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Register a new userMultipolar Modeling of Spatially Dispersive Metasurfaces
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There is today a growing need to accurately model the angular scattering response of metasurfaces for optical analog processing applications. However, the current metasurface modeling techniques are not well suited for such a task since they are limited to small angular spectrum transformations, as shall be demonstrated. The goal of this work is to overcome this limitation by improving the modeling accuracy of these techniques and, specifically, to provide a better description of the angular response of metasurfaces. This is achieved by extending the current methods, which are restricted to dipolar responses and weak spatially dispersive effects, so as to include quadrupolar responses and higher-order spatially dispersive components. The accuracy of the newly derived multipolar model is demonstrated by predicting the angular scattering of a dielectric metasurface. This results in a modeling accuracy that is at least two times better than the standard dipolar model.
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Non-uniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected plane waves and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of incident plane waves into arbitrary directions. In particular, ideal refraction is possible only if the metasurface is bianisotropic (weak spatial dispersion), and ideal reflection without polarization transformation requires spatial dispersion with a specific, strongly non-local response to the fields.
Optical metasurfaces consist of a 2D arrangement of scatterers, and they control the amplitude, phase, and polarization of an incidence field on demand. Optical metasurfaces are the cornerstone for a future generation of flat optical devices in a wide range of applications. The rapidly growing advances in nanofabrication have made the versatile design and analysis of these ultra-thin surfaces an ever-growing necessity. However, despite their importance, a comprehensive theory to describe the optical response of periodic metasurfaces in closed-form and analytical expressions has not been formulated, and prior attempts were frequently approximate. Here, we develop a theory that analytically links the properties of the scatterer, from which a periodic metasurface is made, to its optical response via the lattice coupling matrix. The scatterers are represented by their polarizability or T matrix, and our theory works for normal and oblique incidence. We provide explicit expressions for the optical response up to octupolar order in both spherical and Cartesian coordinates. Several examples demonstrate that our analytical tool constitutes a paradigm shift in designing and understanding optical metasurfaces. Novel fully-diffracting metagratings and particle-independent polarization filters are proposed, and novel insights into the response of Huygens metasurfaces under oblique incidence are provided. Our analytical expressions are a powerful tool for exploring the physics of metasurfaces and designing novel flat optics devices.
We study the guided modes in the wire medium slab taking into account both the nonlocality and losses in the structure. We show that due to the fact that the wire medium is an extremeley spatially dispersive metamaterial, the effect of nonlocality plays a critical role since it results in coupling between the otherwise orthogonal guided modes. We observe both the effects of strong and weak coupling, depending on the level of losses in the system.
We show that the dispersive force between a spherical nanoparticle (with a radius $le$ 100 nm) and a substrate is enhanced by several orders of magnitude when the sphere is near to the substrate. We calculate exactly the dispersive force in the non-retarded limit by incorporating the contributions to the interaction from of all the multipolar electromagnetic modes. We show that as the sphere approaches the substrate, the fluctuations of the electromagnetic field, induced by the vacuum and the presence of the substrate, the dispersive force is enhanced by orders of magnitude. We discuss this effect as a function of the size of the sphere.
The repulsion between free electrons inside a metal makes its optical response spatially dispersive, so that it is not described by Drudes model but by a hydrodynamic model. We give here fully analytic results for a metallic slab in this framework, thanks to a two-modes cavity formalism leading to a Fabry-Perot formula, and show that a simplification can be made that preserves the accuracy of the results while allowing much simpler analytic expressions. For metallic layers thicker than 2.7 nm modified Fresnel coefficients can actually be used to accurately predict the response of any multilayer with spatially dispersive metals (for reflection, transmission or the guided modes). Finally, this explains why adding a small dielectric layer[Y. Luo et al., Phys. Rev. Lett. 111, 093901 (2013)] allows to reproduce the effects of nonlocality in many cases, and especially for multilayers.
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