption can be controlled and maximized to perfect absorption. The reported effects depend crucially on the coherent nature of the exciting radiation. Achieved with a single (two or more) incident plane wave (waves) the effect carries the name of critical coupling (coherent perfect absorption). Thus in a system supporting critical coupling (CC) or coherent perfect absorption (CPA) all the incident radiation can be absorbed leading to null scattering. In particular all the incident light energy can be channeled into a specified mode of a multimodal structure if such modes are supported by the system. We present a brief overview of CC and CPA in linear systems to recount their underlying concepts as time-reversed lasing and some of their futuristic applications. Next we review our work on the nonlinear extensions of CC and CPA where one or more of the layered media could be nonlinear with Kerr-type nonlinearity. The dispersive nonlinearity is shown to offer a practical handle over the process of perfect absorption by incident laser power. Further we show that the nonlinear periodic structure can support gap solitons which absorbs all the incident energy and do not scatter any light outside the hetero-guide. 2b1af7f3a8