*2.4. Micro-Electromechanical-System (MEMS)*

In contrast to other tuning mechanisms that alter the properties of materials, microelectromechanical-system (MEMS) metasurfaces directly change the structural geometry of the unit cell, transforming the electromagnetic wave responses. Moreover, advanced and developed MEMS manufacturing makes it attractive for reconfigurable THz devices [107]. The simplest microstructure cantilever has been extensively studied and was embedded in a metasurface for active tuning [44]. A wideband spatial light modulator was built using an array of 768 actuatable mirrors, with a length of 220 μm and a width of 100 μm (Figure 10a) [108]. These dimensions were selected to reduce diffraction from individual mirrors and to increase the pixel-to-pixel modulation contrast of the spatial light modulator. A cantilever consisting of chrome –copper–chrome multilayers had intrinsic residual stress forcing it to tilt up with an angle of 35◦ (Figure 10b), which minimized back-diffracted waves in the incident direction. The mirrors were pulled down to the substrate by applying a bias voltage of 37 V. The SLM was built with micromirror arrays based on the grating concept to have a wide operational spectral range. The authors designed a spatial light modulator with 4 × 6 independently switchable pixels. Each pixel consisted of 4 × 8 micromirrors with a pixel size of 1 mm × 2 mm (Figure 10c). The modulation contrast was higher than 50% over the frequency range from 0.97 THz to 2.28 THz, with a peak modulation contrast of 87% at 1.38 THz. This method allows for almost arbitrary spatial pixel sizes by collectively switching the corresponding group of mirrors.

Beam steering was demonstrated by a cantilever designed for electrical LC resonance (Figure 10d) [109]. By controlling the suspension angle (from 2◦ to 0◦) of the bimorph cantilever through biasing voltage (from 0 V to 30 V), a phase coverage of 310◦ was achieved at 0.8 THz (Figure 10e). Steering angles of ±70◦ and ±39◦ were demonstrated through simulation by grouping twelve columns as a super cell and controlling each column individually (Figure 10f). Each unit cell could potentially be addressed separately to have unit-cell-level control to enable a programmable MEMS metasurface. A MEMSbased metasurface for wireless security encoding was demonstrated using square double SRRs, forming a Fano resonator (Figure 10g) [110]. Independent control of the two SRRs provided four transmission states for an exclusive-OR (XOR) logic operation (Figure 10h). An application performing the XOR logic operation for one-time-pad (OTP) security in wireless transmission is illustrated in Figure 10i. A private message (m) was encoded with a secret key (k) using the XOR logic operation before sending it out for public-channel transmission, and the original message was decrypted at the destination through the inverse XOR operation. This fast encryption method could be extended to other wireless communication networks requiring high security.

**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].

## **3. Discussion**

THz RISs can dynamically modify the wave propagation direction, thereby enhancing wireless network efficiency, are crucial for actualization of 6G communication links. Reconfigurable metasurfaces with the function of pixel-level amplitude modulation and tunable beam steering are summarized in Tables 1 and 2, respectively, according to tuning elements. Electrical control allows for more precise, spatially selective pixel-level modulation but requires additional wires and complex control circuits. Optical control usually modulates the whole surface with a laser pump, but localization may be possible through an additional coding mask [101]. Tuning elements can be combined to add additional reconfigurable freedom [111,112]. Other reconfigurable methods, such as mechanical [113] and microfluidbased [114] tuning, are also feasible. RISs realized through industry-standard fabrication processes have greater potential for large-scale construction, which is required for future 6G smart cities. Moreover, emerging 2D van der Waals materials using surface-plasmon polaritons provide new approaches for dynamic tunning in the THz spectrum [115–120]. Graphene plasmons demonstrate lower loss than conventional metal-based plasmonic metasurfaces [121,122]. Recently discovered phonon polaritons in van der Waals crystals exhibit remarkably low losses [123–125]. These phonon polaritons can be tunned through chemical intercalation [126–128], twisted stacking [129], and heterostructures [130–133].

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**Table1.**Pixel-levelamplitude-modulationperformancecomparisonofvarioustuningelementsforTHzRIS.

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Since the emergence of multiple-input, multiple-output (MIMO) technology for wireless communication, polarization modulation (PoM) has gained more and more attention thanks to its excellent signal distinguishability, thereby increasing spectrum efficiency [134,135]. Metasurfaces with active elements for polarization modulation at THz frequencies were investigated in [136–139] and were found to have great potential to be utilized for future 6G wireless communications. Another promising area for future THz RIS applications is space-time-coding digital metasurfaces with have multifrequency control for beam shaping and steering [140]. Cell-free massive MIMO for mobile access is expected in 6G based on massive MIMO in 5G. Hybrid RISs with MIMO constitute an important future research direction.

**Author Contributions:** Writing—original draft preparation, F.Y.; writing—review and editing, P.P. and N.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Science and Engineering Research Council of A\*STAR (Agency for Science, Technology and Research), Singapore, under Grant no (A18A5b0056) and the project titled "Terahertz Nanogap Metasurface for enabling 6G communications".

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