Waves in resonant metasurfaces

The underlying significance of exotic electromagnetic phenomena in the operation of metasurfaces is primarily due to certain resonant conditions. The ratio of wavelength in use and the dimensional feature of metasurface remains the determining factor for the observed spectral characteristics with certain features [1–3]. As such, the designing and fabricating artificially engineered structures that are capable of manipulating electromagnetic waves remain greatly attractive. In view of this, the engineering of mediums has been one of the greatly focused topics in the research frontline during the span of a decade or so [4–6].

In the operation of metasurfaces, which are specially designed periodic arrays of micron- or nano-sized objects (or meta-atoms) forming the lattice geometry (of engineered mediums), the electromagnetic phenomena of surface lattice resonance reveals the propagation of surface waves [7–9]. These periodic structures can be considered as effective mediums [10,11], wherein the existing surface waves result in certain lattice modes that the metasurfaces support and exhibit extraordinary electromagnetic response – the feature that can be exploited to a broad spectrum of applications in the current technological scenario. Some of the exemplified applications would be the energy harvesting devices, perfect absorbers, nonlinear mediums for communications, sensors for environmental monitoring and/or medical diagnostics, antenna structures, superlenses, etc. [12–17].

Considering the importance of this field of research, the joint editors-in-chief of the Journal of Electromagnetic Waves and Applications (JEMWA) had the thought of bringing out a Special Issue – Waves in Resonant Metasurfaces – on this topic so that some of the recently reported results relevant to this fascinating area of research can be brought together to focus on. This Special Issue encapsulates 2 review articles and 11 research papers. Through these, the authors report research results highlighting the different aspects of metasurfaces – the prime constituent of varieties of metamaterial-based devices for different kinds of applications.

This Special Issue starts with 2 review articles. One of these discusses the resonance in metamaterials observed in the terahertz (THz) regime of the electromagnetic spectrum. In the article, Banerjee et al. touch upon the fundamentals of resonance conditions exhibited by the inductor-capacitor (LC) circuits and relate the situations with those that happen in the case of split-ring resonators-based metamaterials. Apart from these, the authors also discuss the importance of relevant phenomena, such as Fano resonance, electromagnetically induced transparency, plasmon-induced transparency, in the context of metamaterials. Overall, this review article throws a glimpse of the different resonance phenomena observed in THz metamaterials and may serve as ready reference for investigators dedicated to this area of research. The other review article by Rhee et al. emphasizes more on the recent developments in metamaterial-based perfect absorbers. The authors talk about the investigation of varieties of such perfect absorbers operating in different frequency regimes with enough stability in performance. They also describe different numerical techniques usually exploited in the investigation of metasurface-based structures and highlight the merits and demerits of those. The ways to study the performance characteristics are also emphasized in this review, followed by future perspectives.

The phenomenon of surface plasmonics remains greatly important in the operation of metasurface-based devices. Apart from the incidence excitation, the partnering materials in designing these determine the generation and propagation of surface plasmon polariton waves. The paper by Shabani et al. discusses the impact of material characteristics on the optical response of certain forms of artificial mediums containing the metal-dielectric interface. To be more explicit, the authors highlight the importance of material characteristics in determining the optical behavior of surface plasmon configurations and the ways to achieve the tuning properties for certain photonic applications.

Electromagnetic cloaking has been one on the prudent applications of metamaterials that makes the possibility for an object to be invisible in certain spectral regime [18,19]. Pendry et al. [20] and Leonhardt [21] theoretically proved the existence of an ideal cloaking device. In such studies, the property of incidence radiation remains important which, to a great extent, determines the scattering width that accounts for the invisibility of object [22]. However, apart from the spectral regime of operation, the obliquity of incidence waves is also equally important. In one of the papers in this Special Issue, Bisht et al. propose an approach for improving the invisibility performance of mantle cloak by operating it under tilted conditions with respect to the incident radiation and demonstrate significant reduction in scattering width and performance robustness. They used a particular form of metasurface with a capability to dynamically adjust the impedance. The authors claim the findings to be useful in establishing the mantle cloaking as a potential technique for practical applications, such as camouflaging, low-observability, etc.

The paper by Yu et al. presents a one-dimensional nonreciprocal structure comprising asymmetric multilayers filled with magneto-optical dispersive mediums along with a defect layer. The authors discuss about the excitation of nonreciprocal optical Tamm states and determine theoretically and numerically the one-way transmission and reflection of waves – the characteristics that can be affected by the parametric and operational conditions, such as the defect layer thickness, incidence obliquity and the externally applied magnetic field. The authors claim the obtained results to have promising potential for practical application in designing tunable nonreciprocal optical structures, such as multichannel optical isolators and photodiodes.

