Unlocking complex optical vortices with flat optics

Optical vortices (OVs) possess spiral wavefronts and quantized orbital angular momentum (OAM), allowing them to act as key carriers of information and enabling light to possess high-dimensional degrees of freedom. In the past decade, there has been a growing interest in investigating complex OVs. Nevertheless, conventional methods for generating such beams are limited by bulky setups, high costs, and restricted control. Optical metasurfaces have emerged as promising candidates to overcome these challenges in a compact and cost-effective platform with enhanced control. This review begins by presenting an advanced platform designed for manipulating OVs and then discusses emerging types of OVs, including composite, grafted, and comb-like structured light beams, as well as multispectral and arbitrarily shaped OVs, along with nonlinear metasurface approaches. Next, we review the recent progress in OAM detection and mode discrimination, highlighting current challenges for high-capacity OAM systems. After that, we emphasize key applications ranging from OAM-based holography for optical encryption to super-resolution microscopy, structured-light trapping, and quantum state engineering. Finally, we critically identify the key challenges and future prospects for metasurface-enabled OV technologies.