Hiroshi Yokoyama

Photoinduced in-plane switching of a photochromic nematic liquid crystal

Photoinduced fast in-plane switching of the optic axis of a photochromic nematic liquid crystal is found in a sandwich cell with substrates promoting a twofold degenerate anchoring. The switching process is governed by the modification of the anchoring conditions associated with the photoisomerization of the photochromic nematic liquid crystal. Photoinduced in-plane reorientation of the sample optic axis of about 80° has been found. Together with the model of photoinduced in-plane switching, some implementations of this effect are briefly discussed.

Determination of surface nematic liquid crystal anchoring strength using nano-scale surface grooves.

Based on several nano-scale groove models, we propose a new technique to simultaneously determine the azimuthal and polar surface anchoring strengths of nematic liquid crystal (LC). The optical analysis of LCs on a grooved surface made by nanoimprinting lithography was performed on special alignment material, using a typical rubbing process. In our approach, using a polarizing microscope, we can determine the LC alignment exactly as it is in a parallel state, rather than a twisted state. This simple proposed method gives an accurate value of the surface LC anchoring of various surfaces, as well as simultaneously measuring the azimuthal and polar anchoring energy.

Molecular dynamics simulation of condensed-phase chiral molecular propellers.

Molecular dynamics simulations were performed for an axial-chiral liquid crystalline (LC) monolayer under trans-monolayer gas flow. The rotational dynamics of the monolayer chiral LC molecule along its long-molecular axis were analyzed at the molecular level. We found a precise correspondence between the flow-driven molecular rotation direction and molecular chirality as well as between the rotation direction and the trans-monolayer flow direction. The rotational direction exactly corresponded to what was expected in the proposed chiral molecular propeller model (Tabe, Y.; Yokoyama, H. Nat. Mater. 2003, 2, 806). Among the four trans-monolayer gas species we investigated, we found argon to be the most efficient at driving the chiral molecular propeller and helium the least efficient.

Artificial web of disclination lines in nematic liquid crystals

Disclinations are topological singularities of molecular arrangement in liquid crystals, which typically occur when the average orientation of molecules makes a π rotation along a fictitious closed loop taken inside the liquid crystal. Depending on the sense of molecular rotation, the disclination lines are either of 1/2 or −1/2 strength. When two disclination lines with the opposite strength meet, they are annihilated without trace. It is hence generally considered difficult in the nematic phase to stabilize a condensed array of free-standing disclination lines without the aid of topological objects like colloidal inclusions. Here we show that a free-standing web of 1/2-strength twist disclination lines can be stably formed in thin liquid crystal cells by means of a judicious combination of orientationally patterned confining surfaces fabricated by the micropatterned photoalignment technique. Theoretical model indicates that disclination lines are held apart at the intersection by a repulsive force generated by the Frank elasticity.Disclination lines are topological defects in molecular orientation widely found in liquid crystals. Here Wang et al. use a surface patterning technique to produce a very stable freestanding 3D array of ½ twist disclinations, which could be exploited in a variety of nanometre scale applications.

Azimuthal Dependence of Switching Field Strength for Nematic Liquid Crystal Bistability on Patterned Alignment Layers

We investigate the azimuthal dependence of the switching field strength for nematic liquid crystal bistability on patterned alignment layers to determine the validity of a switching theory proposed by Kim et al. [Appl. Phys. Lett. 78, 3055 (2001)] and to evaluate higher-order azimuthal anchoring coefficients. Director behavior during switching is described in detail for general azimuthal directions of an applied electric field and the experimental results are explained on the basis of the theory. We obtain azimuthal anchoring coefficients up to the second higher-order term in an expansion of the anchoring energy function. Detailed analysis of the director behavior reveals a close analogy between the proposed model of nematic liquid crystal bistability and a coherent rotation (Stoner-Wohlfarth) model of ferromagnetism. This suggests a reversal-asymmetric property of the substrate-nematic interface director, differing from the reversal symmetry of the bulk nematic director, as far as the nematic bistability...

