Metasurfaces for ultrathin planar optical devices

Miniaturization and integration are two continuing trends in the production of photonic devices. The functionality of a traditional photonic 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. The thickness of photonic devices usually remains comparable to the wavelength of the light. However, further reduction in the thickness of the corresponding element is hindered by the design theory since it is based on phase accumulation along the optical path. Metamaterials can usually be engineered to exhibit electromagnetic properties that may not be found in nature or its constituent components, thus providing an unconventional alternative to optical design. Metasurfaces, the emerging field of metamaterials, which consist of a single layer of artificial "atoms", have recently captured the attention of the scientific community since they do not require complicated three-dimensional nano-fabrication techniques but can control light propagation in equally dramatic ways. Unlike the phase change by the accumulated optical path in traditional optical elements, the abrupt phase change takes place at the metasurfaces, meaning that a new freedom for controlling light propagation is introduced. In this talk, I am going to talk about the recent development in the field of metasurfaces, and highlight our previous and current contribution to this research area.