“…As introduced in Section 2 , the basic design methods of dielectric metalens have been limited to the two semi-analytical approaches (geometric phase and propagation phase) in which dielectric nanoantennas are considered as single- or dual-mode waveguides. However, recently, the photonic inverse design methods by numerically optimizing arbitrary geometry of nanoantennas have been successfully verified via advanced electromagnetic optimization [ 61 , 62 , 63 , 64 , 65 , 66 ] and machine learning techniques [ 67 , 68 , 69 ]. Based on elaborate combination of the fast electromagnetic simulation methods and the robust frameworks of optimization and learning, improvements including bandwidth, NA, diffraction efficiency, and aberration correction have been achieved in a single flat optic device [ 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 ].…”