## 4 Antenna Array for Energy Harvesting Applications  
An extraordinary amount of work has been done in the field of energy harvesting applications through antennas in the last decade, to minimize the huge consumption of fossil fuel [46]. For energy harvesting applications, the antenna must have a high gain and radiation efficiency, which can be attained by making an array of several antennas [47]. The idea of collecting solar energy from the sun's and earth's radiation using nano-antennas is based on the fact that when a light source impinges the surface of a nano-antenna, the surface plasmons resonate and a time-varying current is induced on the antenna's surface. This phenomenon generates a voltage at the feeding point of the antenna. The induced electric field is then transferred via a feeding line and converted into an electric current to be supplied to a load [48]. By designing a \(2\times 1\) array from the aforementioned nano-antenna, a good amount of improvement can be achieved in terms of gain and directivity. The array design for a basic \(2\times 1\) configuration is shown in Figure 12, where for a single array with multiple elements, the spacing D is an important parameter that defines the main behavior of the array. The return loss and gain of a single element and \(2\times 1\) array are plotted in Figure 13 for comparison. The figure shows that increasing the elements in the array provides a higher gain, but the frequency response of the antenna slightly changes due to the dependence on the value of D. However, both configurations work in the large optical frequency span and can be considered for energy harvesting applications.  
Figure 11: Surface current plot for the maple shape.  
Figure 10: Electric field surface plot of the maple-shaped nano-antenna.  
Figure 16: The \(2\times 1\) array gain plots in the E and H planes at (**a**) 193 THz; (**b**) 229 THz; (**c**) 353 THz; (**d**) 390 THz.  
Figure 15: Radiation efficiency for the \(2\times 1\) antenna array.  
Researchers have designed various antennas for nano-photonics applications. The proposed antenna has a higher gain and directivity to fully fill the same application compared with the state-of-the-art published work in Table 2.  
As the simulation analysis shows an increase in gain and directivity with the increase in the number of antenna elements, the higher-order array size is required to achieve a gain above 20 dBi. By performing the calculations using the array tool kit in CST MWS, which uses the array factor formula [53] to calculate the array pattern from a single nano-antenna, a 64 \(\times\) 1 array is required to achieve a 3 dB bandwidth gain of 20 dBi. The polar plot and 3D radiation pattern of the 64 \(\times\) 1 element array are depicted in Figures 18 and 19, which show that the beam width becomes narrower and a peak gain of about 26 dBi can be achieved. Table 3 presents a summary of the results with the single, 2 \(\times\) 1, and 64 \(\times\) 1 array nano-antennas for comparison.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**References** & [47] & [44] & [49] & [50] & [51] & [52] & **This Work** \\ \hline Frequency (THz) & 193.5/229/352.9 & 193.5 & 193.5/229/352.9 & 193.5 & 193.5 & 193.5/229/352.9/393 \\ Gain (dBi) & 5.03/4.64/2.02 & 5.6 & 4.67/4.8/7.26 & 7.99 & 7.5 & 9 & 8.02/8.41/10.6/11.4 \\ Efficiency (\%) & *** & 87 & *** & 84.56 & *** & 89 & 60 \\ Bandwidth (GHz) & 234 & 15.6 & 240 & 203 & 20 & 40 & 400 \\ \hline \hline \end{tabular}
*** Not Given.  
\end{table}
Table 2: Comparison of the proposed antenna with the existing antennas.  
Figure 17: The 2 \(\times\) 1 array 3D far field gain pattern at **(a)** 193 THz; **(b)** 229 THz; **(c)** 353 THz; **(d)** 390 THz.  
Figure 19: The 64 \(\times\) 1 array 3D far field gain pattern at (**a**) 193 THz; (**b**) 229 THz; (**c**) 353 THz; (**d**) 390 THz.  
Figure 18: The 64 \(\times\) 1 array gain plots in the E and H planes at (**a**) 193 THz; (**b**) 229 THz; (**c**) 353 THz; (**d**) 390 THz.