  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 $10 GHz$ - $14
GHz$. 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., $12 GHz$ 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 $380
nm$ - $620 nm$ wavelengths.
