Terahertz Reconfigurable Intelligent Surfaces (RISs) for 6G Communication Links

Abstract

The forthcoming sixth generation (6G) communication network is envisioned to provide ultra-fast data transmission and ubiquitous wireless connectivity. The terahertz (THz) spectrum, with higher frequency and wider bandwidth, offers great potential for 6G wireless technologies. However, the THz links suffers from high loss and line-of-sight connectivity. To overcome these challenges, a cost-effective method to dynamically optimize the transmission path using reconfigurable intelligent surfaces (RISs) is widely proposed. RIS is constructed by embedding active elements into passive metasurfaces, which is an artificially designed periodic structure. However, the active elements (e.g., PIN diodes) used for 5G RIS are impractical for 6G RIS due to the cutoff frequency limitation and higher loss at THz frequencies. As such, various tuning elements have been explored to fill this THz gap between radio waves and infrared light. The focus of this review is on THz RISs with the potential to assist 6G communication functionalities including pixel-level amplitude modulation and dynamic beam manipulation. By reviewing a wide range of tuning mechanisms, including electronic approaches (complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high electron mobility transistors (HEMTs), and graphene), optical approaches (photoactive semiconductor materials), phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), as well as microelectromechanical systems (MEMS), this review summarizes recent developments in THz RISs in support of 6G communication links and discusses future research directions in this field.

Keywords: 6G communication; reconfigurable intelligent surface (RIS); reconfigurable metasurface; terahertz (THz).

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].

Figure 2

CMOS transistor-enabled reconfigurable metasurface. (a–d) GHz-speed programmable metasurfaces using CMOS-based chip tiles. (a) A single silicon chip tile consists of a 12 × 12 array (left). The enlarged portion (right) shows the unit-cell structure with active NMOS transistors embedded in the gap of inductive microloops. Each unit cell has an eight-bit control, enabling 256 states for amplitude and phase control. (b) Photo of the fabricated 2 × 2 tiled chips, which were wire-bonded to a customized printed circuit board for external voltage control. (c) Amplitude modulation was experimentally demonstrated as a holographic projection of the letter ‘P’. (d) Beam steering at ±30° with the corresponding three different phase profiles and meta-element digital settings. Reprinted from Ref. [52]. (e–g) Reconfigurable metasurface based on CMOS structures. (e) A bias voltage is applied for transmitted amplitudes and phase modulation. The reconfigurable metamaterial can be divided into subsections for greater functionality. (f) Cross section of the unit cell, consisting of six layers for the CMOS transistor configuration. (g) Layout with wire connection for the biasing control. Reprinted from Ref. [53].

Figure 3

Schottky-diode-structure-enabled reconfiguration. (a,b) A 4 × 4 pixel amplitude modulator. (a) Schematic of the cross section of the unit cell incorporating SRR with Schottky gate structure (top). The gray scale of the depletion region indicates the free charge-carrier density. A single pixel on the THz SLM for amplitude modulation (bottom). (b) THz SLM consisting of $4\times 4$ pixels. Reprinted from Ref. [55]. (c,d) An 8 × 8 four-color spatial light modulator. (c) Schematic of the metamaterial absorber with a flip-chip-bonded, n-doped GaAs epitaxial layer. (d) An example of the spatial light modulator with different frequencies for each pixel. Reprinted from Ref. [56]. (e) A diffractive modulator with grating configuration realizing 22 dB amplitude modulation at $36.1°$. Reprinted from Ref. [57]. (f–h) A phase-modulated deflector. (f) An array consisting of eight unit cells realized $2\mathsf{\pi }$ phase control with nearly the same transmission efficiency. (g) Microscopic image of the fabricated metasurface. (h) An illustration of the deflected wave transmission. Reprinted from Ref. [58].

Figure 4

High-electron-mobility-transistor (HEMT)-enabled reconfigurable metasurface. (a,b) An HEMT-embedded metamaterial modulator with a speed of 10 MHz. (a) A simulation unit-cell model with HEMT beneath each split gap. The cross-sectional view is shown on the right. Reprinted from Ref. [61]. (b) A 2 × 2 spatial light modulator with a modulation depth of 33% at 0.46 THz. Reprinted from Ref. [62]. (c–e) Metasurface with 1 GHz modulation speed combining a dipolar array with a double-channel heterostructure. (c) Image of the fabricated metasurface. (d) Depth modulation of 85%. (e) Phase modulation of $68°$. Reprinted from Ref. [66]. (f,g) Large phase modulator with HEMT embedded and an enhanced-resonance metasurface. (f) Schematic of the unit-cell structure. (g) Phase modulation of more than $130°$. Reprinted from Ref. [67].

Figure 5

Graphene-based reconfigurable metasurface. (a,b) A 4 × 4 reflection modulator. (a) A schematic of the graphene-${\mathrm{SiO}}_2$-Si structures. The substrate has an optical thickness of an odd quarter wavelength. (b) Optical image of the graphene-enabled reflection modulator. Reprinted from Ref. [71]. (c,d) A 256-pixel spatial light modulator. (c) Photo of the modulator (left). The enlargement (right) shows the graphene−electrolyte–graphene unit-cell structure. (d) A THz transmission image at 0.1 THz with two rows and columns biased at +1.0 and −1.0 V, respectively. Reprinted from Ref. [72]. (e) Unit cell consisting of a square graphene patch for a tunable reflective metasurface at 1.3 THz. The cell has dimensions of a = b = 14 µm, ${\mathrm{a}}_{\mathrm{p}}$ =${\mathrm{b}}_{\mathrm{p}}$ =$10 \mathsf{\mu }\mathrm{m}$. Reproduced with permission from [68]. (f) Beam steering with graphene patterned in SRRs. Reprinted from Ref. [73]. (g) Digital metasurface using a graphene–insulator–graphene stack for beam steering. Reprinted from Ref. [81]. (h) Column-level controlled beam steering with graphene embedded with SRR structures. Reprinted from Ref. [82]. (i–j) Experimentally demonstrated beam-steering metasurface with graphene embedded with a bowtie structure. (i) Cross section of the experimentally demonstrated beam-steering metasurface (left) and its unit-cell structure (right). (j) Schematic of the metasurface with the individually biased column. Reprinted from Ref. [83].

