Near-field birefringence response in thickness direction of liquid crystal thin film

Abstract

Information of molecular orientation in nematic LC (liquid crystal) is attractive and important for application in the field of display device. In this paper, we demonstrate a novel method using Birefringence Scanning Near-field Optical Microscope (Bi-SNOM) with a probe which is inserted into the LC thin film to detect the molecular orientation from its birefringence responses in the thickness direction of LC thin film. The probe is laterally vibrated while going forward into LC thin film, and the retardation and azimuth angle are being recorded as the probe going down. Since the affection of shear force acts as a stimulation to LC molecules, the orientation of molecules is changed and reorientated. In this study, LC thin film on homeotropic alignment LC film and homogenious alignment LC film were measured. In the case of homogenious alignment LC film, we propose two experiments; one is the experiment in which the vibration direction of probe is vertical to the alignment direction of PI film, and in the other experiment, we vibrated the probe in the direction parallel to the rubbing direction of alignment layer. We also compared the data measured with no vibration probe and the data measured with probe vibrated vertical to the alignment direction. As results, we obtained the orientation of molecules above the alignment layer by the birefringence response of LC molecules to the disturbance of vibrating probe and the anchoring extrapolation length by Polyimide (PI) alignment substrate. Ultimately, the LC thin film can be modeled in thickness direction from all the results using this method.