**Metamaterials at the THz**

**Chapter 5**

Provisional chapter

**Terahertz Leaky-Wave Antennas Based on**

Terahertz frequencies are increasingly gaining attention due to the recent efforts made in narrowing the technological gap among microwave and optical components. Still the demand of efficient THz antennas is high, due to the difficulty in obtaining directive patterns and good radiation efficiencies with planar, low-cost, easy-to-fabricate designs. In this regard, leaky-wave antennas have recently been investigated in the THz range, showing very interesting radiating features. Specifically, the combination of the leakywave antenna design with the use of metamaterials and metasurfaces seems to offer a promising platform for the development of future THz antenna technologies. In this Chapter, we focus on three different classes of leaky-wave antennas, based on either metasurfaces or tunable materials, namely graphene and nematic liquid crystals. While THz leaky-wave antennas based on homogenized metasurfaces are shown to be able to produce directive patterns with particularly good efficiencies, those based on graphene or nematic liquid crystals are shown to be able to dynamically reconfigure their radiating features. The latter property, although being extremely interesting, is obtained at the expense of an increase of costs and fabrication complexity, as it will emerge from the

DOI: 10.5772/intechopen.78939

Keywords: terahertz, leaky-wave antennas, metasurfaces, graphene, liquid crystals

Metamaterials and metasurfaces [1] are, respectively, three-dimensional (3-D) and twodimensional (2-D) engineered man-made materials, which may exhibit electromagnetic properties commonly unaccessible with materials available in nature. Since the introduction of

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Terahertz Leaky-Wave Antennas Based on

**Metasurfaces and Tunable Materials**

Metasurfaces and Tunable Materials

Walter Fuscaldo, Silvia Tofani, Paolo Burghignoli,

Walter Fuscaldo, Silvia Tofani, Paolo Burghignoli,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Paolo Baccarelli and Alessandro Galli

Paolo Baccarelli and Alessandro Galli

http://dx.doi.org/10.5772/intechopen.78939

results of the presented study.

Abstract

1. Introduction

#### **Terahertz Leaky-Wave Antennas Based on Metasurfaces and Tunable Materials** Terahertz Leaky-Wave Antennas Based on Metasurfaces and Tunable Materials

DOI: 10.5772/intechopen.78939

Walter Fuscaldo, Silvia Tofani, Paolo Burghignoli, Paolo Baccarelli and Alessandro Galli Walter Fuscaldo, Silvia Tofani, Paolo Burghignoli, Paolo Baccarelli and Alessandro Galli

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78939

#### Abstract

Terahertz frequencies are increasingly gaining attention due to the recent efforts made in narrowing the technological gap among microwave and optical components. Still the demand of efficient THz antennas is high, due to the difficulty in obtaining directive patterns and good radiation efficiencies with planar, low-cost, easy-to-fabricate designs. In this regard, leaky-wave antennas have recently been investigated in the THz range, showing very interesting radiating features. Specifically, the combination of the leakywave antenna design with the use of metamaterials and metasurfaces seems to offer a promising platform for the development of future THz antenna technologies. In this Chapter, we focus on three different classes of leaky-wave antennas, based on either metasurfaces or tunable materials, namely graphene and nematic liquid crystals. While THz leaky-wave antennas based on homogenized metasurfaces are shown to be able to produce directive patterns with particularly good efficiencies, those based on graphene or nematic liquid crystals are shown to be able to dynamically reconfigure their radiating features. The latter property, although being extremely interesting, is obtained at the expense of an increase of costs and fabrication complexity, as it will emerge from the results of the presented study.

Keywords: terahertz, leaky-wave antennas, metasurfaces, graphene, liquid crystals

#### 1. Introduction

Metamaterials and metasurfaces [1] are, respectively, three-dimensional (3-D) and twodimensional (2-D) engineered man-made materials, which may exhibit electromagnetic properties commonly unaccessible with materials available in nature. Since the introduction of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Transformation Optics in 2006 by Pendry [2], metasurfaces have represented a privileged platform for achieving a considerable control of electromagnetic waves propagation. Electromagnetic cloaking and metasurfing, i.e., controlling surface or guided waves through tunable metasurfaces [3], are one of the most known applications of metasurfaces. However, in this Chapter, we focus on the application of metasurfaces for the realization of reconfigurable leakywave antennas (LWAs) [4].

2. Fabry-Perot cavity leaky-wave antennas (FPC-LWAs)

of FPC-LWA designs, especially in the microwave range.

Bragg reflector made of tunable nematic liquid crystals cells.

