Reconstructing Focal Topological Curves through Vortex Spots from Multifocal Metasurfaces

Structured light plays an important role in modern photonics. Here, two approaches with distinct properties are proposed for reconstructing focal topological curves using an array of vortex spots generated by synthetic multifocal metasurfaces. A focal vortex lattice can create spatial topological photonics, where the tightly overlapping vortex spots annihilate within the lattice, leaving a closed multi-contoured boundary state as a closed focal curve defined by the lattice periphery. Alternatively, a chain of tightly overlapping vortex spots directly reconstructs a focal topological curve. The resultant curves are of multi-contours depending on the vortex topological charge, and the whole may coalesce into an overall vortex if a curve pattern is simple. Multichannel multifocal silicon metasurfaces are designed and fabricated to realize the required focal topological curves, and upconverted fluorescence is used to directly visualize the morphology of the near-infrared focal fields. This method opens new avenues for high-dimensional manipulation in focal optical fields with metasurfaces.