Numerical modeling of H-PDLC devices for diffractive optical elements with variable properties

Abstract

Holographic polymer dispersed liquid crystal devices (H-PDLC) are involved in many applications, e.g. diffraction lenses, optical data storage, and image capture devices. H-PDLC is based on a light-sensitive monomer and liquid crystal (LC) mixture exposed to an interference pattern. The monomer concentration rises in the illuminated area, whereas in the dark zones, the LC is concentrated, setting up LC droplets of fewer nanometers. Accurate knowledge of the elastic behaviour of the LC director distribution and the influence of the boundaries with the polymer matrix can help to optimise the diffraction efficiency or the angular selectivity of these devices, keeping the driving voltage low. Here, a review of the latest analysis for accurately estimating the director distribution in LC-based devices is shown. This analysis is carried on in three steps. Firstly, an accurate model based on creating a considerably high number of droplets surrounding the polymer matrix fringes is performed. In this step, the user can modify the ratio between the areas filled with LC and polymer and the size and aspect ratio of the droplets. This packing step can be very demanding depending on the thickness of the grating and the domain dimensions considered for the analysis (2D or 3D). Secondly, a formulation based on estimating the director distribution is performed to derive the permittivity tensor from the LC director. Thirdly and last, from the information obtained in the previous step, a Finite-Difference Time-Domain simulation is performed to estimate the electromagnetic field distribution inside the domain considered accurately. The diffraction efficiencies and many indirect parameters can be computed from this rigorous analysis.