Focal Shift of Nano-Optical Lens Affected by Periodic Resonance With Substrate

A method of focal-shift resonance with substrate based on two commonly used structures of nano-optical lens is presented with numerical simulations. This resonance exhibits periodicity with the variation of substrate refractive index. Both types of lenses are based on gradual changed nanoslits and analyzed by the extraordinary optical transmission theory. This displacement of focal point can reach the order of microns. A periodic intensity change of focal light is observed between two absorption peaks in metal nanolens. The origin of this focal shift is also discussed with the analysis of the Fresnel number in diffraction. The finite element method is utilized in this paper.