Design and simulation of achromatic metalens based on topology optimization

In this paper, we propose a novel reverse design method using topology optimization for the overall structure of a rotationally symmetric concentric nanoring metalens with a continuously variable height. The optimize objective is to maximize the focal field intensity of a predetermined numerical aperture lens. Compared to nano-pillar array structures and single thickness structures, this approach offers advantages in terms of high design flexibility in phase distribution and superior imaging performance. Leveraging the rotational symmetry inherent in concentric nanorings, a combined methodology of near-field electromagnetic simulations and far-field calculations is employed to verify the two-dimensional optical field characteristics. This method significantly improves computational efficiency compared to full-wave simulations in three-dimensional optical fields, as it reduces simulation time and has lower CPU requirements without compromising accuracy. The simulation results demonstrate that the designed continuous variable height concentric nanoring metalens achieves imaging close to the diffraction limit within the visible light range (400-700 nm).