Scott Meng

Holographic Polymer-Dispersed Liquid Crystals and Polymeric Photonic Crystals Formed by Holographic Photolithography

The present article describes the experimental and theoretical observations on the formation of holographic, polymer-dispersed, liquid crystals and electrically switchable, photonic crystals. A phase diagram of the starting mixture of nematic liquid crystal and photo-reactive triacrylate monomer was established by means of differential scanning calorimetry (DSC) and cloud point measurement. Photolithographic patterns were imprinted on the starting mixture of LC/triacrylate via multi-beam interference. A similar study was extended to a dendrimer/ photocurative mixture as well as to a single component system (tetra-acrylate). Theoretical modeling and numerical simulation were carried out based on the combination of Flory-Huggins free energy of mixing and Maier-Saupe free energy of nematic ordering. The combined free energy densities were incorporated into the time-dependent Ginzburg-Landau (Model C) equations coupled with the photopolymerization rate equation to elucidate the spatio-temporal structure growth. The 2-D photonic structures thus simulated were consistent with the experimental observations. Furthermore, 3-D simulation was performed to guide the fabrication of assorted photonic crystals under various beam-geometries. Electro-optical performance such as diffraction efficiency was evaluated during the pattern photopolymerization process and also as a function of driving voltage.

Three‐dimensional switchable polymer photonic crystals via various optical wave interference techniques

A theoretical study has been undertaken to elucidate the three‐dimensional pattern formation during holographic polymer‐dispersed liquid crystal fabrication employing various optical wave interference techniques. Initially miscible mixtures of nematic liquid crystal and reactive multifunctional monomer with a photosensitive initiator were exposed to geometrically arranged interfering beams of light, producing a spatially dependent intensity distribution within the sample. To mimic the spatio‐temporal evolution of periodic photonic structures in three dimensions, the time‐dependent Ginzburg–Landau Model C equations, coupled with spatially variant reaction rate equations, have been solved numerically incorporating the local free energy densities pertaining to isotropic mixing, nematic ordering, and network elasticity. The simulated results reveal some key observations during the formation of electrically switchable photonic crystals with few defects. It appears that the network elasticity term exerts profound effects on resultant structures, indicating that photonic crystals with fewer point defects may be fabricated in shorter times. The simulated results are in good qualitative agreement with reported experimental observations in respect of emerged patterns, length and time scales.