**7. Conclusions**

adding an auxiliary dielectric slab parallel to the metamaterial absorber and varying the gap between the metamaterial and the slab. They also demonstrated the possibility of creating multiple absorption bands by smartly adjusting the size and shape of the dielectric slab.

**Figure 8.** (a) Cross-shaped unit cell with graphene wires and (b) schematic view of the metamaterial absorber. (c) Absorptivity of the metamaterial absorber under different bias voltages. Reproduced from [79] with permission.

Graphene has also been utilized for designing tunable metamaterial absorbers due to its tunability of surface conductivity [80, 81]. Zhang et al. [79] combined the metamaterial absorber having cross-shaped metallic unit cells with graphene wires, as shown in **Figures 8(a)** and **8(b)**. Such a structure was realized for polarization insensitive absorption and the absorption spectral could be tuned at terahertz frequencies. As shown in **Figure 8(c)**, they demonstrated that the absorption peak frequency is able to be tuned within a 15% frequency range with nearly uniform peak absorptivity, by simply controlling the Fermi level of graphene. The Fermi level in graphene can

It is known that one of drawbacks of a typical metamaterial absorber is that the absorptivity is commonly fixed after the initial design of the metamaterial absorber. As a consequence, it is not suitable for environments that require the flexible tunability of absorption. The presence of coherent perfect absorption (CPA) is a solution for this issue [82, 83]. Mathematically, CPA corresponds to a zero eigenvalue of the scattering matrix S at a specific frequency, which can

be conveniently controlled by adjusting the bias voltage on the graphene layers.

**6. Coherent metamaterial absorbers**

142 Metamaterials and Metasurfaces

Since the first design in 2008, metamaterial perfect absorbers with deeply subwavelength profiles have received significant attention in the last decade. In this chapter, we have presented a comprehensive review of the recent progress on the theories and designs of planar metamaterial absorbers. We reviewed the fundamental theories and design guidelines for achieving perfect absorption in subwavelength metamaterials. Different structures of unit cells have been studied for achieving nearly complete absorptions. The realizations of broadband and frequency-tunable metamaterial absorbers were also discussed. Moreover, we introduced the concept of coherent perfect absorbers and the coherent control of absorptivity via phase modulation in such metamaterial absorbers. A significant number of works reviewed in this chapter were done in our research group.
