## 4 Structural Optimization
Instead of increasing the thickness further in the same material, we have added GaAs material at the back of this solar absorber, which has a different refractive index and also helps in gaining further absorption. The results for GaAs thickness change are given in Figure 5. The bottom layer added to the substrate is increasing the absorption slightly because of the difference in refractive indices between the two materials. The variation in thickness of this material is not affecting the absorption further, and the same is visible in Figure 5. The absorption level around 1 m shows perfect absorption, with 99.5% absorption as the result. The absorption levels are in three peaks, with the highest having perfect absorption with more than 99% absorption. The results are also showing a similar performance for all the variations in GaAs thickness. The results of the optimization clearly show that there is an increase in absorption with increasing substrate thickness. The thickness improvement also shows that nonlinear parametric optimization is essential and that there is an increase in absorption for higher thicknesses. The increased thickness is the key to improving absorption. The increased thickness influences the overall efficiency of the solar absorber design. The thickness increase was kept to a limit to keep the design complexity and cost lower. The angle of incidence variation is observed by varying the different angles of incidence from 0 degrees to 80 degrees. The angle of incidence is varied to observe its effect on the results. The angle of incidence variation is not affecting the results much, as presented in Figure 6. The result clearly shows that absorption is high for most of the angles except the 30\({}^{\circ}\) angle. The blue color shows the reduction in absorption for an angle range between 30\({}^{\circ}\) and 80\({}^{\circ}\). The red color around other angles indicates high absorption of the red color. The results show that the absorption is high for most of the angles of incidence. The angle at which the absorption is less is also important. The absorption for different angles clearly shows that the solar absorber is giving better results for most of them. The low angles are also showing 30-50% absorption, which is good for solar absorbers as it is going to increase the absorption of the solar absorber. Figure 6: Angle of incidence variation for wavelengths from 500 nm to 3000 nm. For most of the range, absorption is high. The 30\({}^{\circ}\) does not have high absorption until 2500 nm, after which absorption improves until 3000 nm. The absorption level is good for most of the angles. The 30\({}^{\circ}\) angle angle angle exhibits 30–50% absorption. The comparison of the I-shaped metamaterial design is observed in Table 1. The I-shaped metamaterial design is showing 96% average absorption in the visible and infrared regions. This absorption is higher compared to the rest of the references presented in the table. The absorption results show that the I-shape metamaterial design is showing the maximum absorption. The comparison of the proposed I-shape metamaterial design with other published designs shows its improvement in terms of the absorption of the solar absorber. There is one published design with 95% average absorption, but it gives absorption in a lower wavelength range compared to our proposed design results, which give results in the wide wavelength range of 500 nm to 3000 nm.