This paper demonstrates the thermally-switched liquid crystal (LC) alignments based on a rubbed poly(N-vinyl carbazole) (PVK) film, and their application for polarization rotators. The mechanically rubbed PVK layer can induce a planar alignment of LCs with their director axis perpendicular to the direction of rubbing. This direction can be switched toward the rubbing direction by thermal treatment. Experimentally, the angle of re-orientation of the director axis increases with the temperature in a specific range of temperatures. In this study, the optical properties of linear and concentric polarization rotators, fabricated using a rubbed PVK film with thermal treatment, are examined.
© 2011 OSA
Recently, liquid crystal (LC) applications, such as polarization rotators [1,2], axially symmetric LC devices [3–6], LC displays [7–9], and others, have been developed. A linear polarization rotator can continuously change the direction of polarization of linearly polarized light according to the position of the incident beam onto the linear polarization rotator. Methods for fabricating a linear polarization rotator have been widely studied. They include two-direction rubbing  and photoalignment by azo dye-adsorption onto substrates . Linear polarization rotators that are fabricated using the above two methods can rotate the direction of polarization of incident linearly polarized light by between 0° and 90°.
As is well known, many scientists have recently paid much attention to a photoconductive polymer material, poly(N-vinyl carbazole) (PVK) [10,11]. Notably, the mechanical rubbing of not only a PVK layer but also a polystyrene layer can induce a homogeneous alignment of LCs with their easy axes perpendicular to the rubbing direction . The properties of LCs that aligned by rubbed PVK are similar to those of LCs that are aligned by rubbed polystyrene. The key to aligning LCs perpendicular to the rubbing direction is the anchoring that is induced by the phenyl rings in the side chain fragments of PVK and polystyrene. Hasegawa et al. reported that the position of phenyl rings in polystyrene determines the direction of alignment of LCs . Accordingly, LCs are aligned along the phenyl rings by dipole-dipole interaction. Nakajima et al. also demonstrated that the unidirectional alignment of LCs provided by the polystyrene film can be changed toward the direction of rubbing by thermal treatment at various temperatures, because of the micro Brownian motion of the side and main chains .
This paper reports thermally-switched LC alignment based on rubbed PVK films, and their application for polarization rotators. Initially, a twisted nematic (TN) cell was fabricated by two indium-tin-oxide (ITO)-coated glass slides; the inner surface of one of the slides was coated with an alignment film of poly(vinyl alcohol) (PVA), while the other was coated with a PVK film. Both films were rubbed in the direction, R (Fig. 1 ). The fabricated TN cell was then heated to study the thermally-switched LC alignment. Experimentally, thermal treatment reduced the twisted angle of the TN LC cell, and finally a TN aligned LC cell became a homogeneously aligned one. Linear and concentric polarization rotators were demonstrated using this approach. The details of the fabrication process and the performance of the polarization rotators are presented below. It should be noted that the continually rotating range of the demonstrated polarization rotators can be varied if the LC material is properly selected. Additionally, pure PVK films cannot absorb the energy of visible light. Therefore, the polarization rotators can be operated in the broadband range of incident light, and have potential for the fabrication of phase modulators, spatial filters, LC lenses, and other optical elements because of the property of electrically controllable birefringence. The LC phase modulators can perform both phase-only and amplitude modulations, depending on the polarizations of the incident beam . Several reported LC lenses resulting from the spatial distribution of refractive indexes of the used LCs are polarization-dependent because the incident polarized light with various linear polarization experiences a different refractive index variation . Additionally, several polarization-independent LC lenses have also been developed, based notably on polarization-dependent refractive index change [17,18].
To coat a PVK (from Aldrich) film onto an ITO-coated glass substrate, a solvent (chlorobenzene) and PVK were mixed in a weight ratio of 98.36:1.64, and the solution was spin-coated onto the surface of the slide, called the command surface (SC). After coating, the substrates were pre-baked in an oven at 80 °C for 20 minutes and post-baked at 120 °C for 120 minutes. On the other ITO-coated glass slide, the reference surface (SR) was covered with a homogeneous alignment film of PVA. Both films were rubbed in the same direction, R. All empty cells, separated by 12-μm ball spacers, were fabricated by combining these two substrates with anti-parallel rubbing direction. Finally, an empty cell was homogeneously filled with the used nematic LC, E7 (from Fusol-material, with a clearing temperature (TC) ~61 °C), and the edges of the cell were sealed with epoxy to fabricate a TN LC sample.
