High-Density Topological Defect Array by Two-Step Interference Photoalignment.

With nematic liquid crystal (LC) topological defects can be created that are important for the generation of laser beams with orbital angular momentum. Arrays of defects have been realized through photoalignment, by projecting images of a spatial light modulator or digital mirror device. However, such pixel-based approaches seriously limit the achievable density of defects. Here, we propose a scalable method based on two-step interference illumination to achieve a much higher density. In the first step, a rotating director pattern is generated by the interference of two circularly polarized beams. In the second step, amplitude modulation by two-beam interference is used to rewrite the pattern and introduce the defects. By adjusting the illumination doses and angles of incidence in both steps, 2D defect patterns are obtained, with spacing down to 1.25 micrometer. The resulting disclination lines are analyzed, and the proposed structure is supported by numerical simulations. For the first time, a high defect density defect grid is obtained through a scalable two-step photoalignment procedure. This approach enables optical components with large-angle diffraction and bridges the gap between LC topological optics and metasurfaces.