  Sub-wavelength diffractive optics, commonly known as metasurfaces, have
recently garnered significant attention for their ability to create ultra-thin
flat lenses with extremely short focal lengths. Several materials with
different refractive indices have been used to create metasurface lenses
(metalenses). In this paper, we analyze the role of material refractive indices
on the performance of these metalenses. We employ both forward and inverse
design methodologies to perform our analysis. We found that, while high
refractive index materials allow for extreme reduction of the focal length, for
moderate focal lengths and numerical aperture (<0.6), there is no appreciable
difference in focal spot-size and focusing efficiency for metalenses made of
different materials with refractive indices ranging between n= 1.25 to n=3.5.
