**About the Editors**

## **Naser Ojaroudi Parchin**

Naser Ojaroudi Parchin (SMIEEE) is currently an assistant professor (lecturer) at Edinburgh Napier University, Edinburgh, UK. He obtained his Ph.D. in Electrical Engineering from the University of Bradford, UK, where he was a postdoctoral research assistant at the Faculty of Engineering and Informatics and worked as a research fellow for the SATNEX V project, funded by the European Space Agency. From 2018 to 2020, Naser was a Marie Curie Research Fellow of the H2020-ITN-SECRET project funded by the EU Commission, targeting 5G mobile small cells. His research interests include phased arrays, MIMO systems, smartphone antennas, SAR/user impact, full-duplex diversity antennas, 5G antennas, implementable and biomedical sensors, RFID tag antennas, millimeter-wave/terahertz components, fractal structures, metamaterials and metasurfaces, PCB realization, Fabry resonators, EBG/FSS-inspired radiators, microwave filters, reconfigurable structures, and wireless propagation. He has over 12 years of research experience in antennas and microwave engineering. He has participated in several international IEEE conferences across the world, where he presented his articles through oral presentations. Dr. Naser is the author and co-author of several books/book chapters and more than 350 technical journals and conference papers. He is a member of the Institute of Electrical and Electronics Engineers (IEEE), the Marie Curie Alumni Association (MCAA), the European Association on Antennas and Propagation (EurAAP), and the Engineering Professors' Council (EPC). He is also an active reviewer for various high-ranking journals and publishers. He has been appointed as a Guest Editor and Topic Board Member for several Special Issues. He was included in the World's Top Scientists list in 2016, 2020, 2021, 2022, and 2023. His papers have more than 6600 citations with a 46 h-index, as reported by Google Scholar. Naser's score is higher than 95% of all RG members' scores.

## **Mohammad Ojaroudi**

Mohammad Ojaroudi received his Ph.D. in electrical engineering from Shahid Beheshti University, Tehran, Iran. Over the past two decades, his research background has flourished at renowned academic institutions, where he actively took on the role of project manager in diverse research undertakings. The fruitful outcomes of these investigations have been disseminated through the publication of over 150 articles. Guided by an unwavering passion for comprehending consciousness and information processing within the brain, he has directed his focus toward pioneering advancements in neurotechnology. This pursuit led him to establish COGNISCAN in 2021, with an ambitious vision for harnessing the potential of electromagnetic waves in medical brain imaging. As the CEO of COGNISCAN, his primary responsibility lies in leading the executive team. Drawing upon his discerning judgment, he critically evaluates circumstances, establishes strategic objectives, and develops operational plans. He is also proficient in talent acquisition, nurturing a team of skilled professionals, and motivating and rewarding them to foster innovation and achieve tangible results.

### **Raed A. Abd-Alhameed**

Raed A. Abd-Alhameed received his B.Sc. and M.Sc. degrees from the University of Basrah, Basrah, Iraq, in 1982 and 1985, respectively, and his Ph.D. degree from the University of Bradford, Bradford, U.K., in 1997, all in electrical engineering. He is currently a professor of electromagnetic and radiofrequency engineering. He has also been an investigator and co-investigator in several funded research projects. He has many years of research experience in the areas of radio frequency, signal processing, propagation, antennas, and electromagnetic computational techniques; he has published more than 500 academic journal articles and conference papers, and has coauthored three books and several book chapters. His research interests include computational methods and optimizations, wireless and mobile communications, sensor design, EMC, beam-steering antennas, energy-efficient PAs, and RF pre-distorter design applications. He is a Fellow of the Institution of Engineering and Technology, U.K., and of the Higher Education Academy, as well as being a registered Chartered Engineer in the U.K. He received the Business Innovation Award for his successful KTP with Pace and Datong's companies on the design and implementation of MIMO sensor systems and antenna array design for service localizations. He is the chair of several successful workshops on the topic of "Energy Efficient and Reconfigurable Transceivers: Approach Towards Energy Conservation and CO<sup>2</sup> Reduction" that address the biggest challenges for future wireless systems.

## *Editorial* **Special Issue "Metamaterials and Metasurfaces"**

**Naser Ojaroudi Parchin 1,\* , Mohammad Ojaroudi <sup>2</sup> and Raed A. Abd-Alhameed <sup>3</sup>**


Metamaterials and metasurfaces have emerged as promising technologies in the field of antennas and wireless applications. They offer unprecedented control over electromagnetic waves, enabling the design of novel antenna structures with enhanced performance capabilities. By integrating metamaterial structures into antenna designs, it is possible to achieve electrically small antennas without sacrificing efficiency or bandwidth. Metasurfaces, on the other hand, are planar arrangements of subwavelength elements that manipulate the properties of incident electromagnetic waves. They are constructed by patterning a surface with precisely engineered meta-atoms, tailored to achieve specific functionalities. Metasurfaces can be used to control wavefronts, as well as the polarization and reflection properties of antennas. Metasurface antennas offer numerous advantages in wireless applications. They can achieve beamforming and steering capabilities without the need to use bulky and complex phased arrays in future wireless networks. Furthermore, metasurfaces can be used to create conformal and flexible antenna structures. This opens up new possibilities for the utilization of Internet of Things (IoT) applications and wearable devices. The use of metamaterials and metasurfaces can lead to significant advances in wireless communication systems, including improved signal quality, increased data rates, and seamless integration into various devices and environments.

