Danding Huang

Photoalignment of Ferroelectric Liquid Crystals by Azodye Layers

The photoinduced alignment of ferroelectric liquid crystals (FLCs) onto photochemically stable azo-dye films was studied. The alignment quality of FLC display cells depends mainly on the difference between the FLC surface energy and the aligning substrates surface energy; however, the structure and thickness of FLC layers are also important. The effect of the thickness of photoaligning azodye layer on the alignment quality and multiplex operation of passively addressed FLC display cells has been investigated. An optimal (about 3–5 nm) azo-dye layer thickness that provides both the highest multiplex operation steadiness and the best contrast ratio of the FLC display cells was found. The photoaligned FLC display cells showed the contrast ratio CR>500:1 at the wavelength λ=0.63 µm both in surface stabilized and deformed helix FLC electrooptical modes.

Photoinduced ordering and anchoring properties of azo-dye films.

We study both theoretically and experimentally the anchoring properties of photoaligning azo-dye films in contact with a nematic liquid crystal depending on the photoinduced ordering of azo-dye molecules. In the mean field approximation, we found that the bare surface anchoring energy depends linearly on the azo-dye order parameter and the azimuthal anchoring strength decays to zero in the limit of vanishing photoinduced ordering. From the absorption dichroism spectra measured in azo-dye films that are prepared from an azo-dye derivative with polymerizable terminal groups we obtain the dependence of the dichroic ratio on the irradiation dose. We also measure the polar and azimuthal anchoring strengths in nematic liquid crystal (NLC) cells aligned by the azo-dye films and derive the anchoring strengths as functions of the dichroic ratio, which is proportional to the photoinduced order parameter. Although linear fitting of the experimental data for both anchoring strengths gives reasonable results, it, predicts vanishing of the azimuthal anchoring strength at some nonzero value of the azo-dye order parameter, in contradiction with theory. By using a simple phenomenological model we show that this discrepancy can be attributed to the difference between the surface and bulk order parameters in the films. The measured polar anchoring energy is found to be an order of magnitude higher than the azimuthal strength. Our theory suggests that the quadrupole term of the spherical harmonics expansion for the azo-dye-NLC intermolecular potential might be of importance for the understanding of this difference.

Effect of aligning layer thickness on photo-aligned ferroelectric liquid crystal displays

Effect of photoaligning azo-dye layers thickness on the alignment quality and multiplex operation of passively addressed ferroelectric liquid crystal (FLC) display cells has been investigated. An optimal (about 3-5 nm) azo-dye layer thickness, that provides both the highest multiplex operation steadiness and the best contrast ratio of FLC display cells was found.

Novel photoaligned twisted nematic liquid crystal cell

A novel photoaligned twisted nematic liquid crystal display (TN-LCD) cell is fabricated by one-step illumination with oblique nonpolarized UV light of an empty cell with azo-dye layers coated on indium tin oxide (ITO) substrates. If the incident angle is sufficiently large (>75°), the p-polarization of the light is more pronounced for the bottom azo-dye layer in comparison with that for the upper one. Thus, the photo-alignment of the azo-dye layer on the bottom substrate is perpendicular to that of the upper one, and LC directors on the top and bottom substrates also become perpendicular. The method provides a new simple way of manufacturing photoaligned TN-LCDs.

Design of polarizing color filters with double-liquid-crystal cells

A method of designing polarization rotators with double-liquid-crystal (LC) cells is presented. When placed between a polarizer and an analyzer, the polarization rotator becomes a polarizing color filter. Any required color can be generated by optimization of the parameters of the double-LC layers. One specific example of a green filter is given. This filter is analyzed in terms of the optical performance, including transmission spectrum, color coordinates, and viewing angle. A sample green polarizing color filter was made and compared with the theoretical results.

37.3: Optimization of Photo-Aligned Asymmetric Ferroelectric Liquid Crystal Display Performance: Effect of Planar Layer

Photo-aligned ferroelectric liquid crystal (FLC) display reveals a perfect alignment in case of asymmetric boundary conditions, when the competition in aligning action of two solid surfaces is avoided. In this case a stable uniform photo-alignment on one of the cell substrates spreads out to the other, which is prepared to sustain planar alignment with a degenerate azimuthal direction. In the paper, a thin polyimide film was involved to improve smoothness of planar substrate, resulting in the better electrooptical properties of FLC display. A contrast ratio of 300:1 in a static addressing mode and 82:1 in a multiplex mode was demonstrated together with a perfect bistable switching of FLC display in static and multiplex addressing regimes.

44.3: Photo‐Aligned Passively Addressed Ferroelectric Liquid Crystal Display Cells: Effect of Aligning Layer Thickness

of photoaligning azo-dye layers thickness on the alignment quality and multiplex operation of passively addressed ferroelectric liquid crystal (FLC) 1.5µm display cells has been investigated. An optimal (about 3~5 nm) azo-dye layer thickness, that provides both the highest multiplex operation steadiness and the best contrast ratio of FLC display cells was found. simply washed in N,N-dimethylformamide (DMF) and covered with 1.5µm calibrated spacers. The various azo-dye SD-1 layer thickness onto ITO surface has been provided by different azo- dye concentrations in DMF solutions from 0.2% up to 1.3%. The solutions were spin-coated onto ITO electrode at 3500 rpm, and dried at 100°C. The UV light was irradiated onto surfaces of SD-1 layers during 1 hour using a super-high pressure Hg lamp, an interference filter at 365 nm, and a polarizing filter. The light intensity on the surface of layer was 6 mW/cm 2 for the polarized light. The thickness of the azo-dye layer has been measured using the AFM method. The SD-1 film was formed on surfaces of test flat quartz plates, then a scratch line of about 10µm in width was produced using a tungsten tip. The surface with scratch line was scanned by AFM using a tapping mode (i.e. AFM tip is tapping onto the surface with a high frequency rather than contacting the surface continuously to prevent the damage of the film). The thickness was obtained from the cross-section analysis of the AFM scanning image. The film was discontinuous around dust particles with typical explosion regions of several micrometers, so one can measure the step of the film, and it agreed well with the scratch line method. The SD-1 layer thickness was varied from 0 to 12 nm dependent on the concentration of SD-1 in DMF as shown in Fig. 2.