Doo-Han Chung

Competitive effects of grooves and photoalignment on nematic liquid-crystal alignment using azobenzene polymer

Effect of an anisotropic photoalignment layer and microgrooves on nematic liquid-crystal (LC) alignment was quantitatively examined using azobenzene polymer thin film with surface relief grating (SRG) of about 1 μm pitch. The SRG with various modulation depths was treated with polarized light irradiation to align molecules at 45° from the groove. Nematic LC molecules, 4′-n-pentyl-4-cyanobiphenyl, orient to the photoaligned direction on the SRG being shallower than 200 nm. The orientation rather sharply deviates from the photoaligned direction toward the groove direction with increasing grating depth into the deeper region than 200 nm and finally becomes parallel to the grooves on the SRG of 400 nm deep. This behavior is successfully simulated by the consideration of anisotropic surface interaction and an elastic energy of LCs.

Alignment Mechanism of Nematic Liquid Crystal on Rubbed Polymer Surface Studied by Subsequent Processes of Rubbing and Photoalignment

A novel method for understanding the alignment mechanism was motivated by the texture observation of a nematic liquid crystal (LC) contacted with a photoaligned layer after rubbing. Reorientation of director occurs by subsequent photoalignment to different direction from that forced by rubbing. Moreover, it was found by polarized absorption spectra that the preferential average main chain axis over whole the alignment layer does not change, indicating that the orientation change by photoalignment occurs only at very top surfaces. This experiment without changing surface morphology indicates that the alignment priority for the nematic LC is mainly governed by the anisotropic short-range intermolecular interaction between alignment films and LC molecules and the effect of microgrooves plays a minor role.

Comparative Study of Anchoring Strengths for Photo- and Rubbing-Aligned Liquid Crystals on Photoisomerizable Polyimide Alignment Films

The high-electric-field technique was used to measure the out-of-plane (polar) anchoring strength at the interface between a nematic liquid crystal, pentylcyanobiphenyl (5CB), and a photosensitive polyimide (PI) alignment layer possessing an azo group in the main chain. The anchoring energy of the photoaligned state was found to be rather strong, on the order of 0.1 mJ/m2, being at least half of the anchoring energy on the rubbed surface of the same azo PI. The anchoring energy exhibited a conspicuous peak near the clearing temperature of 5CB, particularly in the photoaligned system, indicating a significant role of molecular motion in anchoring behavior.

Liquid Crystals Align Liquid Crystals

We have investigated the molecular orientation of a liquid crystal (LC), pentylcyanobiphenyl (5CB), evaporated on a rubbed polymer by means of polarized ultraviolet absorption spectroscopy and surface second-harmonic generation. The results unambiguously reveal that the in-plane anisotropy is diminishingly small before the molecules form a monolayer, although films thicker than one monolayer have a finite in-plane anisotropy. This drastic change indicates that LC molecules on rubbed surfaces do not align well without the influence of the intermolecular interaction of LCs. Actually we confirmed that nonliquid-crystalline butylcyanobipheny (4CB) exhibits no orientation on the rubbed polymer surface regardless of thickness.

Visualization of Rubbing Nonuniformity by Double Surface Treatments of Polyimide-Coated Substrate for Liquid Crystal Alignment

A new method for characterizing rubbing nonuniformity in liquid crystal cells is proposed. Homogeneously aligned cells made by rubbing apparently give a uniform texture under a polarizing microscope. However, nonuniformity of rubbing becomes apparent as scratched patterns, if the rubbing is preceded by rubbing or photoalignment processes. On the other hand, it was found that photoalignment gives quite a uniform texture even on a priori photoaligned surfaces. Thus the double surface treatments along different directions visualize the nonuniformity of rubbing, supplying a tool to examine the condition of the rubbing machine used including buffing cloths.

Alignment control of a nematic liquid crystal on a doubly treated substrate

We have investigated the molecular orientation of a nematic liquid crystal (LC) on a doubly treated substrate. One of the substrate surfaces was first treated with the photo-exposure or rubbing process and then subsequently treated with the second photo-exposure process as a function of the irradiation time while the other was prepared with a rubbed polyimide layer for strong anchoring. The LC orientation on the doubly treated substrate was well controlled either by the irradiation time or by the rubbing strength. The observed molecular orientation was consistent with the simulation results based on a torque balance equation. The anchoring energies of the treated substrates were determined from the least-squares fits of the experimental data to the torque balance equation.

Enhanced second-harmonic generation by use of the photonic effect in a ferroelectric smectic C* liquid crystal.

Special phase matching of second-harmonic generation (SHG) by use of the photonic effect was experimentally confirmed for an obliquely propagating fundamental beam in the helical smectic C>(*) liquid-crystal phase, as suggested by Belyakov [JETP Lett. 70, 811 (1999)]. The enhancement of SHG occurs when the second-harmonic wave is close to a photonic bandgap (half-pitch band) and consequently the fundamental wave is close to another photonic bandgap (full-pitch band). The enhanced signal was observed from both the transmitted and the reflected directions and was compared with our recently developed simulation.

Enhancement of Second-Harmonic Generation in Helical Ferroelectric Liquid-Crystal Cells under Oblique Incidence

A numerical method was introduced to study the second-harmonic generation (SHG) in helical ferroelectric liquid crystals. The level of approximation is the same as that of the 4×4 matrix method and any polarization conditions can be analyzed. A special phase matching of the SHG, which occurs when the fundamental and SH waves are located near the photonic band edges, was analyzed under the oblique incidence condition. For the material used in previous studies, numerical results showed some features at an incidence angle of 30°. (1) Enhanced SHG is possible when the wavelength of SHG matches the dips near the edges of the total-reflection half-pitch band in the linear reflection spectra. (2) The SH peak intensity is expected to show d3 dependence of cell thickness d.

Axial and Polar Orientational Changes by Rubbing/Photoalignment Processes in a Liquid Crystal Alignment Layer Studied by Optical Second-Harmonic Generation

Molecular reorientation by rubbing and photoalignment processes in polymer alignment layer for liquid crystals has been studied by linear optical absorption and surface optical second-harmonic generation. It was found that rubbing produces polar and axial orders in the photoisomerizable polyamic acid (PAA) containing azobenzene groups in the main chain, while photoalignment after rubbing destroys the polar order but is capable of reorienting the PAA main chain without creating further polar order.

Remarkable SHG enhancement by helical distributed feedback cavity spontaneously formed in ferroelectric liquid crystals

Remarkable enhancement of second-harmonic (SH) generation in a ferroelectric liquid crystal was theoretically studied. The enhancement occurs when SH wavelength coincides with the wavelength at odd-number-th dips of subsidiary oscillation in one edge of the selective reflection band. To interpret this behavior, the electric field of inhomogeneous and homogeneous SH light in a cell was simulated for a right-handed (R) ferroelectric liquid crystal. It was found that R-circularly polarized light forms a standing wave inside the cell at the odd-number-th dips, when two counter-propagating beams with the same polarization (e.g., R-R) are incident. On the other hand, inhomogeneous and homogeneous waves are out of phase to each other at the bounding surface at the even-number-th dips, so that no enhancement occurs. As for an incidence of R- and L-polarized light, enhanced SH light is generated only toward the R-incidence side. The SH light is L-polarized in this case, while R-polarized SHG is observed from both sides for the incidence of counterwaves with the same polarization.