and F. Martin, Novel small resonant electromagnetic particles for metamaterial and filter design, Int Conf Electromagn Adv Applic (ICEAA), Torino, Italy, 2003. 13. M. Shamonin, E. Shamonina, V. Kalinin, and L. Solymar

Extraordinary optical transmission of light or electromagnetic waves through metal plates periodically perforated with subwavelength holes has been exhaustively analyzed in the last ten years. The study of this phenomenon has attracted the attention of many scientists working in the fields of optics and condensed matter physics. This confluence of scientists has given rise to different theories, some of them controversial. The first theoretical explanation was based on the excitation of surface plasmons along the metal-air interfaces. However, since periodically perforated dielectric (and perfect conductor) slabs also exhibit extraordinary transmission, diffraction by a periodic array of scatterers was later considered as the underlying physical phenomenon. From a microwave engineering point of view, periodic structures exhibiting extraordinary optical transmission are very closely related to frequency-selective surfaces. In this paper, we use simple concepts from the theory of frequency-selective surfaces, waveguides, and transmission lines to explain extraordinary transmission for both thin and thick periodically perforated perfect conductor screens. It will be shown that a simple transmission-line equivalent circuit satisfactorily accounts for extraordinary transmission, explaining all of the details of the observed transmission spectra, and easily gives predictions on many features of the phenomenon. Although the equivalent circuit is developed for perfect conductor screens, its extension to dielectric perforated slabs and/or penetrable conductors at optical frequencies is almost straightforward. Our circuit model also predicts extraordinary transmission in nonperiodic systems for which this phenomenon has not yet been reported.

At microwave frequencies, hollow metallic waveguides behave in certain aspects as a ''onedimensional plasma.'' This feature will be advantageously used for simulating the propagation of electromagnetic (EM) waves in left-handed metamaterials provided the hollow waveguide is periodically loaded with split ring resonators. It will be shown that EM transmission in this structure is feasible within a certain frequency band even if the transverse dimensions of the waveguide are much smaller than the associated free-space wavelength. This effect can be qualitatively and quantitatively explained by the left-handed metamaterial theory, thus providing a new experimental validation for such a theory. DOI: 10.1103/PhysRevLett.89.183901 PACS numbers: 41.20.Jb, 42.70.Qs, 78.20.Ek In 1968 Veselago [1] analyzed electromagnetic (EM) wave propagation through media having simultaneously negative electric permittivity and negative magnetic permeability. Since the E, H fields and the wave vector k of a propagating plane EM wave form a left-handed system in these materials, Veselago referred to them as ''lefthanded'' media. Other interesting properties of such media are, among others, negative refractive index and reversed Doppler effect [1]. The nonexistence of transparent left-handed media in Nature made Veselago's analysis and predictions remain for a long time as a mere theoretical curiosity. However, recently, microwave propagation through an artificial left-handed medium (or metamaterial) has been demonstrated by Smith et al. [2]. The left-handed metamaterial reported in this paper was, in fact, the superposition of two different artificial media: a two-dimensional plasma working below its plasma frequency and an artificial negative magnetic permeability medium (NMPM). Artificial plasmas are well-known materials that can be realized, for instance, by using a regular array of conducting wires [3]. Artificial NMPM have recently been proposed by Pendry et al. [4] and experimentally demonstrated by Smith et al. [2]. These latter media are composed of electrically small resonant particles, which show a very high diamagnetic susceptibility above and around its resonance frequency. When these particles are arranged in a regular lattice and illuminated in a proper way, their high diamagnetic susceptibility causes the resulting composite medium to have a negative effective magnetic permeability within a certain frequency range. In this range the resulting artificial medium is called an artificial NMPM. The split ring resonator (SRR) proposed by Pendry et al. [2,4] (see Fig. 1) fulfills all the requirements for the design of the aforementioned NMPM.The artificial plasma medium proposed in [2] was a simulated two-dimensional plasma, made by placing a regular array of metallic posts between two parallel metallic plates. Nevertheless, a significant simplification of the plasma simulation is possible by using a hollow metallic waveguide, as suggested by the dispersive behavior of these waveguides. It is well-known that hollow metallic w...

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Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters. In addition, metasurfaces have recently been proposed to confine electromagnetic waves, thereby avoiding undesired leakage of energy and increasing the overall efficiency of electromagnetic instruments and devices. The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Due to these advantages, they are promising candidates for real-world solutions to overcome the challenges posed by the next generation of transmitters and receivers of future high-rate communication systems that require highly precise and efficient antennas, sensors, active components, filters, and integrated technologies. This Roadmap is aimed at binding together the experiences of prominent researchers in the field of metasurfaces, from which explanations for the physics behind the extraordinary properties of these structures shall be provided from viewpoints of diverse theoretical backgrounds. Other goals of this endeavour are to underline the advantages and limitations of metasurfaces, as well as to lay out guidelines for their use in present and future electromagnetic devices. This Roadmap is divided into five sections: 1. Metasurface based antennas. In the last few years, metasurfaces have shown possibilities for advanced manipulations of electromagnetic waves, opening new frontiers in the design of antennas. In this section, the authors explain how metasurfaces can be employed to tailor the radiation properties of antennas, their remarkable advantages in comparison with conventional antennas, and the future challenges to be solved. 2. Optical metasurfaces. Although many of the present demonstrators operate in the microwave regime, due either to the reduced cost of manufacturing and testing or to satisfy the interest of the communications or aerospace industries, part of the potential use of metasurfaces is found in the optical regime. In this section, the authors summarize the classical applications and explain new possibilities for optical metasurfaces, such as the generation of superoscillatory fields and energy harvesters. 3. Reconfigurable and active metasurfaces. Dynamic metasurfaces are promising new platforms for 5G communications, remote sensing and radar applications. By the insertion of active elements, metasurfaces can break the fundamental limitations of passive and static systems. In this section, we have contributions that describe the challenges and potential uses of active components in metasurfaces, including new studies on non-Foster, parity-time symmetric, and non-reciprocal metasurfaces. 4. Metasurfaces with higher symmetries. Recent studies have demonstrated that the properties of metasurfaces are influenced by the symmetries of their constituent elements. Therefore, by controlling the properties of these constitutive elements and their arrangement, one can control the way in which the waves interact with the metasurface. In this section, the authors analyze the possibilities of combining more than one layer of metasurface, creating a higher symmetry, increasing the operational bandwidth of flat lenses, or producing cost-effective electromagnetic bandgaps. 5. Numerical and analytical modelling of metasurfaces. In most occasions, metasurfaces are electrically large objects, which cannot be simulated with conventional software. Modelling tools that allow the engineering of the metasurface properties to get the desired response are essential in the design of practical electromagnetic devices. This section includes the recent advances and future challenges in three groups of techniques that are broadly used to analyze and synthesize metasurfaces: circuit models, analytical solutions and computational methods.