Bistable Switching of Diffractive Smectic-A Liquid Crystal Device

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Bistable Switching of Diffractive Smectic-A Liquid Crystal Device between Haze-free Transparent and High-haze Translucent States Jae-Won Huh, Tae-Hoon Choi, Jin-Hun Kim, Jae-Hyeon Woo, Jeong-Ho Seo, and Tae-Hoon Yoon ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00373 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Bistable Switching of Diffractive Smectic-A Liquid Crystal Device between Haze-free Transparent and High-haze Translucent States Jae-Won Huh, Tae-Hoon Choi, Jin-Hun Kim, Jae-Hyeon Woo, Jeong-Ho Seo, and Tae-Hoon Yoon* Departments of Electronics Engineering, Pusan National University, Busan 46241, Korea Abstract Bistable operation of a phase grating device using smectic-A liquid crystals (SmA-LCs) is presented. The SmA-LC cell can be operated with very low power because it consumes power only while the state is being switched. This device uses the simple dielectric switching mechanism, and it is free from degradation by the impurity ions. For the switching of an SmA-LC cell, we formed interdigitated electrodes on both the top and bottom electrodes to apply the in-plane and vertical electric fields. We can control the diffracted light intensity easily by the change in the magnitude or duration of the applied voltage wave. We have shown that 98.1% of the incident light can be transferred from the zeroth to higher orders. Owing to the high diffraction efficiency, this device can provide an excellent translucent state. Moreover, it can provide the haze-free transparent state because an SmA-LC cell does not contain a polymer structure or impurity ions. We have shown that the device can provide outstanding performance, such as low power consumption, strong diffraction, high transmittance, high-haze translucent state, and high reliability for over two weeks. We expect that the device can be applied to switchable windows and window displays.

KEYWORDS: Low-power consumption, Liquid crystal, Smectic A, Phase grating, Diffraction

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Liquid crystal (LC) devices have been developed for applications in various photonic devices and information displays. Recently, LC phase grating devices have been studied actively because of their high diffraction efficiency [1-14]. To obtain strong diffraction in an LC device, several methods to build a periodic structure in an LC cell, such as the formation of the patterned alignment layer [2-4], application of an in-plane electric field [7-14], and use of LC materials with intrinsic diffraction properties such as cholesteric LC (ChLC) [5] and ferroelectric LCs [6], have been reported recently. Among the various approaches to realize an LC phase grating device, an LC phase grating driven with interdigitated electrodes is actively studied for their outstanding merits, such as the strong diffraction, large diffraction angle, no diffraction at the initial state, and simple fabrication process [10-14]. It can be used to control the haze value owing to its high diffraction efficiency [13,14]. Although it can be operated with low power, the power consumption needs to be reduced because it requires power to maintain the diffraction. To reduce the power consumption in a phase grating device, bistable operation, which consume power only while it is switched between the states, is essential. SmA-LCs have been studied for bistable devices [15-18]. Owing to the high viscosity of SmA-LCs, it can maintain its state after the applied electric field is removed. Two kinds of the switching mechanism, the electro-hydrodynamic effect or dielectric switching, can be used for switching [15, 16]. The electro-hydrodynamic effect occurs only when specific ionic and LC materials are used [19], and frequency modulation is required for switching. To avoid the complicated frequency modulation, a simple driving method for an SmA-LC device using an in-plane field has been studied [17,18]. In this work, we report the bistable operation of a phase grating device using SmA-LCs. To control the phase difference electrically in an LC cell, interdigitated electrodes can be formed on both the top and bottom substrates. Unlike previously reported SmA-LC devices that use both dielectric switching and electro-hydrodynamic instability [15,16], this device relies only on dielectric switching. The diffracted light intensity can be controlled by applying in-plane and vertical electric fields. Owing to the high viscosity of SmA-LCs, this device can maintain its state after the applied electric field is removed. We have shown that 98.1% of the incident light can be transferred from the zeroth to higher orders. We expect that it will be used for switchable windows and window displays because it can provide a translucent state with a high haze value (~ 92%) owing to the high diffraction efficiency. Compared with light shutter technologies using LC/polymer composites [20-22], this device 2


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exhibits a haze-free transparent state because it does not contain any polymer structure. We confirmed that it could maintain the state for over two weeks and any degradation after repeated switching was not observed.

