Evolution of the Stokes parameters, polarization singularities, and optical skyrmion
A physical theory is presented for polarized light from an aspect of polarization singularity.This is carried out by analyzing the evolution equation of the Stokes parameters that is derived from the nonlinear Schrodinger type equation. The problem is explored from two aspects:The first is concerning the single mode Stokes parameter that is described by the propagation distance $ z $. The trajectory of the polarization singularities is simply given by the points satisfying a rule: $ S_3 (z) = +(-)S_0 $ and $ 0 $ ($ S_3 $ represents the third component of the Stokes parameters), which form a tube like surface centered with the C-line surrounded by L-surfaces in the presence of the linear as well as the nonlinear birefringence. It is pointed out that the optical activity (or chirality) plays an "obstacle" to reveal the polarization singularity. As the other aspect, we consider the field theoretic aspect of the Stokes parameters, for which a special solution, called an optical skyrmion, is constructed to explicate the polarization singularity.The skyrmion forms a {tube}, and the dynamical content of which is briefly discussed. On the basis of the work presented here further analysis would be expected to reveal hidden aspects of polarization optics.