**4. Future trends: reconfigurable absorbers based on plasmonic, graphene, and beyond**

Electromagnetic wave absorbers are mainly utilized as boundary structures to prevent the scattering of electromagnetic fields. They can be divided into two main categories based on their operating bandwidth, i.e., resonant and broadband

absorbers [95, 96]. Resonant absorbers typically depend on the designed material assemblies interrelating with the incident waves around certain resonance frequencies [96]. On the other hand, the wideband absorbers rely on material damping characteristics that are largely independent of electromagnetic frequencies and typically made of lossy dispersive materials [96]. Such broadband absorbing materials along with structural transition are commonly used in anechoic chambers to effectively emulate a non-reflecting unbounded medium suitable for testing radiating antenna elements [95]. Lately, there is an emphasis in the scientific community to design efficient plasmonic metamaterial-based absorbers. Light matter interaction in subwavelength metamaterial structures allowed various other applications including perfect lenses [45, 97], chiral surfaces [98, 99], transformational surfaces [100, 101], optical cloaking [100, 102, 103], spatial light switching [104, 105] and IR camouflage and microwave antennas because of certain useful characteristics [96]. In addition to these applications, the perfect metamaterial absorber (PMA) is designed as a tool to efficiently absorb electromagnetic waves utilizing plasmonic resonator elements embedded within its assembly.

Generally, metamaterial absorbers are composed of a patterned metal film over a continuous thin metallic film with a dielectric substrate sandwiched between them [96]. It should be noted that the definition of total absorptivity considers reflected electric field vector, i.e., **R** ! <sup>¼</sup> *Riia*^*<sup>i</sup>* <sup>þ</sup> *Rjia*^ *<sup>j</sup>*, that considers both co- (*Rii* <sup>¼</sup> *Eref <sup>i</sup> =Einc <sup>i</sup>* ) and cross (*Rji* <sup>¼</sup> *Eref <sup>j</sup> =Einc <sup>i</sup>* ) polarized components of reflected fields [21]. Here, subscript f g *i*, *j* ¼ f g *x*, *y* refer to x or y coordinates. Oftentimes, the plasmonic metasurface absorbers inhibits only co- polarized reflection (i.e., *Rii* ¼ 0) and converts incident fields to cross polarized reflected fields components (*Rji*) [21, 106], as shown in **Figure 12**. Such metasurface is designed with certain structural anisotropy that allows the flow of electric currents that suppresses co- polarization states (*Rii*) for the reflected fields. As a result, such plasmonic metasurface converts the polarization of incident electromagnetic fields to orthogonal reflected field component (*Rji*). Special cases of circular or elliptical polarized reflected fields may also be attained by specific vector combination of co- (*Rii*) and cross (*Rji*) polarized components [107, 108]. In the case of a full absorber as shown in **Figure 12(a)**, the normalized perfect absorption *A*ð Þ *ω* due to the plasmonic metasurface can be found from the amplitude of reflected fields i.e., *<sup>A</sup>*ð Þ¼ *<sup>ω</sup>* <sup>1</sup> � j j *<sup>R</sup>*ð Þ *<sup>ω</sup>* <sup>2</sup> � j j *<sup>T</sup>*ð Þ *<sup>ω</sup>* <sup>2</sup> . Here, ∣*R*ð Þ *ω* ∣ and |*T*ð Þ *ω* ∣ are amplitudes of reflected and transmitted fields, respectively. The ideal back reflector typically supports negligible transmission i.e., ∣*T*ð Þ *ω* ∣ ¼ 0.

#### **Figure 12.**

*(Left) full absorber to inhibit any scattering from the metasurface. (Right) cross polarizer to suppress the co-polarized component R*ð Þ *xx* <sup>¼</sup> <sup>0</sup> *while not affecting cross-polarized reflection component Ryx .*

#### *Artificial Surfaces and Media for Electromagnetic Absorption and Interference Shielding DOI: http://dx.doi.org/10.5772/intechopen.99338*

The response of PMA is often described as an effective medium and characterized by properties such as complex electric permittivity (ε) and magnetic permeability (μ) [109]. Although much of the work on effective medium properties of metamaterials has been focused on the real part of ε and μ, as it contributes to negative refractive index properties, it is equally important to reduce losses represented by the imaginary part of ε and μ for practical applications related to wave propagation within negative refractive index medium [110, 111]. In contrast, the metamaterial absorbers rely on high material losses (large value of imaginary parts of ε and μ) and impedance matching with the background medium. Landy et al. provided the first experimental demonstration of near perfect metasurface absorption at GHz frequencies. The working principle of PMA relies on impedance matching between effective medium forming metasurface with dielectric and background medium to the free space background, rejecting the reflection and therefore efficiently absorbing the incident EM wave [19, 20]. Ever since, the design of PMAs has attracted significant attention ranging from microwave to optical frequencies [112, 113]. Apart from resonant absorption characteristics, the plasmonic effects also provide tremendous near field enhancement to improve the efficiency of solar cells [114], support for sensing [31], and enhanced thermal emission and photo-detection [115].

