Holographic Polymer Dispersed Liquid Crystal Bragg Grating Mechanism And Performance Optimization

Good photon sensitivity, large index modulation and high resolution make monomer in a number of applications such as holographic polymer grating, photonic crystal and light phase modulator. In fabrication of such devices, diffusablity and reaction of monomer have a great effect on the performance of devices. Hence, how to determine the diffusion and reaction coefficients is of significance on the improvement of devices performance. Thus, it is desirable to determine the diffusion and reaction coefficients of monomer universally under convenient conditions. Besides, high diffraction efficiency, low threshold and turn-off voltages and fast response time are necessary gauges to evaluate the holographic polymer dispersed liquid crystal Bragg volume grating. Therefore, this article can be summarized into the following two sides:1. Theoretically, analytical expressions of diffusion and reaction coefficients were derived according to Zhao and Mouroulis's one dimensional diffusion-reaction model and in combination with two-wave coupling theory. Small discrepancies among parameters values determined in multiple experiments under same condition and good agreement with theoretical prediction suggested the feasibility of this method. Thereafter, simulations of the build-up of grating diffraction efficiency were conducted by adjusting the decay constants through solving the finite difference equations under implicit scheme. Taking post curing into account, decay constants were specific for curing and post curing processes. The results showed great agreement between the simulated curves and experimental real-time traces. Moreover, the one dimensional diffusion-reaction model was revised by introducing the radicals' concentration and lives instead of the reaction dependence on recording beam intensity, which interpreted theoretically the post curing very well.2. Experimentally, grating with diffraction efficiency as high as 96% was fabricated by optimizing the recording time making use of post curing effect. In addition, threshold and turn-off voltages of the grating were reduced to 1.5 V/μm and 3.5 V/μm respectively by means of introducing some amount of NOA65 into the recipe. Response times of the up and down slopes were measured to be 250ms and 153 ms respectively. In the end, alignment of liquid crystal molecules was investigated through optically polarized microscope and confocal microscope.