A Broadband Polarization-Insensitive Diffractive Lens Design for Subwavelength Focusing of Light

In this study, we propose the metaheuristic design of a polarization-insensitive diffractive lens. The designed lens structure consists of concentric dielectric rings with optimally modulated widths and heights. Here, the evolutionary optimization approach is conducted to find the optimum values of concentric rings' widths and heights to obtain polarization-insensitive subwavelength focusing effect of the incident light. Three-dimensional finite-difference time-domain (3D FDTD) method is incorporated with the optimization algorithm to evaluate the focusing ability of instantly designed lens structures throughout the optimization process. The presented lens structure is designed at operating frequency of 10 GHz, and has a diameter of 18.45 cm with thickness of 2.4 cm. The detailed analysis of the polarization-insensitive focusing effect of designed diffractive flat lens is numerically investigated at microwave frequency regime considering the scenario of experimental verification to be conducted at the same spectral region. Possible experimental verifications are discussed along with the 3D-printing of the designed lens structure. As a further step, the presented design concept may be applied to design metasurfaces operating at visible wavelengths.