Theory and implementation of focal shift of plasmonic lenses.

We utilize three diffraction theories to investigate the focal-shift phenomena of plasmonic lenses. By taking the Fresnel number into account, a better accuracy in predicting the relative focal shift can be achieved for low-Fresnel-number systems. On the basis of this focal-shift effect, we present a method to realize an active plasmonic lens. Two-dimensional finite-difference time-domain simulation is performed to verify the method proposed.