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

Abstract

Recently, the multilevel diffractive lenses (MDLs) have attracted considerable 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 of high-numerical aperture (NA) ($\sim 0.99$), diffraction-limited achromatic multilevel diffractive lens (AMDL) operating in microwave range $10GHz$ - $14GHz$. A multi-objective differential evolution (MO-DE) algorithm is incorporated 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 structural parameters of the zones for the first time. In other words, 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 as an ultra-compact, the radius is $3.7{\lambda }_c$ where ${\lambda }_c$ is the center wavelength (i.e., $12GHz$ frequency), and flat lens which has a thickness of ${\lambda }_c$. The numerically calculated full-width at half-maximum (FWHM) values are below $0.554\lambda$ and focusing efficiency values are varying between $28\%$ and $45.5\%$. To experimentally demonstrate the functionality of the optimized lens, the AMDL composing of polylactic acid material (PLA) polymer is fabricated via 3D-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 wavelengths to observe achromatic and diffraction-limited focusing behavior between $380nm$ - $620nm$ wavelengths.

12 pages, 7 Figures, 1 Table