Analysis of surface anchored lattice plane orientation in blue phase liquid crystal and its in-plane electric field-dependent capacitance response

Uniformly oriented macroscopic monodomain of cholesteric blue phase liquid crystal has been realised by the influence of surface anchoring. Orientation of the lattice planes in surface-treated (ST) and non-surface-treated (NST) cell were analysed and compared by Kossel diagram technique. NST cell has revealed the green and blue domains corresponding to reflection from oriented (110) and (112) planes of the body-centred cubic lattice. However, in the ST cell only the lattice plane (110) oriented uniformly and tailored the macroscopic monodomain. Electric field driven reorientation of the (110) lattice plane was noticed in NST cell whereas for ST cell such reorientation was absent. Two distinct electric field-induced capacitive responses have been observed in the two different cells. In NST cell anomalous electrostriction was observed, whereas for ST cell normal electrostriction was observed. Interestingly, the capacitance has decreased with an increasing electric field for anomalous electrostriction in NST cell, whereas for normal electrostriction in ST cell it was increased with increasing the field. Such a capacitive change behaviour is explained by dielectric anisotropic change followed by the electric field induced elongation and contraction of the cubic unit cell along and perpendicular to the electric field.

Reflective liquid crystal display for better productivity.

Most reflective LCDs so far proposed require a very thin cell gap of approximately 1.5xa0μm to satisfy the quarter-wave retardation condition when used with a half-wave retardation film to obtain an acceptable broadband performance over the entire visible range. On the other hand, the inevitable difficulty associated with precise manufacturing of thin cell gap devices is likely to deteriorate the yield, thereby increasing the production cost. This paper proposes a reflective LCD with a larger cell thickness to achieve better productivity. The proposed reflective LCD consists of a tactically arranged stack of a half-wave retardation film, a quarter-wave retardation film, and a liquid-crystal (LC) layer whose optical performance has been confirmed both by simulation and experiment. The optimal optical configuration to obtain an excellent dark state in the visible range was determined by the Mueller matrices calculus as applied to each optical component. The simulated and experimental results showed that the proposed reflective LC structure has excellent electro-optical properties and is expected to be useful for the next generation LCD industry.

Optical design for single-mode and single-cell gap transflective liquid crystal displays

Generally, for transflective liquid crystal displays with different modes and different cell gaps between the refractive and transmissive parts, precise process control to pattern the electrode and match the cell gaps may reduce the yield and thus, require high cost. This paper proposes a simple transflective liquid crystal display with a single-mode and single-cell thickness without a patterned electrode to achieve better productivity. The proposed transflective liquid crystal display consists of three half-wave retardation films, two quarter-wave retardation films, and an LC layer, whose optical performance was confirmed by both simulation and experiment. The optimal optical configuration to obtain an excellent dark state in the visible range was determined by the Mueller matrices calculus, which was applied to each optical component. The calculated and experimental results showed that the proposed transflective LC structure has excellent electro-optical properties and is expected to have many liquid crystal display applications.

Pre-organized liquid crystals: biaxial nature of laterally-connected dimer

Biaxial nematic liquid crystals have attracted much attention from both fundamental and application points of view, because the fast response based on the rotation of the minor director is expected. So far, different molecular designs have been proposed for the emergence of the biaxial nematic phase. Among that, we have been interested in applying preorganization concept on generating the biaxiality. Dimeric liquid crystal compounds have been prepared in line with this concept in which two mesogenic parts are linked by the biphenyl connecting group. The pre-organized dimmer shows an anomalous textural change, for vertically-aligned and free-standing film samples, at the smectic C (SmC)- nematic (N) phase transition, in which the Schlieren texture of the SmC changes into the other Schlieren texture of the N phase. There are two possible explanations for this textural change, i.e., the occurrence of the director change at the SmC-N phase transition or the emergence of biaxiality in the N phase. The electric-field-induced birefringence has also been measured in detail for investigating the biaxial nature of the sample.

On the emergence of mysterious liquid crystal phases: chirality and non-chirality issues

The structure–property correlation has been systematically investigated by the modification of the central core part of the dichiral materials so that the chiral and/or non-chiral issue having an effect on the formation of the layered or inter-connected structure can be studied. The central phenyl core of the dichiral compound showing the inter-connected ordered smectic Q phase has been modified in the following three ways: introduction of the substituent in the central phenyl core, introduction of the alkyl spacers between the central phenyl core and the peripheral mesogenic groups, and replacement of the central phenyl ring by the alkyl spacer. It has been found that (1) a symmetric structure stabilizes the tilted molecular ordering and the inter-connected structure, whereas the non-symmetric structure stabilizes the smectic A ordering; (2) a flexible molecular structure stabilizes the layered phase rather than the inter-connected; (3) increase in the degree of the segregation in the molecular assemblies stabilizes the anticlinic layered ordering; (4) increase in the local liquid-crystallinity stabilizes the layered structure; and (5) the ordered inter-connected structure is more sensitive to the chirality of the system than the disordered inter-connected structure.