Principle of operation Figure 1(a) shows the operation principle of a bistable SmA-LC cell. To switch an SmA-LC cell by dielectric switching, we formed interdigitated electrodes on both the top and bottom substrates. By applying in-plane and vertical electric fields, we can control the diffracted light intensity of an SmA-LC cell. In the initial state, most of the incident light passes through the cell without diffraction because SmA-LCs are aligned uniformly perpendicular to the substrates because of the natural ability of the SmA-LC material [15]. When an in-plane electric field is applied between the adjacent interdigitated electrodes on the top and bottom substrates, the LC molecules with positive dielectric anisotropy tilt down along the electric field direction so that the refractive index is modulated periodically, which results in the diffraction of the incident light. When a vertical electric field is applied between the top and bottom common electrodes, the SmA-LCs are reoriented perpendicular to the two substrates so that the diffraction disappears. Owing to the high viscosity of the SmA-LC, the proposed LC cell can maintain the state after the applied voltage is removed. We calculated the LC director distribution using commercial software (Techwiz LCD 3D, Sanayi System Company, LTD., Korea). Figure 1(b) shows the top view of the director distribution in an SmA-LC cell. In the initial state, we can identify that the LC molecules are aligned perpendicular to the substrates. When an in-plane electric field is applied to the cell, they are oriented in various directions. In this state, strong diffraction can be observed owing to the refractive index change in the LC layer regardless of the direction [14].

Experimental condition To investigate the electro-optical performance, we fabricated an SmA-LC phase grating cell with crossed interdigitated electrodes. We used an LC (8CB, Sigma-Aldrich, ∆n: 0.170 and ∆ε: 8.4), which is in the SmA phase for temperatures between 21.5 °C and 33.5 °C. The width of the interdigitated electrodes was 2.8 µm, and the gap between them was 4 µm. The cell gap was maintained to 20 µm using silica spacers. SmA-LCs were injected into the cell via capillary force at 100 °C, at which the SmA LC is in the isotropic phase. 3


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To compare the optical characteristics with an SmA-LC phase grating cell, we fabricated polymer-stabilized LC (PSLC) cells and an ion-doped ChLC cell. For PSLC cells, we mixed negative nematic LCs (BHR28300-400, Bayi, ∆n: 0.230 and ∆ε: -9.3) with two different concentrations of reactive mesogen (RM257, Merck, 3 wt% and 7 wt%). The cell gap was maintained at 10 µm using silica spacers. To form the polymer structure in the cell, UV light with an intensity of 40 mW/cm2 was exposed to the cell for 30 min. For an iondoped ChLC cell [23], we mixed positive nematic LCs (E7, Merck, ∆n: 0.237 and ∆ε: 14.1) with 10 wt% of chiral dopant (S811, Merck, reflection wavelength: 1600 nm), 0.1 wt% of ionic dopant (Hexadecyltrimethylammonium bromide, Sigma-Aldrich), 5 wt% of reactive mesogen (RM257, Merck), and 1 wt% of dichroic dye (S-428, Mitsui). The cell gap was maintained at 10 µm using silica spacers. For the polymerization, the LC cell was exposed to UV light with an intensity of 25 mW/cm2 for 30 min.

Experimental results and discussion Diffraction characteristics of an SmA-LC phase grating cell To verify the diffraction characteristics of the fabricated cell, we measured the diffraction efficiency of the zeroth order and checked the diffraction patterns, as shown in Fig. 2. Here, we focused on the amount of the incident light that was transferred from the zeroth to higher orders. We used a linearly polarized He-Ne laser beam (543.5 nm) as the light source. The intensity of the zeroth-order beam was detected with a photodiode placed 30 cm away from the fabricated cell. All the measurements were carried out at 25 °C to ensure that LCs in the fabricated cell are in the SmA phase. We measured the diffraction efficiency while applying an electric field (marked as a filled circle in Fig. 2(a)) and after the applied voltage was removed (marked as an empty circle in Fig. 2(a)). To eliminate the effect of the pulse duration of the applied electric field on the light diffraction efficiency, all the measurements were made at 10 s after the voltage was applied or removed. The intensity of the zeroth-order beam decreased as the magnitude of the applied inplane electric field was increased. When the applied voltage was 45 V, 98.1% of the incident light was transferred from the zeroth to higher orders. The fabricated LC cell maintained its state after the applied voltage was removed. The non-uniform distribution of the electric field may cause some of the LCs not rotated. When the applied voltage is removed, LC molecules tend to be oriented along the direction of the un-rotated LCs to minimize the elastic energy, 4


