**1. Introduction**

Metamaterials, also known as artificially structured materials, have attracted extensive attention in the last decade, owing to their exotic properties that are not readily available in nature [1–4]. The basic idea of a metamaterial is to design subwavelength unit cells, also known as meta-atoms or meta-molecules, having novel electric and/or magnetic responses to the incident electromagnetic waves [5–7]. This enables the availability of artificial mediums with arbitrary effective material parameters. The development of metamaterials results in a series of intriguing applications, such as a cloak of invisibility [8, 9], giant optical chirality [10, 11], wave-front control [12, 13], surface plasmon manipulations [14–16], as well as antennas of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

compact sizes and enhanced directionalities [17–19]. It is well known that one of the main obstacles toward practical engineering applications is the inevitable intrinsic loss in metamaterials. A significant amount of effort has been devoted to achieving low-loss devices through optimizing structural geometries [20–22]. Loss compensation using gain elements [23–25] is another scheme which requires external excitation sources.

ground plane. The electric permittivity and magnetic permeability of the metamaterial are ε =

are the free space permittivity and perme-

http://dx.doi.org/10.5772/intechopen.78581

135

, (1)

, (2)

is the effective refractive index

, (3)

\_\_\_\_\_ *μ*0 /*ε*0

or *μr* <sup>=</sup> *<sup>ε</sup><sup>r</sup>*

, so that these equations

being the impedance

, is a critical condition

*.* (4)

and *μ*<sup>0</sup>

the medium, which are unitless and normalized with respect to the values of free space. Due to the presence of the ground plane, no transmittance could be found on the other side of the

According to the Fresnel formula of reflection, the reflectivity (*R*) from the metamaterial

where the subscripts TE and TM refer to transverse electric (TE) and transverse magnetic

*<sup>Z</sup>* <sup>−</sup> *<sup>Z</sup>* \_\_\_\_<sup>0</sup> *Z* + *Z*0|

*<sup>μ</sup>*/*ε* being the impedance of the metamaterial and *Z*<sup>0</sup> <sup>=</sup> <sup>√</sup>

2

of free space. Since the metallic ground leads to zero transmissivity, the absorptivity arrives:

for achieving perfect absorption. It is worth noting that, to achieve impedance matching in a metamaterial absorber, simultaneous electric and magnetic resonances are required. For a metamaterial with single resonance, either electric or magnetic resonance, its impedance will be strong mismatched with that of free space. As a consequence, no perfect absorber would

A metamaterial absorber can be regarded as a coupled system and, particularly, its magnetic resonance is induced due to the anti-parallel currents between the front and back metallic layers. However, we may also independently consider the functionalities of the front meta-layer and the ground plane on the other side [47]. The front layer with certain metallic patterns functions as a partial reflection surface, which can be utilized to modify the complex reflection and

*| <sup>Z</sup> <sup>−</sup> <sup>Z</sup> \_\_\_\_*<sup>0</sup> *Z + Z*0*|*

= |√ \_\_\_ *μr* − √ \_\_ *<sup>ε</sup>* \_\_\_\_\_*<sup>r</sup>* √ \_\_\_ *μr* + √ \_\_ *εr*| 2

2

*=* 1 *− |√* \_\_\_ *μr − √* \_\_ *<sup>ε</sup> \_\_\_\_\_r √* \_\_\_ *μr + √* \_\_ *εr|* 2

metamaterial. This allows us to focus only on the reflection from the metamaterial.

(*ω*) are the frequency-dependent relative permittivity and permeability of

\_\_\_\_\_\_\_ *n*<sup>2</sup> − sin*θ* \_\_\_\_\_\_\_\_\_\_\_\_\_ *μr* cos*<sup>θ</sup>* <sup>+</sup> <sup>√</sup> \_\_\_\_\_\_\_ *n*<sup>2</sup> − sin*θ*|

\_\_\_\_\_\_\_ *n*<sup>2</sup> − sin*θ* \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>ε</sup><sup>r</sup>* cos*<sup>θ</sup>* <sup>+</sup> <sup>√</sup> \_\_\_\_\_\_\_ *n*<sup>2</sup> − sin*θ*|

2

Electromagnetic Metamaterial Absorbers: From Narrowband to Broadband

2

\_\_\_\_ *εr μr*

(*ω*), respectively. Here, *ε*<sup>0</sup>

*<sup>R</sup>*TE <sup>=</sup> <sup>|</sup>*r*TE|<sup>2</sup> <sup>=</sup> <sup>|</sup>*μr* cos*<sup>θ</sup>* <sup>−</sup> <sup>√</sup>

*<sup>R</sup>*TM <sup>=</sup> <sup>|</sup>*r*TM|<sup>2</sup> <sup>=</sup> <sup>|</sup>*ε<sup>r</sup>* cos*<sup>θ</sup>* <sup>−</sup> <sup>√</sup>

*<sup>R</sup>* <sup>=</sup> <sup>|</sup>

(TM) polarized waves, *θ* is the angle of incidence, and *<sup>n</sup>* <sup>=</sup> <sup>√</sup>

of the metamaterial. For the case of normal incident, we have *θ* = 0°

*A =* 1 *− R =* 1 *−*

The above equation indicates that impedance matching, *<sup>Z</sup>* <sup>=</sup> *<sup>Z</sup>*<sup>0</sup>

*ε*<sup>0</sup> *ε<sup>r</sup>*

ability. *ε<sup>r</sup>*

is [29]

reduce to:

with *<sup>Z</sup>* <sup>=</sup> <sup>√</sup>

be found.

**2.2. Interference theory**

\_\_\_\_

(*ω*) and *<sup>μ</sup>* <sup>=</sup> *<sup>μ</sup>*<sup>0</sup> *μr*

(*ω*) and *μr*

On the other hand, absorption is also highly desired in many applications, such as energy harvesting [26], scattering reduction [27], as well as thermal sensing [28]. By utilizing the full usefulness of loss, metamaterials with nearly uniform absorption are achievable through properly engineering the electric and magnetic resonances [29–35]. Due to the resonance nature, the first reported metamaterial-based perfect absorber is of narrow bandwidth and polarization sensitive, which restrict its usefulness in practical applications [29]. Great efforts have been devoted to expanding the bandwidth of metamaterial absorbers. Metamaterials with multiband absorption have later been developed using multiple resonant unit cells and combining them through a co-plane arrangement [36, 37]. When these resonances are closed to each other in frequency, broadband absorption is achievable [38]. Broadband absorption can also be achieved in metamaterial absorbers with multi-layer structures [39, 40] or using vertically standing nanowires [41, 42]. Moreover, by incorporating active mediums, the absorptivities and frequencies of metamaterial absorbers could be adjusted via external biases [43, 44].

In this chapter, we present a brief review on the fundamental theories and recent evolutions in the research field of electromagnetic metamaterial absorbers, whose operating frequencies cover from microwave, THz, infrared, to visible regimes. The rest of this chapter is organized as follows. Section 2 introduces the general theories on the design of metamaterial absorbers, where impedance matching theory and multiple interference theory are introduced. Section 3 reviews the narrowband metamaterial absorbers of various structures, all of which have nearly uniform absorptivities. Next, the technologies for broadening the bandwidths of metamaterial absorbers are presented in Section 4. Metamaterial absorbers with tunable absorption properties are reviewed in Section 5. Moreover, the coherent control of the metamaterial absorber's absorptivity through phase modulation is discussed in Section 6. Finally, the conclusion is given.
