  The recent emergence of electromagnetic topological defects has attracted
wide interest in fields from topological photonics to deep-subwavelength
light-mater interactions. Previously, much of the research has focused on
constructing specific topological defects but the fundamental theory describing
the physical mechanisms underlying their formation and transitions is lacking.
Here, we present a spin-orbit coupling based theory describing such mechanisms
for various configurations of spin topological defects in confined
electromagnetic fields. The results reveal that their formation originates from
the conservation of total angular momentum and that their transitions are
determined by anisotropic spin-orbit couplings. By engineering the spin-orbit
couplings, we observe the formation and transitions of Neel-type, twisted-type,
and Bloch-type spin topological defects in confined electromagnetic fields. A
stable Block-type spin topological defect is reported for the first time. Our
theory can also describe the transitions of field topological defects. The
findings enrich the portfolio of electromagnetic topological defects, deepen
our understanding of conserved laws, spin-orbit couplings and transitions of
topological defects in confined electromagnetic systems, and predict
applications in high-density optical data transmissions and chiral quantum
optics.
