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

Recently, multilevel diffractive lenses (MDLs) have attracted considerable attention, mainly due to their superior wave-focusing performance; however, efforts to reduce chromatic aberration are still ongoing. Here, we present a numerical design and experimentally demonstrate a high-numerical aperture (∼0.99), diffraction-limited achromatic multilevel diffractive lens (AMDL), operating in the microwave range of 10–14 GHz. A multi-objective differential evolution (MO-DE) algorithm was incorporated with the three-dimensional (3D) finite-difference time-domain method to optimize the heights and widths of each concentric ring (zone) of the AMDL structure. To the best of our knowledge for the first time, in this study, the desired focal distance was also treated as an optimization parameter in addition to the structural parameters of the zones. Thus, MO-DE diminishes the necessity of predetermined focal distance and center wavelength by also providing an alternative method for phase profile tailoring. The proposed AMDL can be considered an ultra-compact and flat lens since it has the radius of 3.7λ_c and a thickness of ∼λ_c, where λ_c is the center wavelength of 24.98 mm (i.e., 12 GHz). The numerically calculated full width at half maximum values are <0.554λ and focusing efficiency values are varying between 28% and 45.5%. To experimentally demonstrate the functionality of the optimized lens, the AMDL composed of polylactic acid material polymer is fabricated via 3D-printing technology. The numerical and experimental results are compared, discussed in detail, and observed to be in good agreement. Moreover, the verified AMDL in the microwave regime is scaled down to the visible wavelengths to observe achromatic and diffraction-limited focusing behavior between 380 and 620 nm wavelengths.