Photopolymerization of liquid crystal monomers initiated by means of a dichroic photoinitiator provides an additional
degree of freedom in controlling the morphology and structure of the liquid crystal networks formed. The absorption of
the dichroic photoinitiator, and thereby its initiation rate, depends on its position towards the transversal light beam used
for polymerization as well as its position towards the polarization of the light beam. The photoinitiator adapts the
director profile of the liquid crystal monomer. As a result planar oriented areas aligned orthogonal to the propagation
direction of the light beam polymerize faster than the ones parallel to it. Similarly, planar aligned areas with their
orientation parallel to the electrical field vector of the light polymerize faster than the planar aligned areas oriented
perpendicular to that. Based on this principle complex lithographic structures are built, not only forming structures in the
plane of the polymerizing film but also in the third dimension along its cross-section. Additionally, applying the dichroic
photoinitiator together with the principle of polymerization induced diffusion in monomer blends provides a wealth of
new structures, especially when combined further with complicated, but well-controlled, morphologies such as those of
twisted, splayed and cholesteric liquid crystal monomers.
The filling is reported of the air holes of an InP-based two-dimensional photonic crystal with solid polymer and with liquid crystal 5CB. The polymer filling is obtained by thermal polymerization of an infiltrated liquid monomer, trimethylolpropane triacrylate. The filling procedure for both the monomer and liquid crystal relies on the capillary action of the liquid inside the ~ 200 nm diameter and < 2.5 μm deep air holes. The solid polymer infiltration result was directly inspected by cross-sectional scanning electron microscopy. It was observed that the holes are fully filled to the bottom. The photonic crystals were optically characterized by transmission measurements around the 1.5 μm wavelength band both before and after infiltration. The observed high-frequency band edge shifts are consistent with close to 100% filling, for both the polymer and the liquid crystal. No differences were observed for filling under vacuum or ambient, indicating that the air diffuses efficiently through the liquid infiltrates, in agreement with estimates based on the capillary pressure rise.
Polymer filling of the air holes of indiumphosphide based two-dimensional photonic crystals is reported. The filling is
performed by infiltration with a liquid monomer and solidification of the infill in situ by thermal polymerization.
Complete hole filling is obtained with infiltration under ambient pressure. This conclusion is based both on cross-sectional
scanning electron microscope inspection of the filled samples as well as on optical transmission
Dynamic secondary ion mass spectrometry (SIMS) is usually applied to measure depth profiles in inorganic multi-layer systems. SIMS on organic multi-layer samples is highly complicated due to the complex fragmentation of the sample which results in fingerprint of masses representing the components in the sample. Using multivariate statistics, we succeeded to interpret the SIMS spectra and were able to identify layers with different compositions in artificially produced two-layer samples. The method is demonstrated for samples of a poly(isobornylmethacrylate) coating on a polymer dispersed liquid crystal consisting of the nematic liquid crystal (E7) and poly(isobornylmethacrylate). Quantification of the E7 concentration is complicated by evaporation in the vacuum system. Infrared spectroscopy proved that the loss of E7 from poly(isobornylmethacrylate) can be prevented by capping the sample with poly(vinyl alcohol). Cooling to cryogenic temperatures will be required to suppress further evaporation during SIMS analysis. The SIMS depth resolution of a two-layered sample was determined by discriminant function analysis to be 130 nm at a depth of one micrometer, which allows the application of SIMS for a typical optical grating.