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We propose a cholesteric liquid crystal device with a three-terminal electrode structure that can be operated in both the dynamic and the bistable modes. Fast switching (less than 5 ms) between the planar and the in-plane-field-induced states can be realized by applying an in-plane electric field, and conventional bistable switching between the planar and focal conic states can be realized by applying a vertical electric field.
©2012 Optical Society of America
1. Introduction
Thus far, cholesteric liquid crystal (ChLC) devices have been employed in various optical switching applications such as reflective display devices and light shutters, because of their bistable and reflective properties [1–18]. ChLCs have two stable states, i.e., planar and focal conic, and one transient homeotropic state. Bragg reflection is exhibited in the planar state because of its chiral and periodic structure, whereas the incident light is scattered in the focal conic state because of the random distribution of the ChLC domains. Because of their bistable properties, ChLC devices are suitable for displaying static images using little power.
At present, the ability to display high-quality moving pictures is increasingly required of display devices. Thus, the conventional ChLC device must use the transient homeotropic state to display moving pictures by using a mode in which the planar and homeotropic states can be employed as the bright and dark states, respectively. However, although switching from the planar to the homeotropic state occurs rapidly when a voltage is applied, relaxation from the homeotropic to the planar state takes approximately 300 ms after the applied voltage is removed, which is too slow for smooth video [6, 15, 17]. Recently, a fast response time was realized in a short-pitch ChLC device using in-plane switching [10, 14]. However, this device cannot utilize the merits of ChLCs, such as their bistable and reflective properties. Furthermore, the operating voltage is very high because of the short pitch. In addition, this device is limited to transmissive displays; therefore, an optical component such as crossed polarizers is required if they are to be used as display devices.
In this paper, we propose a ChLC device with a three-terminal electrode structure that can be operated in both the bistable and dynamic modes. The bistable mode uses vertical electric field switching between the planar and the focal conic states. Moreover, instead of the conventional slow switching between the planar and the homeotropic states, in-plane switching is employed between the planar and the in-plane-field-induced states for a fast response in the dynamic mode. Thus, we can realize a fast-switching ChLC device capable of utilizing the bistable property of ChLCs. We believe that the proposed device is applicable to various emerging optical switching devices that require operation in both the bistable and the dynamic modes.
2. Principle of operation for dual-mode switching
The principle of bistable operation of the proposed ChLC device is shown schematically in Fig. 1 . We can achieve bistable mode operation by applying a vertical electric field. In the planar state, ChLCs selectively reflect green wavelengths via Bragg reflection. By applying a vertical electric field, the planar state can be switched to the focal conic state, which scatters the incident light. Both states are stable at zero voltage. Either the planar state or the focal conic state can be switched to the homeotropic state, which is a transient state that is maintained while an electric field is applied. When the applied voltage is decreased or eliminated, the homeotropic state returns to either the planar state or the focal conic state. Thus, bistable operation can be realized at low power.
Fig. 1 Bistable mode operation of the proposed ChLC device by applying a vertical electric field: (a) planar state, (b) focal conic state, and (c) homeotropic state.
However, it is difficult to realize the dynamic mode using only vertical electric field switching between the planar and the homeotropic states because the relaxation from the homeotropic to the planar state takes a long time (approximately 300 ms) [17, 18]. The dynamic mode operation of the proposed ChLC device is shown in Fig. 1 as well. By applying an electric field between the patterned electrodes and the common electrode, the planar state switches to the in-plane-field-induced state [10, 14], resulting in transparency with no reflections due to no Bragg reflection. Thus, the in-plane-field-induced state can substitute for either the focal conic state or the homeotropic state. As soon as the applied voltage is removed, the ChLCs return to the initial planar state. Herein, fast dielectric response can be achieved because of strong distortion of the texture in each half-period [10, 14].
3. Three-terminal electrode structure and experiments
The three-terminal electrode structure that allows application of both in-plane and vertical electric fields is shown schematically in Fig. 2 . The top substrate includes an electrode without any pattern for vertical electric field switching, whereas the bottom substrate includes patterned electrodes for in-plane switching; using this structure, a three-terminal electrode structure can be achieved [19–21]. The bottom substrate has the same electrode structure as that used in the fringe-field switching (FFS) mode. The electric field distribution in the three-terminal device with the floated top electrode is almost the same as that in the two-terminal device at the same in-plane driving voltage [20].
Fig. 2 Proposed ChLC device with a three-terminal electrode structure. The top electrode is used for vertical electric field switching. The patterned electrodes are used for in-plane switching.
To confirm the electro-optical characteristics of the proposed ChLC device, we fabricated a ChLC device with the three-terminal electrode structure shown in Fig. 2. The width of the patterned electrodes and the gap between them were 4 μm and 6 μm, respectively. The top and bottom indium-tin-oxide glass substrates were spin-coated with homogeneous polyimide alignment layers (AL16301K, JSR Micro Korea), followed by a baking process to polyimidize the polyimide. The substrates were then assembled, maintaining a cell gap of 4 μm using silica spacers. A positive liquid crystal (MLC-6650, Δn = 0.1498, Δε = 52.6, Merck) was mixed with a chiral material (S811, Merck) to produce ChLCs. The mixing ratio was selected so as to reflect green light. The mixture was injected into an empty three-terminal electrode device.
