Generation of tunable longitudinal superchirality by a vector-light metalens

The high sensitivity detection of chirality is of great importance in fields of physics, biology, and life sciences. Emerging strategies attempt to generate the superchiral fields with metasurfaces to maximize the chiral light-matter interaction. Although these studies have mostly considered the transverse superchirality, the longitudinal optical chirality also plays a very important role in chiral interactions. In this paper, the conditions for generating the optimal longitudinal superchirality are demonstrated with vector diffraction theory. Accordingly, a metalens is designed to generate vector beams as a replacement of conventional optical elements. Moreover, by modifying the polarization state of the incident light from linear to circular polarization, the longitudinal superchirality can be continuously tuned from zero to more than 70 times of the intrinsic chirality of circularly polarized light. Our theoretical analysis as well as the numerical demonstration by a vector-light metalens highlights the generation of tunable longitudinal superchirality, paving a nanophotonic way for the longitudinal chiral sensing.