The scope of this Special Issue encompasses a comprehensive exploration of metamaterials and metasurfaces, covering every facet of their design and construction. Moreover, it strives to spotlight captivating advances, prevailing trends, and recent accomplishments in this field. This Special Issue is a collection of 10 papers that are briefly explained in the following.

Lopato et al. [1] examine the impact of fabrication process uncertainties on the quality of terahertz metasurfaces, specifically focusing on how inaccuracies in metasurface fabrication affect resonances. They employ a numerical model to analyze the influence of uncertainties in the different geometric parameters obtained during the fabrication process, including layer deposition, photolithography, and etching processes, with respect to the resonance behavior of the designed metasurface. To validate their findings, the researchers verify the developed numerical model by applying it to a fabricated structure.

Wang et al. [2] investigate the challenge of anti-jamming matching reception for smallsignal anti-jamming in the presence of intense electromagnetic jamming. The authors employ the Charnes–Cooper (CC) transform algorithm to identify the most favorable dynamic metamaterial antenna (DMA) array–element–codeword–state matrix, which maximizes the received signal-to-interference-plus-noise ratio (SINR). Through simulations, it is demonstrated that DMA offers notable advantages over traditional array antennas. Moreover, the utilization of DMA leads to reduced communication overhead, with a promising potential to upgrade existing wireless communication systems.

Liu et al. [3] delve into the exploration of negative group delay (NGD) metamaterials by utilizing split-ring resonators (SRRs). Their theoretical analysis involves the calculation of equivalent circuit parameters for two distinct types of SRRs. The measured results of the

**Citation:** Ojaroudi Parchin, N.; Ojaroudi, M.; Abd-Alhameed, R.A. Special Issue "Metamaterials and Metasurfaces". *Electronics* **2023**, *12*, 2420. https://doi.org/10.3390/ electronics12112420

Received: 18 May 2023 Accepted: 23 May 2023 Published: 26 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

prototypes align closely with the theoretical predictions and simulated outcomes. In addition, through simulations, it is demonstrated that the proposed metamaterials effectively reduce beam walk, showcasing their potential to serve as a solution in this context.

Zhang et al. [4] present the development of an ultra-broadband and angular-stable reflective linear into a cross-polarization converter utilizing a metasurface. The converter's unit cell is constructed using a slant end-loaded H-shaped resonator. Through simulations, is the authors reveal that the proposed design achieves a polarization conversion ratio exceeding 90% within the frequency range of 9.83–29.37 GHz, resulting in a relative bandwidth of 99.69% with high efficiency, ultra-broadband capability, and angular stability.

Amer et al. [5] present a polarization-insensitive broadband metamaterial absorber structure that exhibits wide-angle reception capabilities. The structure is based on square split-ring resonators (SSRRs) and incorporates lumped resistors. The proposed metamaterial absorber achieves absorption levels exceeding 90% over a wide frequency range from 1.89 GHz to 6.85 GHz, with a relative bandwidth of 113%.

Zhang et al. [6] demonstrate an inverse design framework for isomorphic metasurfaces utilizing representation learning. Through the use of autoencoders (AEs) with various architectures, the original high-dimensional space is effectively mapped onto a low-dimensional space with minimal information loss. It achieves a remarkable average accuracy of 94% on test sets, while also providing the design matrix within a matter of seconds, meaning that it significantly saves resources and time compared to traditional methods.

Voronov et al. [7] introduce new configurations of a magnetoinductive device that exhibits directional filter properties. Additionally, a new method is presented to enhance the device's filtering performance by compensating for multipath loss. The authors demonstrate techniques for constructing tunable devices that utilize toroidal ferrite-cored transformers in which experimental results confirm the agreement with the theoretical models.

Tavora de Albuquerque Silva et al. [8] introduce a novel unit cell design for electromagnetic bandgap (EBG) structures, utilizing a HoneyComb geometry (HCPBG). The design offers several advantages, including a reduced occupied area and flexible rejection band properties. Additionally, a strategy for the design of reconfigurable HCPBG filters is presented where the resonance frequency can be adjusted. The behavior and reconfiguration of the HCPBG filter are demonstrated through EM simulations.

Mitra et al. [9] employ transformation electromagnetics/optics (TE/TO) to realize a non-homogeneous, anisotropic material-embedded beam-steering technique without the need for phase control circuitry. The theoretical framework is supported by numerical simulations, validating the feasibility of the proposed approach. These innovative designs and methods have practical applications in various domains such as wireless communications, radar systems, beamforming, and beam steering.

Abdalrazak et al. [10] conduct a comprehensive literature review focusing on the squint phenomena in antenna arrays at mmWave frequencies. The study examines the challenges associated with the squint phenomena. The authors categorize the main effective solutions into beamforming techniques, antenna geometry modifications, and channel estimation algorithms. Additionally, another classification is explored, specifically one that leverages the beam squint phenomenon to improve channel localization and capacity. The literature review provides valuable insights into understanding and mitigating the challenges associated with squint phenomena in mmWave antenna arrays.

We would like to express our sincere appreciation and gratitude to all the authors who have made exceptional contributions to this journal. We would also like to extend our heartfelt thanks to the reviewers for their valuable comments and feedback, which have greatly improved the quality of the articles. Additionally, we would like to acknowledge the editorial board and the editorial office of *Electronics* for their support and guidance throughout the publication process. We hope that our readers will find the articles in this journal informative, insightful, and full of new and valuable information on Metamaterials and Metasurfaces for wireless communications.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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