  The skyrmion, which is characterised 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, the optical skyrmion was discovered and
has some excellent properties. However, these optical skyrmions have been
observed, for example, in surface plasmons that consist of evanescent waves.
This type of optical skyrmion is difficult to manipulate and also difficult to
apply in practice. In this work, we realise 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, e.g. to enable manipulation of tiny
objects or propagation over long distances.
