nt in the optical to terahertz conversion efficiency. The number of elements in the IPCA can be varied (such as, 2 elements, 5 elements, 14 elements) as shown in Figure 12.7, by keeping the area and gap constant, to increase the efficiency of PCA [Rathinasamy, Thipparaju, Edwin, & Monda (2021); Rathinasamy, Thipparaju, Boby, & Mondal (2022)]. 12.7.3.1 Tip-to-Tip PCA The tip-to-tip PCA is another kind of interdigitated PCA where the cathode and anode electrode fingers are distributed like a comb structure, and they meet head-to-head at the central gap. By introducing this tip-to-tip geometry, the electric field distribution is stronger in the central part of PCA where the fingers are placed in a horizontal pos- ition. There are also tip-to-tip trapezoidal fingers which produce two-fold stronger electric field magnitude. The bow-tie antenna along with the rectangular tip-to-tip or trapezoidal tip-to-tip fingers can also be used to produce terahertz pulses [Khiabani, et al. (2013)]. The detected THz power from the trapezoidal tip-to-tip PCA is seven times more than the bare gap antennas and twice as good as the rectangular tip-to- tip geometries [Khiabani, et al. (2013)]. The geometry of rectangular tip to tip and trapezoidal tip to tip interdigitated PCA is given in Figure 12.8(a) and Figure 12.8(b) respectively. The trapezoidal tip-to-tip structure represented in Figure 12.8(b) produces 1.37 uW power and the interdigitated PCA produces 0.68 uW power whilst the dipole PCA produces 0.19 uW power at 0.17 THz frequency. The trapezoidal tip Terahertz Antenna in Future Healthcare Informatics FIGURE 12.8 Different shapes of tip-to-tip interdigitated electrode: (a) Rectangular (b) Trapezoidal. FIGURE 12.9 Structural diagram of split ring resonator. to tip IPCA can produce high electric-field compared to the rectangular tip to tip and dipole PCA. 12.7.4 SPLIT RING RESONATOR PCA designed in the shape of a split ring resonator (SRR). The structure of the SRR is shown in Figure 12.9. The resonant behaviour of the photo-induce