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Illumination & Displays

Binary liquid-crystal alignment and its applications

A surface-treated alignment layer and dye adsorption onto the polymer surface in dye-doped liquid crystals can be combined to fabricate viewing-angle-dependent LCDs.
15 March 2011, SPIE Newsroom. DOI: 10.1117/2.1201102.003527

Liquid-crystal (LC) alignment—including homogeneous, vertical, and hybrid alignment (HA, VA, and HB, respectively), twisted nematics (TN), and HA with pretilt angles—is widely used for device fabrication, including for LCDs and waveplates. In particular, binary LC alignments (the presence of two different, common LC alignments in a specific area) have received considerable attention. They include orthogonal HA/HB/TN, HA/VA, HA/HB, HA/TN, HB/TN, VA/HB, and VA/TN configurations and can be used to produce phase-modulated structures. Here, we present an approach in which we combine a surface-treated alignment layer and a photoalignment film that is dye-adsorbed onto the polymer surface in dye-doped LCs (DDLCs). This enables us to obtain binary LC alignment and develop viewing-angle-dependent LCDs.1,2

We have discussed photoalignment based on dye adsorption1–4 and have shown that the optical adsorption of dyes onto substrates dominates LC photoalignment through the guest-host effect.5 It also yields an easy axis that is perpendicular to both the polarization direction and the propagation direction of the incident beam. In addition, we previously found that the adsorption rate of methyl red (MR) dyes onto a polymer surface, NOA81, markedly exceeds that onto indium tin oxide (ITO)-coated glass substrates.4 Here we show that the experimental and simulated (using 1D-DIMOS modeling software) transmittances of incident light through a DDLC sample (observed from various viewing directions) closely match.

We used an LC:MR mixing ratio of 99:1% by weight. A layer of NOA81 was spin-coated onto an ITO-coated glass substrate that was covered with a homeotropic alignment film of N, N-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP). After coating, the substrate was irradiated under unpolarized UV light through a stripe-type photomask with alternating opaque and transparent stripes. We next removed the prepolymers in the nonpolymerized regions and generated UV-cured stripe-type polymer patterns on the substrate. Each empty cell was fabricated by combining one substrate with polymer patterns and one with DMOAP film. The mixed compound of DDLCs was then injected into the empty cell and illuminated by linearly polarized green-laser light. Notably, the excited dyes are only anisotropically adsorbed onto the polymer but not onto the DMOAP surface because of the difference between the MR-adsorption rates onto NOA81 and DMOAP surfaces. Therefore, we achieved binary LC alignments of HB and VA in regions with and without polymers, respectively.

Figure 1 displays the fabrication steps involved to realize the specially designed picture. First, the picture of Figure 1(b) was striped with transparent lines, resulting in that of Figure 1(c). Next, the striped picture was superimposed onto Figure 1(d) to yield the specially designed picture, Figure 1(a). Finally, we fabricated a viewing-angle-dependent LCD by placing a DDLC sample with HB and VA alignments—sandwiched between two crossed polarizers with transmission axes under ±45° with the long axis of the adsorbed MR—on top of the specially designed picture.


Figure 1. (a) Specially designed picture obtained by striping image (b) with transparent lines to yield (c). Combining (c) and (d) yields the specially designed picture (a).

Figure 2 shows observations in various viewing directions. Figure 2(a) presents the image seen in the normal direction, with polar angle θ = 0°. Clearly, the LC-alignment type in the dark stripes is VA, while that in the bright stripes is HB, since phase retardation—2πd(neffno)/λ = 2πdΔn/λ, where d, neff, no, and λ are the cell gap, the effective and ordinary indices of refraction, and the wavelength of the incident light, respectively—was absent (Δn = 0) in VA regions (dark) but present (Δn ≠ 0) in HB regions (bright) at normal incidence under ±45° crossed polarizers. Figure 2(b) shows the image as observed from a viewing direction of approximately θ = 20° and azimuthal angle φ = 60°. The LC-alignment type in the bright stripes is VA, while that in the dark stripes is HB. The light leakage (darkness) in VA (HB) regions is caused by phase retardation, ≠2nπ (= 2nπ), where n is an integer. In a viewing direction of approximately θ = 30 and φ = 180°, the phase retardations in the VA and HB regions are not 2nπ. Figure 2(a) and 2(b) can be displayed simultaneously: see Figure 2(c). In addition, when the viewing angle is around θ = 85 and φ = 225°, all regions in the DDLC sample are dark—see Figure 2(d)—because the phase retardations in these two LC alignments are 2nπ.


Figure 2. Fabricated LCD observed in various viewing directions. (a) Polar angle θ = 0°. (b) θ = 20 and azimuthal angle φ = 60°. (c) θ = 30 and φ = 180°. (d) θ = 85 and φ = 225°.

We also simulated the transmittance of incident light through a DDLC sample sandwiched between two crossed polarizers using 1D-DIMOS software. Figure 3(a) and 3(b) show the simulated transmittance of VA and HB DDLC samples, respectively. The transmittances in viewing directions A to D are consistent with those presented in Figure 2(a) to 2(d), respectively.


Figure 3. Simulated transmittance of incident beam through (a) VA and (b) HB DDLC samples at various viewing angles. θ and φin regions (viewing directions) A, B, C, and D are θ = 0°, θ = 20 and φ = 60°, θ = 30 and φ = 180°, and θ = 85 and φ = 225°, respectively, corresponding to Figure 2(a) to 2(d).

In summary, we have presented an approach for achieving various binary LC alignments using a surface-treated alignment layer and dye adsorption onto the polymer surface in DDLCs. We have also demonstrated application of binary LC alignments (VA/HB) to fabricate a viewing-angle-dependent LCD. Importantly, the proposed approach is highly promising for use in developing LC-based optical devices, including phase gratings, Fresnel lenses, and single-cell-gap transflective LCDs. This represents the direction of our ongoing research.


Ko-Ting Cheng
Department of Physics, National Cheng Kung University
Tainan, Taiwan

Ko-Ting Cheng is an assistant research fellow. His current research interests are based on LC physics, including photoalignment, biphotonic effects, holography, nanoparticles, polymers, lenses, blue-phase LCs, and flexible LC devices.

Andy Ying-Guey Fuh
Electro-Optical Laboratory, Department of Physics, National Cheng Kung University
Tainan, Taiwan

Andy Ying-Guey Fuh is a SPIE Fellow, Distinguished Professor of physics, and dean of science.


References:
1. A. Y. G. Fuh, J. C. Chen, S. Y. Huang, K. T. Cheng, Binary liquid crystal alignments based on photoalignment in azo dye-doped liquid crystals and their application, Appl. Phys. Lett. 96, pp. 051103, 2010.
2. A. Y. G. Fuh, J. C. Chen, K. T. Cheng, S. Y. Huang, Polarization-independent and electrically tunable liquid crystal Fresnel lenses based on photoalignment in dye-doped liquid crystals, J. Soc. Inf. Disp. 18, pp. 572, 2010.
3. A. Y. G. Fuh, C. C. Chen, C. K. Liu, K. T. Cheng, Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals, Opt. Express 17, pp. 7088, 2009.
4. S. Y. Huang, S. T. Wu, A. Y. G. Fuh, Optically switchable twist nematic grating based on a dye-doped liquid crystal film, Appl. Phys. Lett. 88, pp. 041104, 2006.
5. G. H. Heilmeier, L. A. Zanoni, Guest-host interactions in nematic liquid crystals: a new electro-optic effect, Appl. Phys. Lett. 13, pp. 91, 1968.