Nonparaxial Gaussian beams: 2. Splitting of the singularity lines and the optical magnus effect

The Poynting vector field lines for the lowest modes of a nonparaxial Gauss ian beam exhibit a number of loops and rings in the vicinity of the phase s ingularity lines (Airy's fringes), with negative energy fluxes present insi de these loops and rings. The positions of these fluxes are nonsymmetric wi th respect to rotation about the optical axis. This asymmetry leads to a lo cal splitting of the phase singularity lines, after which the beam cross se ction transforms from circular to elliptic. The asymmetry of the beam cross section can be eliminated by considering a superposition of circularly pol arized even and odd modes. However, this approach only uniformly redistribu tes the negative energy fluxes in the azimuthal direction of the cross sect ion, rather than completely eliminates these fluxes. Any small perturbation of the resulting symmetric beam gives rise to a unique phenomenon-the opti cal Magnus effect in the free space, whereby the beam intensity pattern rot ates upon changing the circular polarization from right to left. This effec t implies the presence of a spin-orbit coupling in the nonparaxial Gaussian beam propagating in the free space. (C) 2000 MAIK "Nauka/Interperiodica".