3. Results and discussion
Figure 1(a) plots the variation with the temperature of the stable transmittance of the TN LC sample that was fabricated from the rubbed PVA and PVK substrates. Notably, the time required to reach stable transmittance was experimentally determined to depend on temperature. A lower temperature setting corresponds to a longer required duration. Initially, the TN LC sample was placed between two parallel polarizers that were in normally black mode, and the probe beam was normally incident onto SR. The transmission axes of the polarizers were set parallel to the rubbing direction. According to the experimental results, when the sample was heated, the transmittance initially remained almost unchanged, and then gradually increased with temperature above the threshold of ~33 °C. Finally, the transmittance (~0.95) saturated at temperature of ~41 °C, which was below the clearing temperature of the used LCs. The twisted angle decreases with the increase of temperature. This result can be easily understood since the LC alignment anchoring resulted from the rubbed PVK film before and after thermal treatments are orthogonal. The effective anchoring (torque) resulted from the combination of the side and main chains of the PVK film to align LCs toward the rubbing direction increases with the temperature. Accordingly, thermally-switched LC alignment anchoring to align LCs at angles from 90° to 0° with respect to R becomes stronger as temperature increases. Moreover, when the heated LC sample (~45 °C) was cooled down to the room temperature, the stable transmittance remained almost unchanged. Therefore, the thermally-switched LC alignment is irreversible. During the cooling process, the variation of the stable transmittance at different temperature is clearly approximately ± 4.2%. The cause can be understood because the thermal disturbance and the temperature-dependent refractive indexes of LCs cause the transmittance variation observed under two parallel or cross-polarizers [19–21]. Additionally, a separate experiment (not shown) revealed that the stable transmittance of the heated LC sample is still invariable after the heated LC sample has been repeatedly heated (45 °C) and cooled (25 °C) more than 30 times. Accordingly, the thermally switched LC alignment layer is definitely stable. Figures 1(b)–1(d) show images of the LC sample after thermal treatment (heating to ~45 °C followed by cooling to room temperature) when the rubbing direction (R) was at 0°, 45° and 90°, respectively, to the transmission axis of the polarizer in a crossed-polarized optical microscope (POM). Clearly, the absence (presence) of the phase retardation (2πdΔn/λ) in Figs. 1(b) and 1(d) [1(c)] was responsible for the dark (bright) state. Hence, the thermally-switched LC alignment anchoring is inferred to have been caused by the rubbed PVK film after thermal treatment changes the LC alignment from TN to homogeneous alignment. Such an alignment film is highly uniform. Notably, the threshold temperatures, or the so-called onset temperatures, defined as the temperatures required starting the switch of LC alignment, of various LCs are different. Experimentally, the onset temperatures of the three types of LCs, E7 (from Fusol-material, TC~61 °C), MDA-00-3461 (from Merck, TC~92 °C) and HFW59200-200 (from Fusol-material, TC~113 °C), were about 33, 75 and 100 °C, respectively. According to the properties of thermally-switched LC alignment that was based on rubbed PVK film, linear and concentric polarization rotators were successfully demonstrated using a thermal gradient.
The first part of the experiment determines the performance of a linear polarization rotator that is fabricated using the experimental setup that is shown in Fig. 2(a) . The LC directors in a linear polarization rotator are continuously twisted from 0° to 90° [1,2]. To obtain such a distribution of LC directors, part [area A, shown in Fig. 2(a)] of the TN LC sample that was fabricated by combining the substrates that were rubbed with PVA and PVK films was placed on a hot plate (at ~42 °C) to heat the LC sample for 1 minute, and was then naturally cooled to room temperature (~25 °C). The thermal diffusion from heating area A to area B yields a thermal gradient. Finally, the linear polarization rotator was formed. Figure 2(b) presents the LC directors in the fabricated linear polarization rotator. The LC directors close to the dotted line are changed from 90° TN to homogeneous, while those in region B, far from the hot plate remained 90° TN during heating. Thus, a continuous twist angle from 0° to 90° is demonstrated.
