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Recently, the multilevel diffractive lenses (MDLs) have attractedconsiderable attention mainly due to their superior wave focusing performance;however, efforts to correct the chromatic aberration are still in progress.Here, we demonstrate the numerical design and experimental demonstration ofhigh-numerical aperture (NA) (${\sim}0.99$), diffraction-limited achromaticmultilevel diffractive lens (AMDL) operating in microwave range $10 GHz$ - $14GHz$. A multi-objective differential evolution (MO-DE) algorithm isincorporated with the three-dimensional finite-difference time-domain (3D FDTD)method to optimize both the heights and widths of each concentric ring (zone)of the AMDL structure. In this study, the desired focal distance ${\Delta}F_d$is treated as an optimization parameter in addition to the structuralparameters of the zones for the first time. In other words, MO-DE diminishesthe necessity of predetermined focal distance and center wavelength by alsoproviding an alternative method for phase profile tailoring. The proposed AMDLcan be considered as an ultra-compact, the radius is $3.7{\lambda_c}$ where${\lambda_c}$ is the center wavelength (i.e., $12 GHz$ frequency), and flatlens which has a thickness of ${\lambda_c}$. The numerically calculatedfull-width at half-maximum (FWHM) values are below $0.554{\lambda}$ andfocusing efficiency values are varying between $28{\%}$ and $45.5{\%}$. Toexperimentally demonstrate the functionality of the optimized lens, the AMDLcomposing of polylactic acid material (PLA) polymer is fabricated via3D-printing technology. The numerical and experimental results are compared,discussed in detail and a good agreement between them is observed. Moreover,the verified AMDL in microwave regime is scaled down to the visible wavelengthsto observe achromatic and diffraction-limited focusing behavior between $380nm$ - $620 nm$ wavelengths.

Ultra-compact, high-NA achromatic multilevel diffractive lens via metaheuristic approach