Small, surface functionalized, Fe-doped LiNbO3 (FeLN) particles were experimentally investigated. Planar thin-cuts (0.5 – 1 mm thick) of FeLN crystals have recently been investigated as field generating substrates in LC test devices. Here, realignment of the liquid crystal (LC) director can be locally driven with photovoltaic fields, generated by the substrates (surface-fields, generated via the bulk photovoltaic effect). Such samples can be exposed with tightly focused light beams and show unexpectedly high non-linear optical responses, light induced defect formation; some samples could even reversibly store patters inscribed by the use of light. Here, it is investigated how the use of field-generating crystals be pushed further. Small particles are still suitable to control LC realignments – at least to a certain amount. Small particles of Fe-doped LiNbO3 and for reference also BaTiO3 were integrated in test devices. The particles were prepared in a top-down approach, that is larger specimen of ferroelectric crystals were first crushed with mortar and pestle to obtain a powder. From these powders, dispersions of particles were fabricated by high energy ball-milling. A small amount of the powder together with an organic solvent (continuum medium) was placed in the milling jars of a planetary ball mill. A small amount of a surfactant was added – both use of a conventional surfactant and in-house synthesized, photochemically active species was investigated. The dispersions obtained were used to coat the surfaces of LC test cells and for doping of polymer films. The samples obtained show many interesting and sometimes unexpected properties.
Polymer-network liquid crystals (LCs), where the response properties of a LC can be enhanced by the presence of a porous polymer network, are investigated. In the reported experiments, liquid crystals were doped with a small amount (< 10%) of photo-curable acrylate monomers. Samples with surface grafted photoinitiators, dissolvable photoinitiators, and samples with both kinds of photoinitiators were prepared. Both conventional (planar electrodes) and diffractive (interdigitated electrodes) test cells were used. These samples were exposed with a UV light source and changes of their capacitance were investigated with an LCR meter during exposure. Due to the presence of the in-situ generated polymer network, the electro-optic response properties of photo cured samples were enhanced. For example, their continuous phase modulation properties led to more localized responses in samples with interdigitated electrodes, which caused suppression of selected diffraction orders in the diffraction patterns recorded in polymer network LC samples. Moreover, capacitance changes were investigated during photopolymerization of a blue phase LC.
Exciting experimental results on the response properties of hybridized photo responsive liquid crystal test cells are reported, where iron doped lithium niobate substrates were used to photo generate electric fields and indium tin oxide coated cover glasses were used to confine these photo generated fields in a liquid crystal layer. Samples were investigated in a modified inverted optical polarizing microscope with white probe light (crossed polarizers) and exposed with a Gaussian laser beam focused to a small spot (14 μm FWHM). Test cells filled with nematic LC showed homeotropic director alignment. Upon exposure, this alignment was maintained at the exposure spot center and the LC director was selectively realigned in a surrounding single ring. This ring had a thickness of a few microns and its diameter increased with increasing exposure intensity (112 μm at 0.7 mW, 204 μm at 1.1 mW). This characteristic director realignment was traced back to the optically generated electric field distributions by simulations. In samples filled with chiral nematic LC, uniformly standing helix alignment was found. Textural transitions were induced at the focus position, which again led to the formation of well-defined circular defects. We could show that these defects can be permanently stored within the chiral nematic LC. Polarized optical microscopy of a rotated sample revealed that a point like defect with +1 topological charge was enclosed in each of these defects. Photovoltaic fields generated in small lithium niobate particles dispersed in a LC were found to cause promising optical responses and particle movement.
Continuous optical phase modulation was systematically investigated in a nematic host liquid crystal (LC), which was
stabilized with in-situ generated photopolymer. Various driving modes were investigated. With presence of a chiral
dopant, polarization independent and fast electro-optic responses were found in both blue phase mode and uniformly
standing helix mode. Anyway, high driving voltages > 100 V were required to achieve phase modulation depths > π in
reflective test cells with planar electrodes. In contrast, much lower driving voltages < 15 V were required in a polymernetwork
LC based on the same host LC if no chiral dopant was present. In this driving mode, high phase contrast and
sums of response times (ton+toff ) ≈ 3 – 4 ms were found, fast enough to achieve 200-400 Hz modulations.