2.1.4. Graphene

Graphene is a two-dimensional (2D) material of honeycomb structures formed by single-layer carbon atoms arranged in hexagonal lattices. Graphene has complex conductivity that supports the propagation of plasmonic modes at THz frequencies [68]. The surface conductivity can be efficiently controlled through a perpendicular-bias electric field that induces charge carriers to shift the graphene chemical potential (Fermi level) away from the Dirac point. Compared with conventional semiconductors, graphene has the attractive advantages of high electron mobility, considerable thermal conductivity, and strong mechanical ductility [69]. Hence, graphene has considerable potential to be applied in the THz frequency for dynamic wave control.

A graphene-based electroabsorption modulator was demonstrated by placing atomically thin graphene layer on top of a dielectric substrate with a reflective metal back gate (Figure 5a) [70,71]. The substrate thickness was designed to be an odd multiple of a quarter-wavelength of the incident wave to enhance the modulation depth. With a biasing voltage of −10 V, the Fermi level in graphene was tuned at the Dirac point, realizing the maximal reflectance. The carrier concentration increased with increased voltage, resulting in enhanced absorption. The graphene layer was patterned using O2 plasma to demonstrate a 4 × 4 pixelated reflectance modulator (Figure 5b) [71]. An alternative approach using electrolyte gating, producing higher charge densities to build a spatial phase modulator, was presented in [72] (Figure 5c). Large electric fields were generated at the graphene–electrolyte interface, giving rise to charge accumulation over a large area without an electrical short circuit and removing the thickness control. In order to build the 16 × 16 pixelated modulator, graphene was grown by chemical-vapor deposition (CVD) on a large area. Selective control of a single pixel was achieved by voltage biasing through the corresponding column and row (Figure 5d).

A reflect array combined with graphene for dynamic beam steering was first proposed using a square graphene patch (Figure 5e) [68]. Due to the slow wave propagation in the plasmonic mode of graphene, a patch was designed with much smaller dimensions (*λ*/10, compared with conventional conductors of *λ*/2) for a resonance response. This reduced interelement spacing allows for more efficient wavefront manipulation. By tuning the chemical potential from 0 to 0.52 eV, a phase coverage of 300◦ was obtained at 1.3 THz with a fixed patch dimension. To have full 360◦ phase coverage for dynamic-gradient phase control, graphene was designed with resonant structures (Figure 5f) [73,74] or combined with resonant metallic patterns [75,76]. A graphene-based coding metasurface for beam splitting was proposed by controlling the Fermi level (Figure 5g) [77–81]. Such coding metasurfaces also have great potential to be applied to security systems for message transmission [81]. Spatially selective column-level tuning was presented with graphene embedded in SRR, resulting in four deflection angles (5◦, 11◦, 17◦, and 23◦) at 1.05 THz (Figure 5h) [82]. Such graphene-based beam steering metasurfaces had only been tested in simulations. The first experimentally demonstrated design was illustrated in [83] (Figure 5i). The graphene strip was placed at the gap of a bowtie structure for field concentration. Each column was individually controlled, resulting in a maximum beam steering of ±25◦ at 1 THz (Figure 5j).

**Figure 5.** Graphene-based reconfigurable metasurface. (**a**,**b**)A4 × 4 reflection modulator. (**a**) A schematic of the graphene-SiO2-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, ap = bp = 10 μm. 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**) Columnlevel 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].
