**1. Introduction**

With the commercial launch of the fifth generation (5G) network in 2020, research into the sixth generation (6G) communication system is eagerly on the agenda [1–3]. In order to meet the communication requirements of modern society in building smart cities, the 6G wireless network is envisioned to provide ultra-fast data rates (~1 Tbps), ultra-low latency (<1 ms), ubiquitous wireless connectivity, superior spectral and energy efficiency, as well as extremely high reliability and security [4–6]. The terahertz (THz) spectrum, ranging from 0.1 THz to 10 THz, is well suited for 6G applications due to its higher frequency, large bandwidth, and short response time [7–9]. However, the THz spectrum is highly absorbed by water molecules in the atmosphere, and its ultra-short wavelength is easily scattered by obstacles along the propagation path. This high path loss limits the THz wave to shortrange transmission. Energy-concentrated directive beam transmission is imperative for the THz network to compensate for propagation losses. In contrast with active phased-array antennas [10,11] applied on the transmitting and receiving end, reconfigurable intelligent surfaces (RISs) are proposed to modify the wireless transmission path [12–16]. RIS consists of periodic elements (meta-atoms or unit cells) altering the phase and magnitude of incident waves to have constructive/destructive interference in desired directions. Based on this, THz wireless communication is capable of being extended to non-line-of-sight (NLOS)

**Citation:** Yang, F.; Pitchappa, P.; Wang, N. Terahertz Reconfigurable Intelligent Surfaces (RISs) for 6G Communication Links. *Micromachines* **2022**, *13*, 285. https://doi.org/ 10.3390/mi13020285

Academic Editor: Dmitri V. Lioubtchenko

Received: 14 January 2022 Accepted: 4 February 2022 Published: 10 February 2022

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transmission (Figure 1a) [17]. Compared with existing relays using power amplifiers for wave receiving and retransmitting, RIS enhances the transmission by phase control using nearly passive elements, requiring less spectrum and energy [4,13]. Therefore, RISs can be embedded within infrastructure cost-effectively, enabling advanced wireless communication in 6G smart cities (Figure 1b) [18]. RIS is also known as reprogrammable intelligent surface [19,20] or intelligent reflecting surface (IRS) [21,22] in some contexts. The latter primarily denotes an intelligent surface operating in the reflective mode. A promising application of such a surface is deploying it on infrastructure to reflect waves in a desired direction [4].

**Figure 1.** RIS-assisted wireless communication. (**a**) RIS-enabled non-line-of-sight (NLOS) transmission. Reprinted from Ref. [17]. (**b**) RIS-embedded smart infrastructures for future 6G communications. Reprinted from Ref. [18].

RIS benefits have been realized as a result of the flourishing development of electromagnetic metasurfaces in the last decade. Metasurfaces are two-dimensional versions of metamaterials and have the advantages of low profile, reduced loss, and ease of fabrication [3,4]. Metasurfaces consist of artificially designed periodic structures, enabling unprecedented wave manipulation that is unattainable with natural materials [23]. In

2011, Yu et al. first proposed the generalized Snell's law, including abrupt phase shifts from the metasurface unit cells to realize nearly arbitrary wavefront shaping [24]. Since then, various fascinating wavefront manipulations have been demonstrated, such as reflectionless wide-angle refraction [25,26], sub-diffraction focusing with ultrathin planar lens [27,28], and wave-impedance matching across different material boundaries [29]. Such metasurfaces have only a static function once fabricated, whereas reconfigurable metasurfaces with tunable functions are gaining attention. The reconfigurable function is achieved by incorporating meta-atoms with tunable components or materials, which alter the electromagnetic response through external stimuli. With the development of digital control, coding metasurfaces were proposed to manipulate waves by changing the coding sequences of digital particles (unit cells) [30–36]. Active electronic components, such as positive-intrinsic-negative (PIN) diodes [30,37–39], have been widely applied in RISs working at 5G networks but are incompetent for applications in 6G systems due to the cutoff frequency limit. To overcome this challenge, reconfigurable technology approaches from both the electronic and optical sides are being explored intensively to fill the "THz gap" [40]. Beyond that, phase-change materials and microelectromechanical systems (MEMS) for THz RISs are also widely investigated.

Most existing review papers focused on summarizing various possible applications of RISs in wireless communication [41] or analyzing different tunable metasurfaces according to their tuning mechanisms [42,43]. Several review papers analyzed active metasurfaces for only one tuning element [44–47]. This paper emphasizes reconfigurable metasurfaces operating in the THz spectrum with the potential to assist 6G communications, i.e., metasurfaces with tunable functions such as pixel-level amplitude modulation or wide-range phase coverage for beam steering. Polarization modulation is briefly discussed as a possible future carrier-wave function. To underline the reconfigurability, RISs are categorized based on their tuning mechanisms, including complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high-electron mobility transistors (HEMTs), graphene, photoactive semiconductor materials, phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), and MEMS.

The rest of this paper is organized as follows: Section 2 presents various reconfigurable metasurfaces with pixel-level amplitude modulation or dynamic-beam-steering functions at THz frequency (0.1 THz–10 THz). Subsections are divided according to the tuning elements. Section 3 summarizes the properties of THz RISs and discusses possible future research directions.

#### **2. Terahertz Reconfigurable Intelligent Surface**

Various THz RISs with the potential to assist 6G communications are discussed in this section. The functionality of the reconfigurable metasurface focuses on amplitude modulation at the pixel-level or phase modulation for dynamic beam steering. The subsections are devoted to the tuning elements, investigating the reconfigurable mechanism for achieving RIS. The analysis begins with electronic approaches (CMOS transistors, Schottky diodes, HEMTs, and graphene), followed by optical approaches involving semiconductor materials, then phase change materials (vanadium dioxide, chalcogenides, and liquid crystals), and finally MEMS-based structural deformation.