4. Results and discussion
The reflection spectra of the fabricated ChLC device, measured using a spectrophotometer (MCPD-2000), are shown in Fig. 3 . Green light was reflected in the planar state, a state in which the reflectance is the highest at wavelength between 530 and 570 nm. When the applied in-plane field is low, the helical twist is distorted only for the region near the bottom substrate where the electric field intensity is highest. It results in the decrease of the Bragg reflection. As the applied in-plane field is increased, the region where the helical twist is distorted is widened towards the top substrate so that the Bragg reflection is decreased further. Bragg reflection can be completely eliminated when the helical twist is distorted strongly in all regions by applying a high voltage. Thus, the in-plane-field-induced state of the proposed ChLC device shows no reflection over the entire range of visible wavelengths when the applied voltage is higher than 50 V.
Fig. 3 Reflection spectra of the fabricated ChLC device with the applied in-plane voltage as a parameter. Initially, the ChLC device is in the planar state.
The measured voltage–reflectance curve of the fabricated ChLC device operating in the bistable mode is shown in Fig. 4(a) . Vertical electric field was applied between the top and bottom common electrodes. The patterned electrodes and the bottom common electrode were held at the same voltage level. The reflectance was measured after removing the applied voltage pulse, which had a fixed width of 40 ms. The initial planar state was maintained for an applied voltage less than 18 V. When the applied voltage was increased to 20 V, the device began to scatter the incident light because of the presence of focal conic domains, and therefore, the reflectance decreased. When a voltage between 30 V and 36 V was applied, focal conic domains were dominant in the ChLC device such that the lowest value of reflectance was exhibited. The ChLC device switched to the homeotropic state at applied voltages higher than 40 V; it began to relax back to the planar state when the applied pulse was no longer present. These results obtained by vertical field switching are almost the same as those that have been reported for conventional ChLC devices [17, 18].
Fig. 4 Voltage–reflectance curves of the proposed ChLC device with the three-terminal electrode structure when (a) a vertical electric field is applied and (b) an in-plane electric field is applied. The insets show images of the fabricated ChLC devices in each state.
The measured voltage–reflectance curve of the fabricated ChLC device operating in the dynamic mode is shown in Fig. 4(b), where the reflectance was measured while applying a voltage between the patterned electrodes and the common electrode. As the applied voltage increased, the reflectance decreased. When we applied a voltage higher than 40 V, there was little Bragg reflection.
The insets show images of the fabricated ChLC devices in the planar, focal conic, and in-plane-field-induced state when placed on a sheet of white paper with the printed text “PNU.” For comparison, the original sheet of white paper with the printed text “PNU” is also shown on the right. Because of Bragg reflection in the planar state and light scattering in the focal conic state, the text “PNU” cannot be seen in either state. However, in the in-plane-field-induced state, we can identify the printed text because of the device’s transparency and lack of reflection.
To confirm the fast switching property of the proposed ChLC device, we measured the transition time between each state. The turn-on time is defined as the transient time during which the reflectance increases from 10% to 90% of the maximum value, and vice versa for the turn-off time. The planar state can be switched to the homeotropic state by applying a vertical voltage of 45 V. The turn-on time for this transition was less than 2 ms, as shown in Fig. 5(a) . When the applied voltage was removed, it took approximately 195 ms for the device to relax to the perfect planar state. Therefore, it is very difficult to switch between the planar and homeotropic states with sufficient speed to allow the device to display moving pictures. This result is almost the same as that obtained via vertical electric field switching on a conventional ChLC device.
Fig. 5 Temporal switching behavior of the proposed ChLC device for (a) vertical switching between the planar and homeotropic states and (b) in-plane switching between the planar and in-plane-field-induced states.
The planar state can be switched to the in-plane-field-induced state by applying an in-plane electric field. A fast turn-on time of less than 2 ms was achieved at an applied voltage of 45 V, as shown in Fig. 5(b). When the applied electric field was removed, the in-plane-field- induced state reverted to the planar state; a fast relaxation time of less than 3 ms was obtained because of strong distortion of the texture in each half-period [10, 14]. Thus, our proposed ChLC device is suitable for displaying moving pictures using in-plane switching. We expect that further studies on optimization of the cell parameters such as the gap distance and width of the patterned electrodes can increase the switching speed. The proposed ChLC device with a three-terminal electrode structure can use either the bistable mode for low power consumption or the dynamic mode for fast response time.
5. Conclusions
In conclusion, we proposed a ChLC device capable of operation in both the bistable and the dynamic modes using a three-terminal electrode structure. Vertical electric field switching between the planar and the focal conic states is employed for the conventional bistable mode. We achieved a fast response time (less than 5 ms) via in-plane switching between the planar and the in-plane-field-induced states. We expect that the proposed device will be applicable to emerging optical switching devices requiring both fast response times and low power consumption.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0029198).