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which resulted in a slight decrease of the diffracted light intensity, as shown in Fig. 2(a). When a vertical electric field was applied, the zeroth-order efficiency increased as the applied voltage was increased. This means that the incident light is no longer transferred to the higher orders such that the diffraction disappears. The electric field induced between the two interdigitated electrodes can be tilted away from the vertical direction, which can affect the orientation of LCs. We can expect that some of the LC molecules may be tilt-oriented. However, the overall behavior in our experiment was not affected very much by the tilt effect. As shown in Fig. 2(a), the vertical threshold voltage is much higher than the in-plane threshold voltage because of the difference between the cell gap and the electrode spacing. An in-plane electric field applied between neighboring interdigitated electrodes depends on the electrode spacing, which was 4 µm in our experiment. On the other hand, the vertical electric field applied between common electrodes on the top and bottom substrates depends on the cell gap, which was 20 µm in our experiment. Therefore, for the same applied voltage, the in-plane field can be stronger than the vertical field, which may result in the difference of the threshold voltage. Figure 2(b) shows the diffraction pattern. When an in-plane field was applied to the cell, the incident light was well transferred to the higher order. When a vertical electric field was applied to the cell, the incident light was no longer transferred to the higher orders. The proposed SmA-LC cell can control the diffracted light intensity by changing not only the magnitude but also the duration of the applied voltage wave. Owing to the high viscosity of the SmA-LC, it requires sufficient time for switching. We measured the diffraction efficiency of the fabricated LC cell as we vary the duration of the applied electric field, as shown in Fig. 3. For high diffraction, we applied an in-plane electric field of 45 V as we vary the pulse duration from 0 to 400 ms. As shown in Fig. 3, the diffracted light intensity can be controlled by changing the duration of the applied electric field. The longer the duration of the applied electric field, the lower is the intensity of the zeroth order. When the pulse duration was 400 ms, which is sufficient for switching of the SmA-LC molecules, we found that 98.1% of the incident light could be transferred to the higher orders. The diffraction patterns also confirm that most of the incident light is transferred to the higher orders. In other words, by changing the magnitude or duration of the applied voltage wave, the fabricated cell can control the transfer of the zeroth order energy to higher orders. In addition, this device can be operated with very low power because it does not require power to maintain the state. 5


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To confirm the switching behavior of an SmA-LC cell, we measured the temporal response as we vary the duration of the applied electric field, as shown in Fig 4. For turn-on switching, we applied an in-plane field of 45 V to the cell. For turn-off switching, we applied a vertical electric field of 90 V to the cell. When an electric field was applied to the cell continuously, the turn-on [turn-off] time, which is defined as the transient time from 10% [90%] to 90% [10%] of the zeroth order intensity was 56.1 ms [11.9 ms]. We can control the diffracted light intensity of the fabricated cell by changing the duration of the applied in-plane electric field, as shown in Fig. 4(a). When the applied electric field was removed, the fabricated cell relaxed to a state with a slightly lower diffracted light intensity. It is affected by the un-rotated LCs caused by the non-uniform distribution of the in-plane electric field. When the applied voltage was removed, LC molecules tend to be oriented along the direction of the un-rotated LCs. When the duration of the applied electric field was longer than 400 ms, most of LC molecules can be oriented along the electric field direction so that there was no change of the diffracted light intensity by the removal of the applied field. By applying a vertical electric field of 90 V for a duration of 20 ms, the LC molecules can be reoriented perpendicular to the substrates so that diffraction disappears. When the duration of the applied electric field was less than 20 ms, the LCs were not aligned perpendicular to the substrates completely, as shown in Fig. 4(b).

