Background

Reference:Zijian Wu, Yan-Qin Lu, & Jung-Tsung Shen, Jun. 23. (2015). Planar-convex metallic gratings as elective refractive lenses. Manuscript sumbitted for publication. 

In typical optical imaging systems, the length of the eyepiece is pre-dominantly determined by the focal length of the lenses. Therefore it is highly desirable to have as short focal length as possible to realize a compact imaging system. The focal length of glass based lens is determined by its refractive index n. The higher the index is, the shorter the focal length is. For a planar-convex lens, the focal length is inversely proportional to (n=1) in an air environment.

Nevertheless, in practice, the refractive index for high quality lens is around 1.5. Due to the limitation of the available range of refractive index, using conventional optical materials would be challenged to significantly miniaturize the optical system.

Recently, it has been shown that a deep subwavelength planar grating be mapped into an elective high index material with the global index determined by the ration of periodicity d and slit width a. The elective index of the grating should be: neff = d/a. Consequently several studies show that by locally perturbing periodicity and slit width the deep subwavelength planar grating can have a position depending refractive index and achieve a shorter focal length.

This is a normal lens with thickness, t, and width, w.

This is a normal lens with thickness, t, and width, w.

This is a metallic nanoslit lens with period, d, and slit width, a.

This is a metallic nanoslit lens with period, d, and slit width, a.

In this project, our results are all from COMSOL Multiphysics simulations.