*Metamaterials*

In Greek, the word meta means "beyond". The metamaterials are new class of engineered materials which exhibit unusual electromagnetic properties that do not occur in natural materials. In general, natural materials like diamond, glass, etc., have positive refractive index, magnetic permeability, and electrical permittivity. Whereas these new engineered materials show negative index of refraction, negative electrical permittivity, and magnetic susceptibility. The metamaterials are also called as left-handed (LH) materials or backward wave (BW) media or negative index materials (NIM) or double negative (DNG) media. In addition, metamaterials have some special properties such as perfect lensing [1], classical electromagnetically induced transparency [2–5], cloaking capability [6], high frequency magnetism [7], dynamic modulation of Terahertz (THz) radiation [8], reverse

**Citation:** Suresh Kumar, N.; Naidu, K.C.B.; Banerjee, P.; Anil Babu, T.; Venkata Shiva Reddy, B. A Review on Metamaterials for Device Applications. *Crystals* **2021**, *11*, 518. https:// doi.org/10.3390/cryst11050518

Academic Editors: Yuri Kivshar, Raghvendra Singh Yadav, Anju Deswal and Kottakkaran Sooppy Nisar

Received: 12 March 2021 Accepted: 30 April 2021 Published: 7 May 2021

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Doppler effect, and reverse Cerenkov effect [9]. These unique properties of metamaterials enable them to fabricate functional devices with switching and tuning capabilities [10–14]. Further, based on the permittivity and the permeability, the metamaterials are categorized as mu-negative material (MNG), epsilon negative material (ENG), double positive material (DPS), and double negative material (DNG) [15]. The MNG and ENG are also called as single negative materials. Whereas the double negative and double positive materials can be engineered at particular frequency band. In the year 1968, Victor Veselago, who is a Russian physicist, first theoretically proposed the metamaterials [16]. He theoretically explored the electrodynamics of the materials with negative values of the magnetic permeability (µ) and relative permittivity (). Moreover, the propagation of electromagnetic waves is discussed in those materials. Nevertheless Smith et al. practically demonstrated for first time a structure which exhibits negative refraction in the microwave region [15]. In addition, J.B. Pendry et al. [17] fabricated the first metamaterial by two interpenetrating subsystems. Particularly, by using an array of thin metallic (copper) wires and rings, they fabricated split ring resonators (SRRs), which provide negative values of permittivity and permeability [17–19]. In metamaterials, the split rings play the role of atoms in natural materials and act as electrically small resonant particles which contribute negative permeability. Herein, copper wire contributes negative permittivity. Combined array exhibits negative permeability and permittivity. Figure 1 gives the pictorial representation of a metamaterial [20] (Figure taken from ref. [20]). Later, Ziolkowski [21] reported another metamaterial which consists of substrate entrenched with capacitively loaded strips (CLSs) and square SRRs. In this material, capacitively loaded strips gives a strong response to electric fields and gives negative and SRRs interacts with magnetic fields and provides negative µ. Afterwards, numerous researchers designed and fabricated different metamaterials by using different methods like shadow mask/etching, clean room etching, etc., in many frequency bands. In addition, based on the presence of SRRs, the metamaterials are available in various forms. That means the metamaterials consisting SRRs are available in one, two, and three dimensions [22–25]. Further, those metamaterials without SRRs (for example, fishnet structure) are available in two, quasi-two, and three dimensions [26–29]. Hence, owing to their unique properties and structures, the metamaterials find their applications in various devices such as sensors [30–32], superlens [33], antennas [34,35], superconductors [36], absorbers [37–42], energy harvesting [43,44], etc. In this article, we have concentrated on recently developed metamaterials and their applications.

**Figure 1.** Pictorial representation of a metamaterial [20].
