Design of Phase Mask Based on Periodic Array of Tapered Slits

A powerful design method for subwavelength-scaled phase mask with enhanced transmission is proposed. In the phase mask, the extraordinary optical transmission is further boosted up by a nanofocusing effect through a plasmonic tapered slit instead of using ordinary straight slit. The optical fields are calculated rigorously by the finite element method and the genetic algorithm is chosen as a global optimization tool. The objective function to evaluate the performance of the phase mask and its robustness are investigated at the same time by employing gradient index as another objective function. It is found that the fundamental order transmissions normalized by the gap-to-period ratio are calculated as 0.6059, 3.25, and 5.14, respectively. The numerical optimization results show the proposed method is found to be a useful design approach for subwavelength-sized phase masks with nanofocused optical field and high ratio of fundamental to first order diffraction transmission.