Thermally switched liquid crystal alignment based on rubbed poly(N-vinyl carbazole) films
Liquid crystal (LC) applications including displays and polarization rotators have received considerable attention in recent decades owing to the potential of the technology for electro-optical devices and consumer products. In particular, methods of fabricating linear polarization rotators—for use in beam splitters, lenses, filters, and phase modulators—have been widely studied. These methods include two-direction rubbing1 and photoalignment by dye adsorption2 typically onto poly(vinyl alcohol) (PVA) or polyimide-coated glass substrates. Linear polarization rotators made using these approaches can rotate the direction of polarization of incident light by between 0 and 90°.
Poly(N-vinyl carbazole) (PVK) is a photoconductive material used in multilayer LEDs to improve their efficiency. Mechanically rubbing a layer of PVK induces homogeneous alignment of LCs with their director axes perpendicular to the rubbing direction,3 whereas conventional PVA and polyimide layers show parallel alignment. The group of Hasegawa reported that the position of phenyl rings in polystyrene (whose chemical properties are similar to those of PVK) determines the direction of alignment of the LCs.4 Nakajima and co-workers further showed that the unidirectional alignment of LCs provided by the polystyrene film can be changed toward the direction of rubbing by heat treatment at various temperatures owing to micro-Brownian motion (i.e., molecular agitation).5 These experimental results suggest that heat-switched LC alignments can be used to make polarization rotators. Here, we report a novel and simple approach using thermally treated PVK to fabricate both linear and concentric polarization rotators.6 Our method has the advantage of being low cost, fast, commercializable, and scalable.
To layer a PVK (Aldrich) film onto an indium tin oxide-coated glass substrate, we mixed a solvent (chlorobenzene) and PVK in a weight ratio of 98.36:1.64. Then, we spin-coated the solution onto the surface of the slide. We then prebaked the substrates in an oven at 80°C for 20min and postbaked them at 120°C for 120min. We fabricated a 90° twisted nematic (TN) cell (cell gap ∼12μm) using one PVK-coated substrate and another that was coated with an alignment film of PVA. Finally, we filled an empty cell homogeneously with the nematic LC—E7 (Fusol Material Co.)—and sealed the cell's edges with epoxy to fabricate a TN LC sample. We heated the cell to study the thermally switched alignment.
Figure 1 plots the variation with temperature of the stable transmittance of the sample. Initially, we placed it between two parallel polarizers that were in normal black mode, with the probe beam normally incident to the sample from the PVA side. The transmission axes of the polarizers were set parallel to the rubbing direction (R). When the sample was heated, the transmittance initially remained almost unchanged, and it then gradually increased with temperature above the threshold of ∼33°C. Ultimately, the transmittance (∼0.95) saturated at ∼41°C, which was below the clearing temperature of the LCs. (The twisted angle decreases with increasing temperature.) The effective anchoring (torque) resulted from the combination of the side and main chains of the molecules of the PVK film. Accordingly, thermally switched LC anchoring, which aligns LCs at angles from 90° to 0° with respect to R, becomes stronger as temperature increases. Moreover, when the heated sample (∼45°C) was cooled down to room temperature, the stable transmittance remained almost unchanged. We concluded, therefore, that thermally switched LC alignment is irreversible. This stability is important for the potential application of our technology to electro-optical systems such as axial polarizers and optical tweezers, among others.
We fabricated a concentric polarization rotator using the experimental setup shown in Figure 2(a). We used a solid copper cone as a medium to transmit heat from the hot plate to the TN LC cell, which we made by combining substrates with rubbed PVA and PVK films. The diameters of the upper and lower contact areas of the cone were 1 and 11mm, respectively. We set the temperature of the hot plate to ∼45°C to heat the sample for 1min. Afterwards, it was left to cool naturally to room temperature. The thermal diffusion in the area in contact with the upper cone established a radial thermal gradient. Figure 2(b) shows the top-view LC configuration in the concentric polarization rotator. The LC alignment of the central region was homogeneous, and the region that was far from the heated cone remained in the 90° TN state.
Figure 3(a–d) presents photographs of the our concentric rotator observed under two polarizers in which P (the polarizer transmission axis) was parallel to R, and A (the analyzer transmission axis) was set at 0, 45, 90, and 135° in relation to R. Clearly, the continually rotating angles of the rotator change from 0° (center) to 90° (margin). The transmittances were also measured at various positions to determine the range of continuous rotation. To verify the experimental results, we calculated the theoretical transmittance of the linear polarization rotator according to the Jones matrix method.1, 2 The transmittance of the LC cell can be expressed as a function of position:
where T0 denotes the maximum transmittance of the LC cell and Y(X) is the distance between y1(x1) and y2(x2) along the y(x) axis. Figure 3(e) and (f) plots the transmittance versus position curves (dotted lines) that were detected along X and Y directions, respectively. As measured, the contrast ratio of the concentric polarization rotator was 240:1. Additionally, setting y1(x1) and y2(x2) to 0.1 (0.1) and 2.6 (2.8), and y1′ (x1′) and y2′ (x2′) to 3.7 (3.9) and 6.7 (6.5) in Equation (1) yields the theoretical transmittance versus position curves—see Figure 3(e) and (f) (solid lines)—which closely matches the experimental results (dotted lines). Applied in a circular variable neutral density filter, such a concentric polarization rotator can change a light beam with spatially uniform intensity to a Gaussian-like or a donut-like beam.
In summary, we described the fabrication of thermally switched LC alignments based on rubbed PVK films and their use in constructing concentric polarization rotators. These rotators have the potential to be used in optical applications such as beam forming, density beam splitters, and circular variable neutral density filters. In the future, we plan to extend thermally switched LC alignments based on rubbed PVK films to optically induced, thermally switched alignments by doping the LCs with dyes.
Ko-Ting Cheng is an assistant research scholar. His current research interests focus on various aspects of LCs, including photoalignment, blue phases, flexible electronics, novel displays, nanoparticle doping, polymers, holography, and lenses.
