Plasmonic focusing in spiral nanostructures under linearly polarized illumination

We have investigated the focusing properties of nanostructured plasmonic spiral lens by using linearly polarized illumination, and analysed its field enhancement effect based on the phase matching theory and finite-difference time-domain simulation. We demonstrate that under linearly polarized illumination, spiral plasmonic lens shows focusing property regardless its polarization directions, and the focal spot is about 250nm when the incident wavelength is 671nm. The intensity of the focal spot could also be controlled by altering the radius, the number of turns and the width of the nanostructured spiral slot which are confirmed by finite-difference time-domain simulation.