# High‐Density Topological Defect Array by Two‐Step Interference Photoalignment

## TL;DR

A new method creates high-density topological defect arrays in liquid crystals using two-step interference photoalignment, enabling advanced optical applications.

## Contribution

A scalable two-step interference photoalignment method is introduced to achieve high-density topological defect arrays in nematic liquid crystals.

## Key findings

- Defect spacing as low as 1.25 micrometers is achieved using the two-step interference method.
- Disclination lines connecting defects on opposing substrates are successfully created and analyzed.
- The method enables optical components with large-angle diffraction and bridges LC topological optics with metasurfaces.

## Abstract

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.

High density arrays of topological defects in nematic liquid crystal are realized by two‐step interference photoalignment, with a minimal distance of 1.25 micrometer and disclination lines connecting defects on opposing substrates. Such layers can be used as reconfigurable templates for photonic devices to achieve large angle diffraction, beam shaping with large numerical aperture, or bistable smart windows.