Effect of Twisting Phases on Linear–Circular Polarization and Spin–Orbital Angular Momentum Conversions in Tightly Focused Vector and Scalar Beams

We theoretically investigated the effect of a new type of twisting phase on the polarization dynamics and spin–orbital angular momentum conversion of tightly focused scalar and vector beams. It was found that the existence of twisting phases gives rise to the conversion between the linear and circular polarizations in both scalar and vector beams during focusing. The linear–circular polarization conversion further leads to an optical spin–orbital angular momentum transformation in the longitudinal component (LC). Therefore, even in a scalar optical field with a uniform linear polarization distribution, a circular polarization (spin angular momentum), and an orbital angular momentum (OAM) can appear in the cross-section and the longitudinal component, respectively, while being tightly focused. The novel distributions of the optical field, state of polarization (SOP) and OAM in the focal region are sensitively dependent on the twisted strength of the twisting phase. These results provide a more flexible manipulation of a structured optical field in the aspects of the optical field, SOP, and OAM.