Recently, the electro-optic tunability of resonant absorption spectrum at THz frequencies is made possible by varying the frequency dependent conductivity of plasmonic materials through the use of graphene metasurface [22, 59]. The frequency dependent optical properties of graphene in the THz frequency range are controlled by its optoelectronic properties. The permittivity (*ε*g) of graphene is function of the surface conductivity (*σ*g).

$$
\epsilon\_{\mathfrak{g}}(\boldsymbol{\alpha}) = \mathbf{1} + j \frac{\sigma\_{\mathfrak{g}}(\boldsymbol{\alpha})}{\varepsilon\_{0} \boldsymbol{\alpha} \boldsymbol{\Delta}}.\tag{40}
$$

Here, *ε*<sup>0</sup> is the vacuum permittivity, and Δ = 1 nm is the monoatomic thickness of graphene.

The surface conductivity of graphene (*σ*g) can be deduced from the Kubo formula, at low THz frequencies (inter-band conductivity can be neglected in this regime),

$$\sigma\_{\rm g}(\omega) = j \frac{e\_0^2 k\_B T}{\pi \hbar (\hbar \omega + j \Gamma)} \left\{ \frac{\mu\_c}{k\_B T} + 2 \log \left[ 1 + e^{-\mu\_c/(k\_B T)} \right] \right\}. \tag{41}$$

Here, *<sup>μ</sup>c*[eV] is the chemical potential of graphene, <sup>Γ</sup> ¼ � *<sup>e</sup>*0ℏ*v*<sup>2</sup> *f* � �*<sup>=</sup> μμ<sup>c</sup>* ð Þ is the damping coefficient, *vf* [m/s] is the Fermi velocity, and *μ* [cm<sup>2</sup> /Vs] is the electron mobility, *e*<sup>0</sup> is the electronic unit-charge,*T* is temperature, *k*<sup>B</sup> is the Boltzmann constant, and ℏ is the reduced Plank's constant. Therefore, the surface conductivity *σ*<sup>g</sup> and permittivity of graphene *ε*<sup>g</sup> can be directly controlled by varying *μc*. The chemical potential can be controlled by doping graphene or by applying an electrostatic bias to the graphene sheet.

The real-time tunability of Graphene Surface Plasmons (GSPs) is a distinct feature offered by graphene metasurface when compared to traditional noble metals. In addition, GSPs offer other benefits such as tighter mode volume confinement and lower intrinsic losses compared to conventional surface plasmon materials. The unique electro-optic tunability of graphene conductivity within the THz band makes it an attractive candidate for plasmonic metamaterial applications. Therefore, graphene enabled the research in surface plasmons to be redirected

toward reconfigurable THz wave optics applications, including GSPP waveguides [116], modulators [24], THz cloaks [117], THz antennas [118], Fourier optics [119], photonic crystal nano-cavities [120], and biochemical sensors [31, 121]. The reconfigurable response of resonant absorption can offer additional functionalities, including wave modulation, polarization conversion, and sensing.

Here, we discuss a few selected applications of reconfigurable THz metasurface absorbers, as shown in **Figure 13**. **Figure 13(a)** shows graphene micro-ribbon metasurface design capable of efficiently absorbing THz radiation [22]. The chemical potential (*μc*) can be used to control graphene conductivity. As a result, the reconfigurable response with near perfect absorption can be utilized for THz wave modulation applications. **Figure 13(b)** shows a design of multilayer graphene

#### **Figure 13.**

*(a) (Top) Schematic illustration of graphene micro-ribbon metasurface. (Bottom) Tunable absorption characteristics due to variation in graphene chemical potential (μc) [22]. (b) (Top) Multilayer graphene metasurface biased at different levels to efficiently absorb incident radiation. (Bottom) Wideband absorption spectrum for THz waves for the normal incident condition [59]. (c) Description of polarization state modulation of a digital M-level signal by chiral graphene metasurface. The 4-level digital stream is fed to a limiter that converts it into the required chemical potential and consequently produces desired polarization state for the reflected field [98]. (d) The chiral biosensor is constructed by graphene metasurface supporting chiral reflection characteristics to test ligand-antigen bindings on the surface. The resonance frequency supports a distinct contour path traversed by locus tip of electric field vector in time for three different strains of influenza viruses H1N1, H5N2, and H9N2 [31].*

*Artificial Surfaces and Media for Electromagnetic Absorption and Interference Shielding DOI: http://dx.doi.org/10.5772/intechopen.99338*

metasurface that operates at THz frequencies [59]. The graphene layers are designed to generate quadrupolar localized surface plasmons that destructively interfere with the dipole mode. The patterned graphene layers are biased to operate at different chemical potential levels and backed up with dielectric substrates stacked on top of each other. Full-wave electromagnetic simulations demonstrate that the absorption spectrum is not only tunable but can be optimized to a large bandwidth of operation, *i.e.*, 6.9 THz bandwidth is obtained for over 90% normalized absorption. **Figure 13(c)** shows a schematic illustration of the design of graphene metasurface supporting polarization state modulation with high spectral efficiency [98]. The structural chirality of metasurface is utilized to generate chiral reflection along with highly dispersive Fano resonance. Several polarization states, including two orthogonal linearly polarized, right-and left-handed circular polarized reflections, are demonstrated for a narrow electro-optic tuning range of chemical potentials between 500 and 700 meV. By exploiting these properties, highly efficient modulation stages of modern communication systems can be designed. **Figure 13(d)** shows a polarization-state sensing setup to distinguish closely resembling optical properties of biomolecules such as viruses [31]. The measurement consists of a plasmonic metasurface with chiral unit cells, and the polarization properties of reflected fields can determine the optical characteristics of the analyte. It is shown that the proposed sensor can distinguish three closely resembling influenza virus strains i.e., H1N1, H5N2, and H9N2 based on the variation of the reflected polarization states.