Haze and transmittance of an SmA-LC cell Recently, several studies using LC phase grating devices have been carried out for the control of the haze value [13, 14]. Owing to the strong diffraction of the proposed SmA-LC device, it can provide a high-haze translucent state. In addition, it can be operated with very low power because it consumes power only for switching between the states. To confirm the optical performance of the fabricated cell in the transparent (without diffraction) and translucent state (with diffraction), we measured the specular transmittance and haze using a haze meter (HM-65W, Murakami Color Research Laboratory). The specular [diffuse] transmittance Ts [Td] refers to the ratio of the beam power that emerges from a sample cell, which is parallel (a small range of angles within 2.5°) [not parallel] to a beam entering the cell, to the power carried by the beam entering the cell. The total transmittance

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Tt is the sum of the specular transmittance Ts and the diffuse transmittance Td. The haze H can be calculated as H= Td/Tt. We measured the specular transmittance and haze when the fabricated cell is switched from transparent [translucent] to translucent [transparent] while applying the voltage (marked as filled circle in Fig. 5(a), (b)) and after the applied voltage was removed (marked as empty circle in Fig. 5(a), (b)). In the transparent state, the specular transmittance and haze of the fabricated cell were 76.7% and 1.3%, respectively. To switch the fabricated cell from the transparent to the translucent state, an in-plane electric field was applied to the cell. As the applied voltage was increased, the specular transmittance decreased, and haze increased. The specular transmittance and haze in the translucent state were 6.1% and 92.2%, respectively. When the applied voltage was lower than 45 V, there was a slight decrease of the haze and increase of the specular transmittance after the applied voltage was removed. The non-uniform distribution of the electric field may cause some of the LCs not rotated. When the applied voltage is removed, LC molecules tend to be oriented along the direction of the un-rotated LCs to minimize the elastic energy, which resulted in a slight decrease of the haze and increase of the specular transmittance. To switch the fabricated cell from the translucent to a transparent state, a vertical electric field was applied. As the applied voltage was increased, the specular transmittance increased, and the haze decreased. The fabricated cell maintained its state even after the removal of the applied voltage in all the states. This means that the proposed device can be operated with very low power because it consumes power only for switching between states. The in-plane threshold voltage is lower than the vertical electric field because the in-plane field is stronger than the vertical field for the same applied voltage. The fabricated LC cell showed a haze-free transparent state and a high haze of 92.2% in the translucent state. With these excellent optical performances, objects behind the fabricated cell can be seen through or hidden. Figure 6 shows the images of the fabricated cell placed on a printed paper. We took photographs of the fabricated cell when the voltage was applied and removed. In the transparent state, we can identify the printed image clearly because the SmA-LC cell is haze-free. When the fabricated cell was switched from the transparent to the translucent state, the objects behind the cell disappeared as the applied voltage was increased. Owing to the high haze of 92.2% in the translucent state, the fabricated cell can hide the objects behind it completely. When the fabricated cell is switched 7


