Structured light with diffractive optics and metasurfaces

Diffractive optical elements (DOEs) such as spatial light modulators (SLMs), binary phase gratings, microelectromechanical systems (MEMS) and metasurfaces, have matured to the extent that they can reshape the scattered wavefront, i.e., altering the phase, amplitude and polarization of light. Metasurfaces — flat optic elements composed of subwavelength-spaced array of scatterers with spatially varying geometric parameters such as shape, size, orientation —are among the recent tools that can mould optical wavefronts into arbitrary shapes, point-by-point, with subwavelength resolution. Owing to the recent advances in nanofabrication, multi-layer metasurfaces have now become feasible. Here, we explore the possibilities offered by bilayer metasurfaces and show how it can overcome some of the intrinsic limitations of the widely adopted single layer metasurfaces. As an example, a new class of reflective bilayer metasurfaces is designed and fabricated. Such a device can, in principle, impart Pancharatnam-Berry phase on linearly polarized incident light as opposed to single layer Berry phase metasurfaces that primarily operate on circular polarization bases. In this process, we analyze the coupling within these bilayer structures and provide a design recipe for which this coupling can be neglected. In addition, we explore the use of programmable SLMs for generating new class of structured light beams; namely, optical vortices whose orbital angular momentum (OAM) can be adiabatically controlled along the optical path. We discuss the theoretical formalism, modeling, and experimental generation of this new class of beams and show that such an evolution in their OAM state is associated with a topological Berry phase factor that is accumulated along the propagation direction, at-will. Lastly, an outlook for this area of research and possible future directions are discussed.