Electromagnetic Metasurfaces. Christophe Caloz

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Preface

      Over the past decade, metasurfaces have been developed into a novel paradigm of modern science and technology. They offer possibilities to manipulate electromagnetic waves that were simply unthinkable even 10 years ago. They have already led to a myriad of new applications, reported in uncountable scientic publications. Moreover, they have recently started to make their way into the industry in a most promising fashion. Such a spectacular development has brought metasurfaces at the time of this writing to a point of maturity that is optimal for their introduction into the syllabus of all researchers and engineers active in the broad eld of electromagnetics across the microwave, terahertz, and optical regimes of the spectrum. This book, the rst textbook on the eld, responds to a pressing need for a unied document as a support to this syllabus, and opens horizons for further developments of the field.

      What is the point about metasurfaces? After all, their visual appearance is essentially similar to that of frequency- or polarization-selective surfaces and spatial light modulators, which have been known and extensively used for several decades. The point is. . . DIVERSITY. Metasurfaces represent generalizations of these devices, which are respectively restricted to ltering or polarizing and phase-only or magnitude-only wave manipulations. Metasurfaces indeed offer a universal platform for unlimited control of the magnitude, the phase, and the polarization of electromagnetic waves to meet transformation specications of virtually unlimited complexity. Moreover, in contrast ix to their voluminal counterparts, they feature a low prole that implies easier fabrication, lower loss, and, perhaps surprisingly, even greater functionality.

      This diversity would be of little practical interest if it were not accompanied by a proper instrument to master it. Fortunately, such an instrument has progressively emerged and crystalized as a combination of bianisotropic surface susceptibility models and generalized sheet transition conditions, the former properly describing metasurfaces, whose subwavelength thickness prohibits Fabry–Perot resonances, as a sheet of equivalent surface polarization current densities, and the latter representing a generalized version of the conventional boundary conditions, via the addition of these currents to the induced sources. This global modeling approach allows to systematically manipulate the enormous number of possible combinations of the 36 degrees of freedom of bianisotropic metasurfaces, both in the analysis and characterization of existing metasurfaces and in the synthesis and design of metasurfaces according to specications. It constitutes, therefore, the backbone of the book, providing, in addition to its design power, deep insight into the physics of metasurfaces and quick information on their fundamental properties.

      The book constitutes a coherent, comprehensive, self-consistent, and pedagogical framework that covers the essential theoretical aspects and practical design strategies of electromagnetic metasurfaces and their applications. It is designed to provide a solid understanding and efficient mastery of the eld to both students and researchers alike. To achieve this goal, we have organized this book in a logical fashion, with each chapter building up on the concepts established by the previous ones. Considering the plethora of publications in the eld, we naturally had to select out what we considered as the most important and relevant developments, hoping that the presented material would be x sufficient to provide comfortable access to all the non-explicitly treated topics.

      Chapter 1 provides a general denition of metasurfaces as well as a historical perspective of their development. Chapter 2 describes the fundamental electromagnetic media properties pertaining to metasurfaces. It presents the general bianisotropic constitutive relations, reviews the concepts of temporal and spatial dispersion, and recalls the Lorentz and Poynting theorems. Chapter 3 extends these concepts to the metasurfaces, establishes their modeling principles in terms of bianisotropic surface susceptibilities and generalized sheet transition conditions, and extracts from this model several fundamental physical properties and limitations. Chapter 4 applies these modeling principles to the synthesis of linear time-invariant, time-varying and nonlinear metasurfaces, and illustrates them with several examples. Chapter 5 overviews four analysis schemes that may be used to simulate and predict the scattering response of metasurfaces. One of these methods is based on the Fourier propagation technique; two are based on nite-difference methods, one in the frequency domain and the other in the time domain; and the last method is an integral equation method, which is particularly well suited for 3D computations. Chapter 6 details several fabrication metasurface technologies that pertain to the different parts of the electromagnetic spectrum. It shows how dielectric and metallic scattering particles may be used to realize metasurfaces at both optical and microwave frequencies and demonstrates several techniques to tune the shape of the scattering particles to achieve desired responses. Finally, Chapter 7 discusses three representative metasurface applications, involving respectively the properties of angle independence, perfect matching, and diffractionless refraction, and constituting representative examples of state-of-the-art metasurfaces.

1 Introduction

      Metasurfaces are subwavelenghtly thick electromagnetic structures that may be used to control the scattering of electromagnetic waves. Because of their subwavelength thickness, they may be considered as the two-dimensional counterparts of volumetric metamaterials. However, compared to metamaterials, metasurfaces offer the advantages