  The miniaturization of current image sensors is largely limited by the volume
of the optical elements. Using a sub-wavelength patterned quasi-periodic
structure, also known as a metasurface, one can build planar optical elements
based on the principle of diffraction. However, it was believed that high
refractive index materials are required for metasurface optics. Here, we show
that one can employ the design principles of a metasurface even with low
contrast materials, such as silicon nitride. We validate our theory by
fabricating and experimentally characterizing several silicon nitride based
lenses and vortex beam generators . The fabricated lenses achieved beam spots
of less than 1 {\mu}m with numerical apertures as high as ~ 0.75. A
transmission efficiency of 90% and focusing efficiency of 40% in the visible
regime was observed. Our results pave the way towards building low-loss
metasurface based optical elements at visible frequencies using low contrast
materials.
