Effect of lens size on the focusing performance of plasmonic lenses and suggestions for the design.

We present a detailed investigation of the effect of lens size on the focusing performance of plasmonic lenses based on metallic nanoslit arrays with variable widths. The performance parameters considered include the focal length, depth of focus (DOF), full-width half-maximum (FWHM) and the maximum intensity of the focal point. 2D FDTD simulation was utilized. The results show that all the lens parameters are greatly affected by the lens size. A larger lens size, with a total phase difference of at least 2π, will produce a better focusing behavior and a closer agreement with the design. The Fresnel number and diffraction theory can be used to explain the effect of lens size. Suggestions are provided for realization of a practical plasmonic lens using the existing nanofabrication techniques.