Multiple Optical Knotting with Multi-Foci Metalens

Taking advantage of metasurfaces that have the unprecedented ability to control light propagation in a desired manner, metalenses can achieve unusual functionalities that are difficult or impossible for traditional lenses. In this work, we have experimentally reported metalenses that can generate optical structures in three-dimensional (3D) space. Our key design is the utilization of geometric metasurfaces based on gold nanostructures to realize multi-foci metalenses to generate more than ten thousand focal spots for creating multiple optical knots in 3D space. Both metalenses creating on-axis and off-axis propagations have been designed in order to compare their generated intensity patterns. As a result, the intensity patterns generated by the off-axis metalens have been observed more clearly than the on-axis propagation. More than four 3D knots in off-axis propagation can be created on the same observation plane, depending on the knot's size and the efficiency of metalenses. Our research findings are relevant to various practical applications, including imaging, sensing, and particle trapping.