Cassegrain metasurface for generation of orbital angular momentum of light

Light beams with a helical phase-front (optical vortices) carry orbital angular momentum (OAM), which provides an attractive degree of freedom for controlling light and developing a series of applications, such as optical tweezers, super-resolution imaging, optical communication, and quantum information processing. Recently, the versatile metasurfaces, two-dimensional arrays of subwavelength structures with space-variant phase responses, provide powerful and convenient approaches for OAM generation. We propose and experimentally demonstrate an efficient method to generate and focus the vortex beam with a planar Cassegrain metasurface. The required helical phase profile is imparted via the secondary reflective metasurface of the planar Cassegrain metasurface. The topological charge of the vortex beam is revealed by measuring the interference pattern at the focal plane of the planar Cassegrain metasurface with a reference Gaussian beam. The combination of OAM generation, Cassegrain system, and ultrathin flat metasurface renders this scheme very attractive for diverse applications at miniature scales.