Chiral mediums are optically active in nature and have been attractive in designing metamaterials [23]. The paper by Feng et al. focuses on the use of chiral metamaterials in realizing the asymmetric transmission of circularly polarized waves in the microwave regime. In particular, the authors propose a kind of bi-layer extrinsic chiral metamaterial, and numerically determine the transmission properties. As the authors mention, the tunable property of the proposed metamaterial has potentials in the polarization control related applications, viz. circular polarizers and polarization rotators.

In another work by Lin et al., simulations of an anisotropic metasurface to be utilized for ultra-wideband circular polarization-maintaining reflector is discussed, followed by experimental validations. The results indicate the proposed reflector design to be useful for both the right- as well as left-handed circularly polarized incidence radiations in the frequency band of 8.79–27.09 GHz. To analyze the physical mechanism, the authors present formula to be used to evaluate the co- and cross-polarization reflection coefficients under the circularly polarized incidence. The authors claim the structure to have potentials in the design of Pancharatnam-Berry phase metasurface.

The paper by Barde et al. deals with fairly simple kind of metamaterial absorber having an array of metallic set-square design deposited over the FR₄ substrate to operate in the X-band regime of the electromagnetic spectrum. The authors present simulation and experimental results, which remain is close agreement. As they mention, the structure can be used in applications, such as phase imaging, spectroscopic detection, antenna engineering, etc. In yet another related work, the reports on a wideband microwave resonating type of absorber for X-band applications are dealt with by Gogoi. In particular, the author exploits expanded graphite-silicone composite encapsulated in flexible silicone rubber matrix to build periodic unit cell of the developed absorber. The length and width of the unit cell and the used meta-atom are kept the same, in order to get a symmetrical design for normal incidence wave. The author investigates resonance and absorption mechanisms evaluating the simulated electric/magnetic fields and current density inside the unit cell of metasurface. The proposed structure exhibits polarization-independent absorption for the nominal incidence of wave. The prototype absorbers are also experimentally studied to validate the simulated results with the measured ones.

Lu et al. present the design and analyses of perfect metamaterial absorber for the THz regime implementing the simulation and numerical techniques. In particular, they touch upon metasurfaces comprising an array of one-side-open metallic circular cavity and compare the absorption results with those when the circular structure is replaced with the elliptical shape. Using the transverse electric and transverse magnetic polarizations of incidence radiation, the results indicate achieving high absorption corresponding to 6.674 THz frequency under a normal incidence of waves. The authors claim the simulation and analytical results to be in close agreement.

Metasurfaces have wide applications in the field of antenna engineering [12]. For low-loss antennas with high radiation efficiency, fulfilling the impedance matching condition becomes necessary. However, the multi- and wideband solutions in engineering antenna structures may suffer from the limitations imposed by the impedance matching requirements. This triggers to exploit metamaterials in antenna designs, in order to achieve the desired results without compromising enough with the antenna parameters. In this context, Sharma and Bhatia present the design of a fidget spinner-shaped metamaterial inspired fractal antenna loaded with parasitic split-ring resonator, and investigate the performance characteristics for varieties of wireless applications in the 1.88–7.0, 7.81–8.85, and 9.6–14.3 GHz frequency regimes. The authors present the simulation results, which are found to be in good agreement with the measured values for the fabricated prototype.

Metasurfaces can be used to achieve polarization conversion [24]. In one paper, Zhang et al. discuss a compact and high-efficiency tri-band artificial electromagnetic polarization manipulating metasurface that can be used as polarization converter. The authors report the polarization conversion ratio, which determines the efficiency of the proposed metasurface, to be over 90% in the 9.8–12.2, 19.0–26.6 and 24.9–29.5 GHz frequency spans. They also report certain values of frequency corresponding to which the polarization conversion ratio reaches almost 100%. The experimental results of the fabricated prototype are found to be consistent with the simulation results. They claim the proposed metasurface design to be potentially useful in high-performance polarization devices.

Artificial magnetic conductors (AMCs)-based metasurfaces are greatly important in the context of microwaves and antenna-related applications [25,26]. These are periodic arrangements of two-dimensional subwavelength-sized metallic scatterers on a grounded dielectric substrate. Pivoted to this area of research, the paper by Brandão et al. discusses on AMC-based metasurfaces that can be exploited in operations, such as antenna ground plane to improve the radiation parameters, radar cross-section (RCS) reduction, and as insulators between antenna and metallic surfaces in RFID applications. Within the context, the authors present analytical studies of surface susceptibility using the generalized boundary conditions and equivalent circuit model. They report the detailed design procedure and demonstrate the applicability through numerical examples. The study reveals the viability of the proposed model.

The above discussions, either on the presented review articles or the usual research papers, essentially determine the technological importance of metasurfaces gained in the recent years. All the inclusions in this Special Issue describe varieties of metasurface-based devices that can be of potentials in some kinds of electromagnetic applications. The joint editors-in-chief are hopeful of getting these research results attractive, which would stimulate the readers to venture for coming up with new design techniques in the context of metamaterials with an even wider scope of technological usage.