Performance Of Holographic Polymer Dispersed Liquid Crystal Grating And Its Crystal Molecular Orientation

The holographic polymer dispersed liquid crystal Bragg grating is a grating structure which liquid crystal and polymer are periodically mixed. Due to its characteristics of high diffraction efficiency, fast response time, relative simple preparation and obvious light control switch, it has potential applications in many areas of photonic devices. The key parameter diffraction efficiency which determine the performance of HPDLC actual device already can stably reach higher than90%, however, as the driving voltage (15V/μm) is too high, its practical application is still restricted. Based on the analysis of polarization photomicrographs of a large number of HPDLC samples, one reason for this is the irregular sort of liquid crystal molecules in the grating. Therefore, the molecular motion is restricted, and a higher voltage is needed to drive the liquid crystal molecules to reach a refractive index. The paper focused on the orientation of liquid crystal molecules. It contains the following aspects:1. Some researches on the method of friction to achieve liquid crystal molecules’ orientation were done. By virtue of large amount of experiments, we made the rubbing process of the liquid crystal in a lab, and optimized the PI (polyimide) and NMP (N-methyl-pyrrolidone) concentration ratio, spin-coated PI film, assembling of liquid crystal cell and other details of the process parameters.2. We optimized the experimental optical system and the ratio of the mixture of materials for preparing a HPDLC Bragg grating. A HPDLC grating with a high diffraction efficiency (92%) was prepared. We probed the two-photon fluorescence of the nematic liquid crystal used in preparation of the grating. What’s more, fluorescence photos were also took.3. We made researches on the alignment of liquid crystals in HPDLC Bragg grating. By virtue of a series of processing, a better alignment of the liquid crystal molecules was formed. After fitting the curve of transmittance of the sample versus rotation angle of liquid crystal cell, we got molecules orientated in the sample. The liquid crystal anisotropy (An) is also well consistent with its nominal value. In addition, in order to do the measurement of the liquid crystal alignment, we also made an optical chopper which cost low and convenient.