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from the translucent to the transparent state, we can identify the objects behind the cell. Moreover, we confirmed that the fabricated cell could maintain any state between the transparent and translucent states after the applied voltage is removed. The repeatable switching and long-term stability are some of the primary issues for practical applications. We tested whether the switching of the fabricated cell between the transparent and translucent states could be repeated multiple times without any degradation in performance. The fabricated cell maintained the characteristics without any degradation after 50 cycles of repeated switching, as shown in Fig. 7(a). To confirm the long-term stability, we measured the haze of the fabricated cell in the transparent, translucent, and two states between the transparent and translucent states every 12 h, as shown in Fig. 7(b). Owing to the high viscosity of the SmA-LC, the fabricated cell can maintain the state without power consumption. It maintained its state for over two weeks without any degradation. We compared the haze value of the bistable SmA-LC cell with other LC devices that can control the haze value, such as PSLC cells with monomer concentrations of 3 wt % and 7 wt%, and a bistable ion-doped ChLC [23] cell, as shown in Table 1. A PSLC cell is not bistable although it has a high-haze value in the translucent state. An ion-doped ChLC cell has the highest haze value in the focal-conic state among the bistable ChLC devices. In the transparent state, ion-doped ChLC, 3-wt% PSLC, and SmA LC cells show almost the same haze value in the transparent state. An SmA-LC cell shows a slightly lower specular transmittance than a 3-wt% PSLC cell because of the low-transmittance indium tin oxide (ITO) substrates that we used, which can be improved by using high-transmittance ITO substrates. In the translucent state, an SmA-LC cell shows a haze value of 92.2%, which is much higher than 74.0% of a 3-wt% PSLC cell and 79.8% of an ion-doped ChLC cell. A 7wt% PSLC cell shows the highest haze value of 94.9% in the translucent state, but its transparent state is hazy because the high concentration of monomer results in the index mismatch between LC and polymer in the transparent state. Unlike a bistable ion-doped ChLC device [23] that exhibits stable homeotropic and focal-conic states only under the limited condition, an SmA-LC cell is free from degradation by ionic impurities and has a wide process window.

Conclusion

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We demonstrated bistable switching of an SmA-LC phase grating device. The SmA-LC device can easily control the diffracted light intensity by dielectric switching. It can be operated under very low power because it can maintain its state after the applied voltage is removed. The diffracted light intensity of this device can be controlled by not only the magnitude but also the duration of the applied voltage wave. The SmA-LC cell can transfer all of the zeroth-order energy to higher orders. Owing to the high diffraction efficiency, an SmA-LC cell can provide a high-haze translucent state. Moreover, it can provide a haze-free transparent state because it does not contain any polymer structure. We found that the device exhibits excellent reliability for over two weeks and no degradation after repeated switching. Compared with the previously reported ion-doped bistable ChLC device, this device has a wide margin of processing window and is free from degradation by ionic impurities. We believe that this device can be one of the new candidates for switchable window and window display applications.

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Acknowledgments This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2017R1A2A1A05001067). References 1.

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12. H. Chen, G. Tan, Y. Huang, Y. Weng, T.-H. Choi, T.-H. Yoon, and S.-T. Wu, “A low voltage liquid crystal phase grating with switchable diffraction angles,” Sci. Rep. 7, 39923 (2017). 13. T.-H. Choi, J.-H. Woo, J.-M. Baek, Y. Choi, and T.-H. Yoon, “Fast control of haze value using electrically switchable diffraction in a fringe-field switching liquid crystal device,” IEEE Tans. Electron Dev. 64 (8), 3213-3218 (2017). 14. T.-H. Choi, J.-W. Huh, J.-H. Woo, J.-H. Kim, Y.-S. Jo, and T.-H. Yoon, “Switching between transparent and translucent states of a two-dimensional liquid crystal phase grating device with crossed interdigitated electrodes,” Opt. Express, 25(10), 11275-11282 (2017). 15. D. Coates, W. A. Crossland, J. H. Morrissy, and B. Needham, “Electrically induced scattering textures in smectic a phases and their electrical reversal,” J. Phys. D Appl. Phys. 11 (14), 2025 (1978) 16. D.J. Gardiner, S. M. Morris, and H. J. Coles, “High-efficiency multistable switchable glazing using smectic A liquid crystals,” Sol. Energ. Mat. Sol. C., 93 (3), 310-306, (2009). 17. H.-Y. Chen, R. Shao, E. Korblova, W. Lee, D. Walba, and N. A. Clark, “A bistable liquidcrystal display mode based on electrically driven smectic A layer reorientation,” Appl. Phys. Lett. 91, 163506 (2007). 18. H.-Y. Chen, R. Shao, E. Korblova, W. Lee, D. Walba, and N. A. Clark, “Bistable SmA liquid-crystal display driven by a two-direction electric field,” J. Soc. Inf. Disp. 16 (6), 675681 (2008). 19. J.-W. Huh, J.-H. Kim, S.-W. Oh, S.-M. Ji, and T.-H. Yoon, “Ion-doped liquid-crystal cell with low opaque-state specular transmittance based on electro-hydrodynamic effect,” Dyes Pigments 150, 16-20 (2018). 20. R. A. M. Hikmet, “Electrically induced light scattering from anisotropic gels,” J. Appl. Phys. 68(9), 4406–4412 (1990). 21. J. W. Doane, N. A. Vaz, B. G. Wu, and S. Žumer, “Field controlled light scattering from nematic microdroplets,” Appl. Phys. Lett. 48(4), 269–271 (1986). 22. P. S. Drzaic, “Polymer dispersed nematic liquid crystal for large area displays and light valves,” J. Appl. Phys. 60(6), 2142–2148 (1986) 23. J.-H. Kim, J.-W. Huh, S.-W. Oh, S.-M. Ji, Y.-S. Jo, B.-H. Yu, and T.-H. Yoon, “Bistable switching between homeotropic and focal-conic states in an ion-doped chiral nematic liquid crystal cell,” Opt. Express 25(23), 29180-29188 (2018).

