ABSTRACT
Traditional optical devices can modify the wave front of light by altering its phase, amplitude, and polarization. For example, a lens or prism is usually realized by reshaping the wave front of the light that relies on gradual phase changes along the optical paths, which is accomplished by either controlling the surface topography or varying the spatial profile of the refractive index. A wave plate utilizes bulk birefringent crystals with optical anisotropy to change the polarization of light, and a hologram is based on the interference of diffracted waves from different parts of the components in the far-field to produce the desired optical pattern. All of these components shape optical wave fronts using the propagation effect. Thus, it is hard to accumulate sufficient phase change once the device size is further reduced to micro- and even nano-scale due to the finite permittivity and permeability of natural materials. While there has been great progress in the miniaturization of optical elements, such photonic integration largely depends on the technical advancement. 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. Therefore, to meet the growing requirement of device miniaturization and system integration, a new design methodology is urgently needed to develop ultrathin optical devices.
