Metallic Planar Lens With Binary Nanoscale Slits

We propose a binary plasmonic lens based on metallic nanoslit array with different fillings. The phase range of π is achieved by changing the refractive indices of the filling materials in the slits. These index-modulated slits are demonstrated to have unique advantages in phase delay compared with the width-modulated ones. The proposed binary structures can keep the advantage in phase modulation and reduce the difficulty in fabrication. Lenses with different functions, such as focusing light to one or two points or correcting oblique incident light, are designed by a simulated annealing algorithm and investigated by the finite-difference time-domain method.