What is an “optical skyrmion?”

Preparing to write the final section for the second edition of my Singular Optics textbook that happens to be on, you guessed it, optical skyrmions! I thought I would try writing a non-technical explanation first to straighten out all the concepts in my head. One of the big evolutions in the field of optical physics over the past twenty years has been the introduction of the mathematics of topology into almost all aspects. This has led to a lot of interesting phenomena in optical wavefields themselves such as optical vortices, optical knots and optical Möbius strips. (What is “topology?” We will come back to that in a moment.) In the past decade, a lot of research has focused on a different sort of topological feature that can arise in optical wavefields, known as a skyrmion. The name comes from Tony Skyrme, who first introduced this topological feature in a paper1 in 1961 as a possible model of a nucleon. Optical physicists, having drawn lots of inspiration from topology previously,…