Fabry-Perot cavity leaky-wave antennas (FPC-LWAs) are partially open waveguiding structures, which support cylindrical leaky waves that radially propagate outward from the source [12, 23]. In this class of structures, radiation occurs through the excitation of the fundamental leaky modes supported by the structure, which are forward fast waves. Interestingly, when the excitation is a horizontal dipole (either electric or magnetic), the fundamental pair of TE, TM leaky modes is excited, and an FPC-LWA may produce a directive pencil beam at broadside or

Terahertz Leaky-Wave Antennas Based on Metasurfaces and Tunable Materials

http://dx.doi.org/10.5772/intechopen.78939

91

The architecture of an FPC-LWA relies on a grounded dielectric slab (GDS) covered with a partially reflecting screen (PRS) [25]. The gain enhancement phenomenon of these structures was originally explained through a ray-optics interpretation based on the Fabry-Perot concept [25] in the 1950s. However, the radiating features of these structures are more conveniently explained under the frame of the leaky-wave theory [12]. Indeed, due to the generality of the leaky-wave interpretation, the PRS can take various forms (e.g., a homogenized metasurface [24, 26] as in Figure 1(a), a denser dielectric superstrate [27, 28] as in Figure 1(b), and a distributed Bragg reflector [29] as in Figure 1(c)) provided that is suitably modeled with homogenized effective materials (when the PRS is bulky, e.g., a metamaterial) or surface impedances (when the PRS is planar, e.g., a metasurface). Regardless of the type of PRS, the leaky-wave approach yields to the same design process. This has allowed for the introduction of a variety

Figure 1. On the left, a typical FPC-LWA example fed by a horizontal magnetic dipole oriented along the y-axis. Depending on the operating point, such an antenna is able to produce either a pencil beam at broadside, or a conical beam. (a)–(f) Illustrative examples of Fabry-Perot cavity leaky-wave antennas (FPC-LWAs). (a)–(c) Conventional FPC-LWAs based on (a) a homogenized metasurface, (b) a high-permittivity dielectric cover layer (substrate-superstrate design), and (c) a distributed Bragg reflector (multiple layers dielectric structure). (d)–(f) Reconfigurable FPC-LWAs based on (a) a tunable graphene sheet, (b) a tunable graphene sheet in a substrate-superstrate configuration, and (c) a distributed

a conical beam with the cone axis along the vertical x-axis (see Figure 1) [12, 23, 24].

In the microwave range, there exists numerous realizations of metasurfaces (see, e.g., [4] and refs. therein), but at terahertz (THz) frequencies (nominally comprised between 300 GHz and 3 THz [5]), very few designs are available. Nowadays, THz technology is recognized as one of the most promising and challenging area of research for a twofold reason: (i) on the one hand, the wide and interdisciplinary character of THz applications, spanning from molecular spectroscopy and astrophysics, to high data rate communications and high-resolution imaging, passing through security screening and drug detection [6]; (ii) on the other hand, the increasing availability of efficient THz sensors and sources [7] that have recently contributed to considerably narrow the so-called THz gap.

Nevertheless, the demand of efficient THz antennas is still high [6]. Indeed, even though various solutions have been proposed (see, e.g., [8–11]), efficient realizations often require high fabrication costs and complexity [11]. To better handle the efficiency vs. cost/complexity tradeoff, leaky-wave antennas [12] have been proposed and experimentally demonstrated as valid alternatives for the design of efficient, low-cost, THz antennas [13, 14]. Such prototypes [13, 14] present many advantages with respect to previous THz antenna solutions, but their lens-like structure does not allow for a planar design, an attractive feature for micromachined packaging of THz systems [11]. In this regard, more recently, a specific class of LWAs based on Fabry-Perot cavities (FPCs), i.e., FPC-LWAs, has been proposed as low-profile, fully-planar, directive, and efficient THz antennas [15]. Even more interestingly, tunable materials such as graphene [16] and liquid crystals [17] have recently been employed for the realization of FPC-LWAs showing beam-steering capabilities at fixed frequency [18–22].

This Chapter is devoted to the analysis and design of novel THz FPC-LWAs. In Section 2, the general properties and the radiating features of FPC-LWAs are briefly reviewed. Conventional and more advanced designs are presented, with a specific focus on the technological constraints that typically affect the design of FPC-LWAs in the THz range. In Section 3, the design of THz FPC-LWA based on a homogenized metasurface is presented. The choice of a fishnetlike unit-cell is motivated by its remarkably low spatial dispersion and the capability of achieving a high reflectivity with a feasible variation of its structural parameters. The THz FPC-LWA considered in Section 3 produces a considerably high gain, but it does not allow for pattern reconfigurability. In Sections 4 and 5, two reconfigurable THz FPC-LWAs are therefore presented. In Section 4, the capabilities of two different THz FPC-LWAs based on graphene are shown and compared. The tunability of the graphene impedance through a DC voltage allows for a wide reconfigurability of the pattern. In Section 5, nematic liquid crystals are employed as tunable materials in a THz FPC-LWA to enhance the control of the beam angle through the application of a low-driving voltage. Finally, conclusions are drawn in Section 6.
