Tunable chirality and helicity of Bessel twisted beams near the Dirac points in topological metacrystal

In this work, we undertake an approach to describe the tunable optical chirality and helicity of twisted beam near the Dirac point in topological meta-crystals. An improved method based on the full vector wave analysis is introduced to investigate the chirality and helicity of Bessel twisted beams near the Dirac point in the interface of topological meta-crystals. It is showed that the giant non-specular effects of chirality and helicity are discovered when a Bessel twisted Beam reflects in the vicinity of a Dirac point, which is sensitive to the incident frequency, angle, and electromagnetic properties of the topological meta-crystal. We can attribute the giant non-specular effect of chirality and helicity to the degeneracy between critical angle and Brewster angle caused by the Dirac point. The results will be of significant interest of study the measurement of Dirac point, and inspire a perspective for manipulating twisted beams with Dirac point.