Directional design of optical lens based on metallic nano-slits with variant widths

The lens constructed with slits perforated on the thin metallic film is designed by simulated annealing algorithm. And two demonstrates are carried out to validate the directional design algorithm. First, the lens with one focal spot is considered. After the width of each slit is obtained, the finite-difference time-domain method is used to check the performance of the designed lens. It is found that the focal length is 800nm and the full-width at half-maximum of the focal spot is 252nm, which corresponds to the preset objective well. Then, the lens with three focal spots is also designed by this algorithm and the performance of the designed lens is quite close to the objective. This is the first work to introduce directional algorithm into design SPP lens. It may pave the way for the design sub-wavelength optic devices.