| Photonic crystals are dielectric substances with well defined periodicity that are
structured to exhibit photonic band gap characteristics analogous to the electronic band
gaps of semiconductor. Photonic band gap materials are fabricated by holographic
lithography involving patterned photopolymerization. Photopolymerization induced
phase transition is a phenomenon of non-equilibrium and non-linear phase transformation
involving liquid-liquid phase separation and mesophase ordering from isotropic liquid (or
melt) driven by photochemical reaction. Basically, holographic lithography technique
operates based on the principles of multi-beam optical interference in creating one- or
multi-dimensional periodic interference patterns with alternating low intensity and high
intensity profiles. These photo-patterns are imprinted onto the mixtures of photo-curable
monomers and liquid crystals to produce switchable photonic crystals. Although the
dielectric contrast in the holographic polymer dispersed liquid crystals (H-PDLC) may be
small relative to the inorganic counterparts, the electrical switchability makes such HPDLC
materials unique. The present talk focuses on experimental and theoretical
elucidation of the formation of polymer dispersed liquid crystals and photonic crystals
induced by photo-patterning. Theoretical modeling and simulation was undertaken in
order to provide guidance to fabrication of photonic crystals. The pattern forming aspects
of photo-polymerization induced phase transition in mixtures of emerging polymer and
nematic liquid crystal have been modeled in the context of the time-dependent Ginzburg
Landau equations (TDGL, Model C) pertaining to a conserved compositional order
parameter, and a non-conserved orientation order parameter coupled with the photopolymerization
reaction kinetics. The spatio-temporal growth of the photonic structure
has been simulated that shows striking resemblance to the observed morphology and
diffraction efficiency.
Biosketch
Kyu received Ph. D. in Polymer Chemistry, College of Engineering, Kyoto
University in 1980. Prior to joining the Department of Polymer Engineering, University
of Akron in 1983, he was a post-doctoral research fellow at the Department of Chemistry,
McGill University, Montreal, Canada, in 1980-81 and a research associate at the Polymer
Research Institute, University of Massachusetts, Amherst, in 1981-83. His research
interest encompasses phase equilibria and kinetics of phase separation in polymer blends;
phase transitions in crystalline and liquid crystalline polymers; molecular composites;
ion-exchange membranes; pattern formation dynamics and electro-optical properties of
dispersed liquid crystal and photonic crystals. |