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Figure and Table Captions Table 1. Measured transmittance and haze of the fabricated PSLC, ChLC, and SmA-LC cells. Figure 1. (a) Operation principle and (b) top-view director profiles of an SmA-LC phase grating cell. Figure 2. (a) Measured diffraction efficiency of the zeroth order and (b) diffraction patterns of the fabricated SmA-LC phase grating cell. Figure 3. Measured diffraction efficiency versus pulse duration of the applied in-plane electric field and diffraction patterns of the fabricated SmA-LC phase grating cell. Figure 4. Temporal switching behaviors of the fabricated SmA-LC phase grating cell when (a) in-plane and (b) vertical electric field is applied. Figure 5. Measured specular transmittance and haze of the fabricated SmA-LC phase grating cell when switched from the (a) transparent to translucent and (b) translucent to transparent state. Figure 6. Images of the fabricated SmA-LC phase grating cells in the transparent and translucent states. Logo use with permission from Pusan National University. Figure 7. Measured haze value (a) under repeated switching and (b) versus time of the fabricated SmA-LC phase grating cell.

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Table 1. Transparent state Specular transmittance

Haze

Translucent state Specular transmittance

Haze

PSLC cell (3 wt%)

82.2%

2.8%

21.2%

74.0%

PSLC cell (7 wt%)

76.1%

9.2%

4.4%

94.9%

Ion-doped ChLC cell [23] SmA-LC cell

71.4 % 76.7%

4.4 % 1.3%

12.8 % 6.1%

79.8 % 92.2%

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Figure 1.

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ACS Photonics

Figure 2.

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Figure 3.

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ACS Photonics

Figure 4.

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Figure 5.

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ACS Photonics

Figure 6.

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Figure 7.

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ACS Photonics

For Table of Contents Use Only Title: Bistable Switching of Diffractive Smectic-A Liquid Crystal Device between Haze-free Transparent and High-haze Translucent States Authors: Jae-Won Huh, Tae-Hoon Choi, Jin-Hun Kim, Jae-Hyeon Woo, Jeong-Ho Seo, and TaeHoon Yoon*

Abstract Bistable operation of a phase grating device using smectic-A liquid crystals (SmA-LCs) is presented. The SmA-LC cell can be operated with very low power because it consumes power only while the state is being switched. This device uses the simple dielectric switching mechanism, and it is free from degradation by the impurity ions. For the switching of an SmA-LC cell, we formed interdigitated electrodes on both the top and bottom electrodes to apply the in-plane and vertical electric fields. We can control the diffracted light intensity easily by the change in the magnitude or duration of the applied voltage wave. We have shown that 98.1% of the incident light can be transferred from the zeroth to higher orders. Owing to the high diffraction efficiency, this device can provide an excellent translucent state. Moreover, it can provide the haze-free transparent state because an SmA-LC cell does not contain a polymer structure or impurity ions. We have shown that the device can provide outstanding performance, such as low power consumption, strong diffraction, high transmittance, 21


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high-haze translucent state, and high reliability for over two weeks. We expect that the device can be applied to switchable windows and window displays.

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Bistable Switching of Diffractive Smectic-A Liquid Crystal Device

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