Tightly autofocusing beams: An effective enhancement of longitudinally polarized fields

We proposed a type of tightly autofocusing beams (TABs) to create and enhance longitudinally polarized fields with subdiffraction beam sizes. It reveals that the TABs can automatically focus and behave with similar focusing properties to those with a high numerical aperture lens; a radially polarized beam generates a strong longitudinal field, while the azimuthally polarized one cannot. By reducing the order of the TAB, the maximum conversion efficiency of light to longitudinal field is as high as 98.32% in theory. As such, we demonstrate that the spin-to-orbital angular momentum conversion is also enhanced by circularly polarized TABs.