Figure 6

Semiconductor materials for temporal modulation. (a,b) Optical active polarization switching and dynamic beam splitting. (a) An illustration of the hybrid circular split-ring resonator (h-SRR) pumped by near-infrared femtosecond pulses. (b) An active polarizing beam splitter. Reprinted from Ref. [84]. (c,d) Spatiotemporal dielectric metasurfaces for beam steering. (c) Ultrafast femtosecond laser pulses (@ 800 nm, 100 fs) pump high-resistivity silicon on a quartz substrate, providing transient photocarriers for temporal modulation. (d) Temporal beam steering of $34.7°$ at 0.586 THz. Reprinted from Ref. [85].

Figure 7

Phase-change materials of a vanadium-dioxide (${\mathrm{VO}}_2$)-enabled reconfigurable metasurface. (a–c) A thermally controlled reconfigurable metasurface with broadband absorption-to-reflection conversion. Reprinted from Ref. [90]. (a) Schematic of the ${\mathrm{VO}}_2$ integrated metasurface. (b) Unit-cell structure (top) and a unit-cell array for 2𝜋 phase control (bottom). (c) Broadband reflection when ${\mathrm{VO}}_2$ is in its fully metallic state. (d–f) An electronically controlled beam-steering metasurface operates at 0.1 THz. Reprinted from Ref. [89]. (d) Top-view scanning electron microscopic image of the metasurface (top left), unit-cell structure (bottom left), and beam steering (right). (e) Horizontal beam steering at $-22°$, $-14°$, $0°$, $12°$, and $22°$. (f) Vertical beam steering at $-14°$, $0°$, and $12°$.

Figure 8

Chalcogenide phase-change materials enabled a reconfigurable metasurface. (a–c) Germanium–antimony–tellurium (GST) incorporated with Fano-resonance mate atoms for multicolor spatial light modulation. (a) A 2 × 2 array for four-color spatial light modulation. (b) Schematic of current biasing. (c) Multilevel Fano-resonance modulation (FRM) results from different input currents (stimulus period $\approx 15 \mathrm{s}$). Reprinted from Ref. [98]. (d–g) Phase-change GeTe material applied for a multifunctional coding metasurface. (d) Illustration of the coding metasurface. (e) GeTe- and gold-integrated unit cell with amorphous (insulating) state of GeTe and crystalline (conductive) state. (f) Reflected phase of $180°$ at 0.3 THz for the coding element at two different states. (g) Multifunctionality (beam tilting, directing, and splitting) is realized through different coding masks. Reprinted from Ref. [101].

Figure 9

Liquid-crystal-enabled reconfigurable metasurface. (a–c) A spatial light modulator based on liquid crystals. (a) Schematic of metamaterial absorbers covered with a layer of liquid crystals. (b) Spatial light modulator device and an enlargement for the meta-atom dimensions. (c) A 6 × 6 pixelated absorption map measured at 3.725 THz. Reprinted from Ref. [102]. (d–g) A spatial phase modulator operating at 0.8 THz. (d) Schematic of the metasurface. (e) An optical microscopic image of the fabricated metasurface. (f) Phase difference as a function of liquid-crystal tilt angle. (g) Calculated beam deflection. Reprinted from Ref. [104]. (h,i) Programmable metasurface for beam steering. (h) Schematic of the beam steering metasurface with the control element (top). The unit cell consists of a liquid-crystal layer embedded between two metallic layers. Schematic of the metasurface with the applied coding sequence of /01.../(bottom). (i) Reflected angles for five different coding sequences with an incident angle of $20°$ at 0.672 THz. Reprinted from Ref. [105]. (j,k) Liquid crystal-based multifunctional transmissive coding metasurface. (j) Schematic of the functional metasurface (top) and the asymmetric unit-cell design (bottom). (k) Measured transmitted pattern for different coding sequences. Reprinted from Ref. [106].

Figure 10

MEMS-enabled reconfigurable metasurface. (a–c) Micromirror array for the wideband spatial light modulator. (a) Schematic of a single-pixel in OFF state (top) and ON-state (bottom) for a bias voltage of 0 V and 37 V, respectively. (b) SEM image of the inclined mirrors. (c) Model of a 2 × 2-pixel SLM with two ON pixels (highlighted by black frames) along one diagonal (left) and its corresponding measured-intensity distribution (right) at 1.38 THz. Reprinted from Ref. [108]. (d–f) MEMS-based metal–insulator–metal metadevices for beam steering. (d) Images of the fabricated metasurface in “ON” and “OFF” states. (e) Simulated phase response as a function of the cantilever angles. (f) Simulated dynamic beam steering with six-digit control. Reprinted from Ref. [109]. (g–i) Reconfigurable MEMS Fano metasurfaces for logic operations in cryptographic wireless communication networks. (g) Unit-cell model of the metasurface. (h) Measured far-field transmission spectra showing the exclusive-OR (XOR) logic feature for various voltage states of the SRRs at 0.56 THz. (i) Implementation of the XOR logic for OTP-secured wireless communication channels. Reprinted from Ref. [110].