Optical devices—such as lenses—have restrictions in their potential for miniaturization and in many cases are ultimately limited in feature size by the wavelength of light. On the other hand, for optical systems to continue to be established as economically viable in a range of emerging application areas, it is necessary to continue the trend of miniaturization and integration. The general function of 92most optical devices can be described as the modification of the wavefront of light by altering the three fundamental properties of light (phase, amplitude, and polarization). One of the key techniques in many optical components is a spatially varying phase response, as illustrated in optical lens and phase modulation holograms. The functionality of a traditional optical device is usually realized by reshaping the wavefront of the light that relies on gradual phase changes along the optical paths, which are accomplished by either controlling the surface topography or varying the spatial profile of the refractive index. However, it is hard to accumulate sufficient phase change once the device size is further reduced to the microeven the nanoscale due to the finite permittivity and permeability of natural materials.