**5. Frequency tunable metamaterial absorbers**

is only around 35–40%, while the absorptivity of the metamaterial absorber reduces to be only around 20–40% when the water is emptied. These results confirm that the ultra-broadband absorption mainly contributes to localized resonances in the structured water resonators.

**Figure 5.** (a) Schematic view of the water metamaterial absorber, (b) layer by layer view of the unit cell, and (c) cut plane view of the water layer. (d) Absorptivity spectra of the water metamaterial absorber, the full water layer backed by a

metal ground, and the metamaterial without water. Reproduced from [61] with permission.

140 Metamaterials and Metasurfaces

Highly doped silicon has relatively low resistivity and behaves as a lossy dielectric at terahertz frequencies, which was employed for achieving broadband absorption [59]. Using a lossy patterned silicon substrate, Yin et al. [64] also experimentally demonstrated a metamaterial absorber with an operating band from 0.9 to 2.5 THz. A silicon-based metamaterial absorber, as shown in **Figure 6(a)**, was proposed for broadband high absorption at visible wavelengths [63]. Such a metamaterial absorber has three functional layers: a subwavelength silicon layer with periodic truncated conical holes, a subwavelength silicon dioxide spacer, and a thick gold substrate. As seen from the numerical results in **Figure 6(c)**, the silicon metamaterial absorber with truncated conical holes has higher absorptivity and wider bandwidth at the frequency band of interest.

**Figure 6.** (a) Schematic view of the silicon-based metamaterial absorber and (b) its unit cell. (c) Absorptivity spectra of

silicon-based metamaterials with conical and circular holes. Reproduced from [63] with permission.

Although metamaterials, in principle, can be designed for having arbitrary electromagnetic properties, these properties are generally fixed after the design of the metamaterials [65–68]. This is also true for metamaterial-based absorbers, whose operating frequencies are very much fixed, restricting their practical applications. Therefore, metamaterial absorbers with frequencytunable characteristics are highly desirable, which allows more fruitful applications. To enable the tunability in a metamaterial absorber, one may integrate a medium with adjustable material properties into a traditional passive metamaterial absorber. Some of the proven methods include having elements, such as varactor diodes [69], ferroelectrics [70], ferrites [71], graphene [72, 73], anisotropic liquid crystals [74], and phase-transition materials [75].

Mechanical bending or shifting was also studied for tunable metamaterial absorbers [74–78]. Zhang et al. [76] experimentally presented a mechanically stretchable metamaterial absorber, which is composed of dielectric resonators on a thin conductive rubber layer, as shown in **Figure 7**. A nearly 100% absorption was found, along with strong localized electric field confinement due to the Mie-type resonance of the dielectric resonators. When stretching the metamaterial absorber under uniaxial stress, the space between dielectric bricks increases gradually, and therefore the resonance frequency undergoes a red-shift of 410 MHz in the X band (see **Figure 7**). Zhu et al. [78] experimentally demonstrated a metamaterial absorber whose resonance frequency can be shifted by mechanical means. The shift was achieved by

**Figure 7.** (a) Experimental and (b) simulated absorptivity spectra of the mechanically stretchable dielectric metamaterial absorber. (c) Schematic of stretching the dielectric resonators on a thin conductive rubber layer. (d) Magnetic field distribution at the resonance frequency. Reproduced from [76] with permission.

be regarded as the time-reversed lasing at threshold. The perfect absorption can be achieved by utilizing the destructive interference in a standing wave system formed by two counterpropagating beams [84]. Moreover, the absorptivity in such a system can be modulated from nearly 0 to 100% by solely adjusting the phase difference between the two counter-propagating incident beams [85]. Owing to this dynamic configurability of absorptivity, such absorbers are very attractive for applications in transducers, modulators, and electromagnetic switches.

Electromagnetic Metamaterial Absorbers: From Narrowband to Broadband

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

143

The concept of CPA was first presented theoretically by Chong et al. [82] and experimentally demonstrated by the same group later [83]. Since then, CPA phenomena have been observed in epsilon-near-zero metamaterials [86], slow light waveguides [87], a metasurface consisting of metallic cross antennas [88], and a Fano resonance plasmonic system [84], just to name a few.

Most of the coherent metamaterial absorbers are based on metallic subwavelength resonators. However, recent researches revealed that CPA could also be achieved in metal-free metamaterials or metasurfaces. Zhu et al. [89] designed a mono-layer fishnet structure made of all dielectric ceramic, which has a thickness of two levels smaller than the operating wavelength. They demonstrated that CPA could be found in such a structure and the absorptivity is controllable within a wide range from 0.38 to 99.85% through phase modulation. A similar monolayer fishnet structure made of water could also be used for achieving high coherent absorption at multiple frequency bands [90]. Moreover, due to intrinsic high loss in water, the

Unlike perfect metamaterial absorbers that require strong electric and magnetic resonances resulting from the artificially structured resonators, few recent works reported that CPA can also be found in naturally existing layer materials with deep subwavelength thickness. Li et al. [89] presented that ultra-thin conductive films could be used for achieving CPA. As demonstrated experimentally, broadband coherent absorption with relative bandwidth close to 100% at microwave frequencies was observed in a conductive film, having a thickness of

1/1000 of the working wavelength. The CPA phenomena in thin graphene and MoS2

coherent absorber to be more flexible in working frequency, which could be controlled by

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.

were also investigated [60, 91]. The tunable conductivity in graphene or MoS2

layers

allows such a

CPA could be designed with wider bandwidths.

adjusting the chemical doping rate or bias voltage.

**7. Conclusions**

**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.

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.

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