Reverse Flux of Light Energy in Sharp Focus
This chapter shows that the reverse propagation of light near the optical axis also occurs in the plane of sharp focus of an arbitrary optical vortex with the topological charge of 3 and with left circular polarization. When non-paraxial light fields with different polarization states propagate in space, local areas can appear in their cross-sections, where the flux of light energy has a reversed direction that is the longitudinal component of the Poynting vector in such areas has a negative value. Reverse flux of light energy along a spiral around the optical axis near the focus has been confirmed by the finite-difference time-domain simulation of sharp focusing of an optical vortex with a phase zone plate. If the beam order is equal to two, then the region with negative values of the projection of the Poynting vector is located in the center of the focal spot.