**6. Metamaterials in Energy Harvesting**

Metamaterials play a vital role in energy harvesting. As we know that acoustic energy is inexhaustible and present in the ambient environment, together with common voice, rustle of falling leaves, sound near jet plane, operating sound of large grinding machine and so on [226,227]. However, if the energy density is low, most of the acoustic energy

is dissipated into thermal energy during its propagation. In recent years, the researchers are focused on energy harvesting technology to convert energy into electrical energy to power up low power electronic devices [228,229]. To scavenge and confine the acoustic energy distinct energy harvesters have been proposed [230]. The advancement of acoustic energy harvesters has begun from classic Helmholtz resonators [231], photonic crystal resonators [232], and quarter-wave resonators [233] to local acoustic metamaterials resonators [234] for large energy focusing on short structural dimensions. Wireless energy harvesting (WEH) from electromagnetic fields is flattering an emerging technology. Recent advances in broadband rectennas for wireless power transfer (WPT) and ambient RF energy harvesting was studied by C. Song [235]. The review of wireless and battery-free platforms for collection of bio signals, biosensors and bioelectronics was done by Tucker Stuart et al. [236]. They summarize present methods to realize such device architectures and deliberates their building blocks. On-site and external energy harvesting in underground wireless was studied by Raza, U [237]. These energy harvesting methods lead to design of a competent wireless underground communication system to power underground nodes for extended field operation. Additionally, key energy harvesting tools are offered that use available energy sources in the fields like vibration, solar, and wind. In this concern, the Electromagnetic (EM)- and Magnetic Induction (MI)-based approaches are important for underground wireless communication system. Recently, WPT and energy harvesting: current status and future prospects was reviewed by J. Huang et al. [238]. A meta-material based on a cubic high-dielectric resonator (CHDR) for coupled WPT (wireless power transfer) system was studied by R. Das et al. [239], they have observed that the proposed CHDR system providing more than 90% power transfer efficiency at a distance of 0.1λ. Amongst, the photonic crystals are suitable harvesters for high frequency acoustic environment, because the Bragg scattering controls the scattering characteristics of the waves in photonic crystals and the wavelength of the harvested sound is of the order of magnitude (periodic parameters) of the scatterer.

The acoustic metamaterials alter the propagation of the wave depends on the local resonance bandgap principle. In which at a specific frequency, the propagation of the incident wave is suppressed due to interaction of the resonant modes of the structural units with the travelling wave [240]. In addition, the acoustic metamaterials exhibit special wave propagation characteristics in wave vector space, spectral space, and phase space [241,242]. In acoustic energy harvesters, the acoustic energy is focused onto piezoelectric crystal and its density is improved significantly to reach the usable magnitude. The piezoelectric material converts the incident energy into electrical energy by inverse piezoelectric effect. The generated electrical energy can be used in low power consuming devices and to power up the wireless sensor network. Different types of acoustic metamaterials have been developed for harvesting of acoustic energy. With the help of lead zirconate titanate transducer and double layered acoustic metamaterial Wang et al. [243] demonstrated the acoustic energy harvester which exhibits the maximum power of 73.1 nW at the incident wave frequency of 318 Hz and pressure of 2 Pa. In addition, using spring-mass resonators Oudich et al. [244] developed acoustic metamaterial thin-plate for harvesting of acoustic energy. The designed harvester shows large out power of 18.1 µW at 519 Hz and 2 Pa. Recently, Ma et al. [245] designed a two-dimensional local resonant acoustic metamaterial for energy harvesting applications. They reported that the designed energy harvester shows voltage enhancement of 950% compared to bare plate energy harvester. In addition, at resonant conditions and the sound pressure of 20 Pa the proposed harvester shows maximum voltage of 291 mV, average power of 28 µW and power density of 1.24 mW cm−<sup>3</sup> . Further, the outdoor studies revealed that the acoustic material-based harvester shows 18 times better open circuit voltage compared to the bare plate harvester. Hence, the outstanding properties of the proposed harvester evinced that it could serve as a promising acoustic energy harvester with improved performance.

In addition, the metamaterials can also absorb and harvest electromagnetic signals. Nowadays, the investigations on the metamaterial-based absorbers have increased

for electromagnetic signal harvesting and absorbing applications in microwave region. Landy et al. [246] in 2008 designed metamaterial-based absorber consisting of two separated resonators to harvest electromagnetic waves. In addition, for high frequency regions of GHz and THz, Dincer et al. [247] introduced a metamaterial-based absorber with the help of square resonator. In addition, Cheng et al. [248] investigated the polarization insensitive metamaterial-based absorber for harvesting electromagnetic energy at various frequency bands. Yagitani et al. [249] proposed and designed an electrical circuit model and mushroom-like electromagnetic band gap [250] (EBG) structures for obtaining the 2D image of distribution of RF power. Finally, Alkurt et al. [251] demonstrated a metamaterialbased absorber for energy harvesting and imaging applications. The proposed harvester is also called as 2 × 2 patch array antenna and microwave image detector. The unit cell of the proposed harvester is shown in Figure 4 (Figure 1 of the Ref. [251]). They reported that, in the proposed absorber, first the absorbed energy is converted into DC signal with the help of Schottky diodes. Subsequently, the obtained DC signals generate image of the absorbed power. Afterwards, they fabricated 2 × 2 patch array antenna and energy harvester. The outcomes of the numerical and experimental measurements are matched with one another. Hence, all these results evidenced that the proposed metamaterial-based absorber can be promising candidate for energy harvesting, imaging, crack detection, and so on.

**Figure 4.** Unit cell of the metamaterial-based harvester (Figure 1 of Ref. [251]).
