Photo-aligned LC cell with weak anchoring energy and specific profiles : physics & applications

One of the main issues in liquid-crystal (LC) displays and tunable optical LC devices is the control of the anchoring on the substrates. A change in time of the anchoring directions leads to a decrease of the optical contrast and/or the formation of shadow images, even a change of the thresholds in LC devices. In this thesis, we investigated two kinds of surface dynamics that characterize property of the substrate to align the nematic on its surface and hence govern the surface anchoring. The slow azimuthal reorientation dynamics and polar (zenithal) director reorientation were studied based on a weak anchoring surface between LC molecule and substrates which is achieved by a special LC alignment technology called photo-alignment. Based on the study about the weak anchoring surface and properties, we expanded the investigation in physics into application fields by applying such weak anchoring surface into hollow optical fiber, silicon waveguides and other specific surface profiles. With the controllable anchoring energy on the sample surfaces and the results of weak anchoring surface study above, we achieved successful liquid crystal alignments in expected orientations, built mathematical model and estimated the possible threshold voltages to make switching inside the fiber and waveguide. By observing the spectrum shift of the silicon waveguide resonator under different LC alignment conditions, we proved LC alignments in such weak anchoring surfaces, can be used broadly in optical tunable devices. The first part of this thesis gave brief introductions about LC fundamentals and surface anchoring studies worldwide so far. Then the following two chapters covered comprehensive experimental data, theoretical calculations, observations and proof of hypotheses on the study of slow azimuthal reorientation phenomena and the polar (zenithal) director reorientation based on the weak anchoring surfaces. The chapter 4 and chapter 5 mentioned the two related applications of such weak anchoring surface study, which were successfully tested in hollow optical fiber and silicon waveguides. Theoretical estimations and experimental results were exciting and provided a possible way of using LC materials as the tunable units in optical devices