d current is due to the inductance nature of the SRR electrode structure based on the lumped cir- cuit model. The current flow is induced between the cathode and anode of the SRR PCA by laser illumination which in turn leads to the THz generation. This vari- ation in current becomes the source of radiation of THz electromagnetic waves. The generated THz field and the photocurrent is described by the Equation 12.1 [K. Lee et al. (2020)]. add dI(t) (12.1) 5G Wireless Communication System in Healthcare Informatics Silver nanoantennas pulse FIGURE 12.10 Illustration of the silver nano-antennas in the gap of log-periodic PCA. The incident plane wave is perpendicular to the SRR surface, and the electric field excites the electro-magnetic modes (EM modes) of the SRR. The structure is sym- metrical, and the equivalent circuit consists of two inductors and one capacitor. The inductance value depends on the whole electrode length (L4) and width (L2) whereas the capacitance of the SRR depends on the gap width G and length W1. 12.7.5 NANO-STRUCTURED PCA To improve the optical to terahertz conversion efficiency, researchers are inclined towards nano-antennas, nano-structured arrays, double layer nano-plasmonic structures, nano-rods, and the like. The screening effect, the low absorption coeffi- cient and lower quantum efficiency are vital factors which affect PCA performance. To overcome these phenomena, nano-structured antennas can be used. In the last few years, the plasmonic effect couples the EM field with surface electrons to enhance electric field distribution as well as the absorption coefficient and also improves the efficiency of PCA. This is also known as hybrid THz-optical PCA. The silver nano- antennas in the active area of the log-periodic antenna are shown in Figure 12.10. The experimental results of nano-antenna based PCAs generate better output power compared to the conventional PCA [Lepeshov, et al. (2017)]. For photonics and optoelectronic devices, such as, photodetectors, optical modulators, optical filters