Synthesis and observation of optical skyrmionic structure in free space

The skyrmion, which is characterized by a topological integer, is a structure that is topologically stable against local disturbances. The huge potential of skyrmions for use in magnetic storage systems has drawn considerable research interest among physicists. Recently, optical skyrmions were discovered to have some excellent properties. However, all of these optical skyrmions have been observed in surface plasmons that consist of evanescent waves. This type of optical skyrmion is difficult to manipulate and also difficult to apply in practice. Here we describe several skyrmionic optical structures with different skyrmion numbers in a free-space linear optical system. Because of the convenience of operation using free-space optics, with the exception of the original applications of skyrmions, skyrmionic optical structures can also be applied widely, ranging from manipulation of tiny objects to propagation with information over long distances.