Figures 3(a) –3(d) display photographs of the fabricated linear polarization rotator observed under two polarizers, in which the transmission axis of the polarizer (P) is parallel to R and that of the analyzer (A) is set to 0°, 45°, 90° and 135° with respect to R, respectively. Clearly, the area observed under parallel- (crossed-) polarizers between A and B exhibits a gradually varying gray level from bright (dark) to dark (bright), as shown in Fig. 3(a) [3(c)]. Restated, the twist angles of the demonstrated polarization rotators do, indeed, continuously change from 0° to 90°. The transmittances of the polarization rotator, plotted in Fig. 3(e) (dots), were measured at various positions using an He-Ne laser (λ = 632.8 nm) to determine the range of the continuous rotation. The polarization of the used probe beam, which was normally incident onto the cell, placed between crossed polarizers, from the rubbed PVA film, was parallel to R. The spot size (diameter) of the probe beam was about 0.7 mm. The transmittance was measured from A (homogeneous) to B (TN). From Fig. 3(e), the continuously rotating length (from homogeneous to 90° TN) and the contrast ratio of the linear polarization rotator were approximately 3.6 mm (0.1 to 3.7 mm) and 350:1, respectively.
To verify the experimental results, the theoretical transmittance of the linear polarization rotator was calculated according to Jones matrix method [1,2]. Since the LC cell satisfied Mauguin’s condition, Δnd >> λ (where Δn, d and λ are the birefringence of LC, the cell gap and the wavelength of the probe beam, respectively), the transmittance of the LC cell can be expressed as a function of position as,Figure 3(e) plots the theoretical results (solid line), which match the experimental results well.
In the second part of the experiment, a concentric polarization rotator was fabricated using the experimental setup that is shown in Fig. 4(a) . A solid copper cone was utilized as a medium to transmit heat from the hot plate to the TN LC cell that was fabricated by combining substrates with rubbed PVA and PVK films. The diameters of the upper and lower contact areas of the solid copper cone were 1 and 11 mm, respectively. The temperature of the hot plate was set to ~45 °C to heat the LC sample for 1 minute. Afterwards, the sample was cooled naturally to room temperature. The thermal diffusion in the area in contact with the upper cone established a radial thermal gradient. Figure 4(b) depicts 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.
Figures 5(a) –5(d) present photographs of the fabricated concentric polarization rotator observed under two polarizers in which P was parallel with R, and A was set at 0°, 45°, 90° and 135° with respect to R, respectively. Clearly, the continually rotating angles of the concentric polarization rotator change from 0° (center) to 90° (margin). Also, the transmittances of the formed concentric polarization rotator were measured at various positions to determine the range of continuous rotation. Figures 5(e) and 5(f) plot the transmittance versus position curves (dotted lines) which were detected along X and Y directions, respectively. The contrast ratio of the concentric polarization rotator was measured to be 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 Eq. (1) yields the theoretical transmittance versus position curves, as shown in Figs. 5(e) and 5(f) (solid lines), which matched the experimental results (dotted lines) closely. Such a concentric polarization rotator can be applied in a circular variable neutral density filter.
In conclusion, the thermally-switched LC alignments based on rubbed PVK films, and their use in the fabrication of linear and concentric polarization rotators were successfully demonstrated. The key to fabricating such polarization rotators is to establish a thermal gradient. The optical properties of the fabricated polarization rotators agree well with theory. The polarization rotators have potential for use as beam forming applications, density beam splitters, circular variable neutral density filters, and other applications.
The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Grant No. NSC 98-2112-M-006-001-MY3 and NSC 99-2112-M-006-002-MY3. Additionally, this work is partially supported by Advanced Optoelectronic Technology Center as well.
