Coherent diffraction and random scattering in thiol-ene–based holographic polymer-dispersed liquid crystal reflection gratings

Abstract

Bragg diffraction and random scattering in reflective holographic polymer-dispersed liquid crystal gratings are modeled using a matrix approach for a stack of low-high index layer pairs and an effective medium theory. Scattering is due to both random roughness of layer interfaces and random index variations within the layers. These are related to random liquid crystal droplet size and location as well as random orientation of the symmetry axes of bipolar droplets. Characteristic parameters governing coherent diffraction efficiency and random scattering are obtained partly from experiments, where possible, and partly from calculations based on a model of an effective medium applied to the grating. Calculations of grating transmittance are then compared to experimental transmittance spectra. Effects of scattering, primarily a decrease in baseline transmittance with wavelength and a small reduction in diffraction efficiency at the Bragg wavelength, are found to be due primarily to index inhomogeneities within the liquid-crystal-rich layers of the grating.