Micro Structure and Optical Performance of Topological Defects in Nematic Liquid Crystal

Sheng-Kai Yao¹, Jieh-Wen Tsung^1*

¹Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan

* Presenter:Jieh-Wen Tsung, email:jiehwen.tsung@nycu.edu.tw

Topological defects are the discontinuous place in ordered material, which can be generated by confinement, frustrations or phase transition. In this study, topological defects in liquid crystal were observed under a polarized optical microscope. I inspected the director field of liquid crystal in three-dimensional (3D) space and then drew the structure of them based on the birefringence of liquid crystal. Vertically aligned (VA) liquid crystal cells were fabricated. Patterned transparent electrodes were designed and manufactured. Pad, hole, coil crossed strip electrodes were fabricated to create various closed boundaries in the liquid crystal cells, enforcing the liquid crystal to have topological defects. The liquid crystal cell was assembled and then the cell gap measured. In this experiment, point defects in 3D space, point defects on two-dimensional (2D) planes, and ring defects were created. Then, the director field of liquid crystal around a topological defect was derived based on the birefringence colors and the transmittance of light under a polarized optical microscope. The experimental result shows that when the liquid crystal molecules are strongly compressed by applying a large electric field, a ring defect is formed instead of a point defect. The result is consistent with the theoretical prediction. In addition, when light passes through the topological defect, the optical phase difference (OPD) at the defect core and the OPD around the defect are different, leading to diffraction of light. The microscope image was carefully analyzed to measure the local transmittance. The optical transmittance profile due to retardation and the one due to diffraction were compared. Finally, radial escaped defects and hyperbolic hedgehog defects were generated in a pair above a hole on the electrode. The annihilation process was recorded, and bistable state can be switched by adjusting the voltage ramp. Topological defects diffract and scatter
light. If topological defects are periodically arranged to form defect arrays, they can be applied to liquid crystal devices such as electronically controlled gratings, smart glass, and optical vortex generators.

Keywords: topological defect, nematic liquid crystal, polarized optical microscopy, phase grating, diffraction