References and links
1. H. Ren and S.-T. Wu, “Liquid-crystal-based linear polarization rotator,” Appl. Phys. Lett. 90(12), 121123 (2007). [CrossRef]
2. C.-Y. Huang, H.-Y. Tsai, Y.-H. Wang, C.-M. Huang, K.-Y. Lo, and C.-R. Lee, “Linear polarization rotators based on dye-doped liquid crystal cells,” Appl. Phys. Lett. 96(19), 191103 (2010). [CrossRef]
3. Y.-Y. Tzeng, S.-W. Ke, C.-L. Ting, A. Y. Fuh, and T. H. Lin, “Axially symmetric polarization converters based on photo-aligned liquid crystal films,” Opt. Express 16(6), 3768–3775 (2008). [CrossRef] [PubMed]
4. S.-W. Ko, Y.-Y. Tzeng, C.-L. Ting, A. Y. Fuh, and T. H. Lin, “Axially symmetric liquid crystal devices based on double-side photo-alignment,” Opt. Express 16(24), 19643–19648 (2008). [CrossRef] [PubMed]
6. S. Nersisyan, N. Tabiryan, D.-M. Steeves, and B.-R. Kimball, “Fabrication of liquid crystal polymer axial waveplates for UV-IR wavelengths,” Opt. Express 17(14), 11926–11934 (2009). [CrossRef] [PubMed]
7. A. Y. Fuh, C. C. Chen, C. K. Liu, and K. T. Cheng, “Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals,” Opt. Express 17(9), 7088–7094 (2009). [CrossRef] [PubMed]
8. A. Y.-G. Fuh, J.-C. Chen, S.-Y. Huang, and K.-T. Cheng, “Binary liquid crystal alignments based on photoalignment in azo dye-doped liquid crystals and their application,” Appl. Phys. Lett. 96(5), 051103 (2010). [CrossRef]
9. W.-Z. Chen, Y.-T. Tsai, and T.-H. Lin, “Single-cell-gap transflective liquid-crystal display based on photo- and nanoparticle-induced alignment effects,” Opt. Lett. 34(17), 2545–2547 (2009). [CrossRef] [PubMed]
10. C.-Y. Huang, J.-M. Ma, T.-S. Mo, K. C. Lo, K. Y. Lo, and C.-R. Lee, “All-optical and polarization-independent spatial filter based on a vertically-aligned polymer-stabilized liquid crystal film with a photoconductive layer,” Opt. Express 17(25), 22386–22392 (2009). [CrossRef]
11. S.-Y. Huang, T.-C. Wung, A. Y.-G. Fuh, H.-C. Yeh, C.-Y. Huang, C.-M. Ma, S.-C. Huang, T.-S. Mo, and C.-R. Lee, “Electro- and photo-controllable spatial filter based on a liquid crystal film with a photoconductive layer,” Appl. Phys. B 97(4), 749–752 (2009). [CrossRef]
12. M. Kaczmarek and A. Dyadyusha, “Structured, photosensitive PVK and PVCN polymer layers for control of liquid crystal alignment,” J. Nonlinear Opt. Phys. Mater. 12(4), 547–555 (2003). [CrossRef]
13. M. Hasegawa, “Key molecular structure determination of photoalignment materials from the effects of linearly polarized deep UV light on several polymers,” Jpn. J. Appl. Phys. 39(Part 1, No. 3A), 1272–1277 (2000). [CrossRef]
14. K. Nakajima, H. Wakemoto, S. Sato, F. Yokotani, S. Ishihara, and Y. Matsuo, “Polystyrene derivative films for liquid crystal alignment,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 180(2), 223–232 (1990).
15. V. Freedericksz and V. Zolina, “Forces causing the orientation of an anisotropic liquid,” Trans. Faraday Soc. 29(140), 919–930 (1933). [CrossRef]
16. H. Ren, Y.-H. Fan, S. Gauza, and S.-T. Wu, “Tunable-focus flat liquid crystal spherical lens,” Appl. Phys. Lett. 84(23), 4789–4791 (2004). [CrossRef]
17. A. Y.-G. Fuh, S.-W. Ko, S.-H. Huang, Y.-Y. Chen, and T.-H. Lin, “Polarization-independent liquid crystal lens based on axially symmetric photoalignment,” Opt. Express 19(3), 2294–2300 (2011). [CrossRef] [PubMed]
18. A. Y.-G. Fuh, J.-C. Chen, K.-T. Cheng, and 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(8), 572–576 (2010). [CrossRef]
19. J. Li, S.-T. Wu, S. Brugioni, R. Meucci, and S. Faetti, “Infrared refractive indices of liquid crystals,” J. Appl. Phys. 97(7), 073501 (2005). [CrossRef]
20. Z. Cao, L. Xuan, L. Hu, X. Lu, and Q. Mu, “Temperature effect on the diffraction efficiency of the liquid crystal spatial light modulator,” Opt. Commun. 267(1), 69–73 (2006). [CrossRef]
21. S.-T. Wu and D.-K. Yang, Reflective Liquid Crystal Displays (Wiley, 2001).