References and links
1. D.-K. Yang, J. W. Doane, Z. Yaniv, and J. Glasser, “Cholesteric reflective display: drive scheme and contrast,” Appl. Phys. Lett. 64(15), 1905–1907 (1994). [CrossRef]
2. L.-C. Chien, U. Muller, M.-F. Nabor, and J. W. Doane, “Multicolor reflective cholesteric displays,” Soc. Inf. Disp. Int. Symp. Dig. Tech. Pap. 26, 169–171 (1995).
3. B. Taheri, J. W. Doane, D. Davis, and D. St. John, “Optical properties of bistable cholesteric reflective displays,” SID Int. Symp. Digest Tech. Papers 27, 39–42 (1996).
4. M.-H. Lu, “Bistable reflective cholesteric liquid crystal display,” J. Appl. Phys. 81(3), 1063–1066 (1997). [CrossRef]
5. M. Xu, F. Xu, and D.-K. Yang, “Effect of cell structure on the reflection of cholesteric liquid crystal displays,” J. Appl. Phys. 83(4), 1938–1944 (1998). [CrossRef]
6. J. Anderson, P. Watson, J. Ruth, V. Sergan, and P. Bos, “Fast frame rate bistable cholesteric texture reflective displays,” SID Int. Symp. Digest Tech. Papers 29(1), 806–809 (1998).
7. A. Khan, X.-Y. Huang, R. Armbruster, F. Nicholson, N. Miller, B. Wall, and J. W. Doane, “Super high brightness reflective cholesteric display,” SID Int. Symp. Digest Tech. Papers 32(1), 460–463 (2001).
8. C.-Y. Huang, K.-Y. Fu, K.-Y. Lo, and M.-S. Tsai, “Bistable transflective cholesteric light shutters,” Opt. Express 11(6), 560–565 (2003). [CrossRef] [PubMed]
9. D.-K. Yang, “Flexible bistable cholesteric reflective displays,” J. Disp. Technol. 2(1), 32–37 (2006). [CrossRef]
10. H. H. Lee, J.-S. Yu, J.-H. Kim, S.-I. Yamamoto, and H. Kikuchi, “Fast electro-optic device controlled by dielectric response of planarly aligned cholesteric liquid crystals,” J. Appl. Phys. 106(1), 014503 (2009). [CrossRef]
11. K.-H. Kim, H.-J. Jin, K.-H. Park, J.-H. Lee, J. C. Kim, and T.-H. Yoon, “Long-pitch cholesteric liquid crystal cell for switchable achromatic reflection,” Opt. Express 18(16), 16745–16750 (2010). [CrossRef] [PubMed]
12. K.-H. Kim, H.-J. Jin, D. H. Song, B.-H. Cheong, H.-Y. Choi, S. T. Shin, J. C. Kim, and T.-H. Yoon, “Switching of liquid-crystal devices between reflective and transmissive modes using long-pitch cholesteric liquid crystals,” Opt. Lett. 35(20), 3504–3506 (2010). [CrossRef] [PubMed]
13. J. Ma, L. Shi, and D.-K. Yang, “Bistable polymer stabilized cholesteric texture light shutter,” Appl. Phys. Express 3(2), 021702 (2010). [CrossRef]
14. D. J. Gardiner, S. M. Morris, F. Castles, M. M. Qasim, W.-S. Kim, S. S. Choi, H.-J. Park, I.-J. Chung, and H. J. Coles, “Polymer stabilized chiral nematic liquid crystals for fast switching and high contrast electro-optic devices,” Appl. Phys. Lett. 98(26), 263508 (2011). [CrossRef]
15. K.-H. Kim, D. H. Song, Z.-G. Shen, B. W. Park, K.-H. Park, J.-H. Lee, and T.-H. Yoon, “Fast switching of long-pitch cholesteric liquid crystal device,” Opt. Express 19(11), 10174–10179 (2011). [CrossRef] [PubMed]
16. P. G. de Gennes and J. Prost, The Physics of Liquid Crystals (Oxford University Press, 1993).
17. S. T. Wu and D.-K. Yang, Reflective Liquid Crystal Displays (Wiley, 2001).
18. D.-K. Yang and S.-T. Wu, Fundamentals of liquid Crystal Devices (Wiley, 2006).
19. J.-I. Baek, Y.-H. Kwon, J. C. Kim, and T.-H. Yoon, “Dual-mode switching of a liquid crystal panel for viewing angle control,” Appl. Phys. Lett. 90(10), 101104 (2007). [CrossRef]
20. J.-I. Baek, K.-H. Kim, J. C. Kim, T.-H. Yoon, S. T. Shin, and H. S. Woo, “Fast turn-off switching of a liquid crystal cell by optically hidden relaxation,” SID Int. Symp. Digest Tech. Papers 39, 1846–1849 (2008).
21. J. H. Lee, T. Kim, T.-H. Yoon, J. C. Kim, C. G. Jhun, and S. B. Kwon, “Transflective dual operating mode liquid crystal display with wideband configuration,” J. Opt. Soc. Korea 14(3), 260–265 (2010). [CrossRef]
