Before this semester, we had confirmed that the electromagnetic constants of the metal nanoslit structures are entirely determined by the d/a ratio, where d is the distance between two adjacent slits and a is slit width. In other words, n = d/a. Thus, we could manipulate the optical properties of our lenses by varying the structures of the lenses. In our simulations, we fixed the distance between two adjacent slits, but vary the slit width. A larger electromagnetic constant n is very effective in shortening the focal length, but at the same time it also causes the transmittance to decrease. In other words, it was not possible to have short enough focal lengths and high enough transmittances simultaneously.
Since this semester, we have worked on how to balance between the focal length and transmittance of our design. Thus, we fix the refractive index n = d/a to 3, and vary the thickness of the lens. Originally, when we simulated with silver and gold with a thickness L = 2*λ, we already get a focal length (~20 μm) shorter than that of the normal lens. Since the transmittance decreases with increasing refractive index, we would use a smaller n for higher transmittance. Then we reduce the thickness of the lens with a increment equal to 1/6λ in order to catch the constructive interference as much as possible. What we have found is that as the thickness of the lens decreasing, the focal length increases a little bit while the transmittance improves greatly. The data table is shown as above.