Analysis of Tapered Nanopillars for Reflective Metalens: The Role of Higher-Order Modes

Despite the widespread interest in the metalens technology, few works show the errors during fabrication and analyze how they influence the focusing performance. In this work, we proposed a reflective metalens design and carry out a fundamental study on how a mild tapering angle (<; 6°) of the nanopillars would influence the performance of a reflective metalens. By analyzing the interaction of Mie resonance in the lateral direction and Fabry-Pérot resonance in the longitudinal direction using numerical simulations, we reveal the detrimental role the tapering angle plays due to the low transmission near the resonance. We observe the weakening of the electric quadrupole and magnetic dipole response, and the increase of the electric dipole response for tapered nanopillars. Moreover, the Fabry-Pérot resonance is disturbed due to the broken symmetry. These factors contribute to lower Q factor resonance and hence the low transmission. The results presented in this work can guide the analysis of fabrication errors and provide insights on how to compensate them.