  We investigate the applicability of the coupled multipole model and its
modification in the framework of the zero-order Born approximation for modeling
of light focusing by finite-size nanostructures of silicon nanospheres,
supporting electric and magnetic dipole and quadrupole resonances. The results
based on the analytical approximations are verified by comparison with the
numerical simulations performed by the T-matrix method. Using the evolutionary
algorithm optimization, we apply the developed approach to design the silicon
nanosphere metalenses with predefined focusing properties. The obtained results
demonstrate a strong optimization potential of the suggested calculation scheme
for engineering ultra-thin metalenses.
