## 3 Simulation and Results  
For a broadband nano-antenna that can operate in the optical and infrared regions to radiate the optical signal into the free space [44], the presented map-leaf shape nano-antenna is a promising solution. The proposed nano-antenna has been designed and analyzed using the CST Microwave Studio software package [45], which is based on the technique of the finite integration (FI) solver. The return loss for the antenna design at each step (shown in Figure 3) is plotted in Figure 4.  
The return loss curve and gain for the final design are plotted against the operational frequency in Figure 5. Here, it can be seen that the antenna operates in all of the three main windows for optical frequencies between 50 to 450 THz with a return loss less than \(-10\) dB. The second observation from the results is that the gain increases with the frequency, which is very useful to compensate for the high propagation loss at higher frequencies. The voltage standing wave ratio (VSWR) of the proposed broadband nano-antenna is depicted in Figure 6. The VSWR value is between 1 and 1.5 for the frequency band of 50 to 450 THz, which is an acceptable value for an antenna operating at these high frequencies. The VSWR value is found to be 1.15, 1.18, 1.14, and 1.4 at 193, 229, 353, and 390 THz, respectively. The map-shaped antenna is evaluated for different lengths of the leaf tips, as shown in Figure 7. The shape with small leaf tips resonates in multiple frequency bands with the reduced bandwidth. The increase in the leaf tips of a common map-shape provides better results, but as the length of the leaf tips increases, the dimensions become very narrow and the proposed design will be extremely difficult to be used for real applications, especially in a large array where such small structure details are impossible to achieve.  
Figure 4: The S\({}_{11}\) of the design evaluation at each step.  
Figure 3: Varied design steps of the proposed antenna.  
The simulation work has been conducted by considering the limitations of the fabrication processes. Therefore, a feasible design is selected for the further analysis.  
Figure 5: S\({}_{11}\) and gain of the antenna versus frequency in THz covering all of the three main optical windows.  
Figure 6: VSWR values for the proposed antenna.  
Figure 7: Performance comparison for different lengths of the leaf tips with the maple-leaf antenna geometry shown as an inset.  
The polar plot and 3D radiation pattern for the simulated broadband nano-antenna are shown in Figures 8 and 9, respectively. The simulation results show that the proposed optical antenna directs the optical signal at 0\({}^{\circ}\) with the half-power beamwidth (HPBW) of 49.5, 41.4, 32.8, and 26.4\({}^{\circ}\), while operating at a frequency of 193, 229, 353, and 390 THz, respectively. From these values, it can be concluded that as the transmitting frequency increases, the beamwidth of the antenna narrows down, and the antenna becomes more directive.  
To observe the resonant area, surface electric fields (E field) recorded at the resonance frequency of 193 THz are indicated in Figure 10, where the upper and lower edge of the maple-leaf shaped broadband antenna show more intensity than the central region. The effective resonant area is highlighted in Figure 11 for the surface current distributions of the proposed nano-antenna. As expected, at the resonance frequency of 193 THz, most of the current density is concentrated at the central area of the antenna.