Open Access
Add to BibSonomy
Abstract
In this study, a twisted nematic mode polymer-stabilized liquid crystal (TN mode PSLC) integrated with a crossed polarizer was used to create a transparent waveguide display. When a voltage was applied, the PSLC scattered the waveguide light with a high polarization selectivity such that no substantial loss of the outgoing light intensity was observed after integrating the polarizer. However, with a crossed polarizer, in the ON state, the background light was not only scattered but also absorbed by the analyzer. Using this device configuration, with a 12 µm cell gap and 7% monomer concentration, we successfully realized a normally transparent waveguide display. The contrast ratio of the waveguide outgoing light was 26 and that of the undesired background reached 90. This device can display images due to waveguide edge-lit light scattering and simultaneously block the background information to improve the image quality.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The next generation displays are expected to possess unprecedented and novel features apart from being lighter, thinner, exhibiting higher resolutions, and showing a more realistic display quality. After adding a “transparent” feature to the display, it can be applied to a wide range of applications. The transparent display can integrate virtual information with the real environment and provide real-time access to the information as well as enable interaction with the environment to achieve augmented reality [1,2]. Transparent displays are suitable for numerous applications that can replace almost all glasses such as those in automobiles, buildings, and shop windows, and they are garnering significant attention. At present, the common transparent display technologies such as thin-film transistor liquid crystal display, [3,4] active matrix organic light-emitting diodes (AM-OLEDs), [5–8] micro light emitting diodes [9,10] and electrowetting display [11–13] can simultaneously display image and background information, but each has its own development difficulties. However, the image quality of most transparent displays is related to their surroundings. In a transparent display, a part of the pixel is a transparent area that cannot shield the light. If the background brightness is excessively strong to exceed the luminous intensity of the transparent display, then the background interferes with the image on the transparent display, resulting in a low image quality and low contrast ratio, as shown in Fig. 1(a). In recent studies, the purpose of shielding the background has been achieved by adding a shutter layer behind an AM-OLED display [14,15]. However, owing to the need for double panels, the manufacturing process is complicated with an increased cost. A schematic of a transparent display with a double-layer structure is shown in Fig. 1(b).
Fig. 1. (a) Image displayed by the transparent display, with background interference. (b) Transparent display with the double-layer structure. (c) Images displayed by a single-layer transparent display. The background interference is effectively blocked in this case.
In recent years, edge light sources combined with polymer-stabilized liquid crystals (PSLCs) have been proposed for the fabrication of transparent waveguide displays [16–20]. These transparent waveguide displays do not require polarizer and color filters to achieve a high transparency and full-color display. When no voltage is applied, the light emitted by the edge light source undergoes total internal reflection at the glass substrate and air interface and propagates in the panel, which is transparent. After the voltage is applied, the liquid crystal is rearranged to scatter the light from the edge light source and that emitted from the panel, and an image is displayed.
A homogenous and aligned PSLC transparent waveguide display has been successfully developed in previously reported studies. Additionally, the correlation between the incident light and the alignment direction, as well as the polarization characteristics and mechanism of the emitted light have been discussed [21]. However, this type of display still exhibits limitations such as background interference and a low image quality, similar to the other transparent displays. In this study, we utilized the polarization-dependence characteristic of the light emitted by a PSLC and integrated it with crossed polarizers to form a transparent waveguide display that can display images on a single-layer structure while effectively blocking the background, as shown in Fig. 1(c). Furthermore, herein, we discuss the electro-optical (EO) performance of this transparent waveguide display under different processes.
2. Material properties and cell fabrication
The PSLC mixture contained a nematic liquid crystal (HTW114200-050; extraordinary index ne ≈ 1.729, ordinary index no ≈ 1.507, dielectric anisotropy Δɛ = ɛ// - ɛ⊥ ≈ 10.9, form HCCH) exhibiting positive dielectric anisotropy, and different concentrations of a reactive mesogen (RM257; extraordinary index ne ≈ 1.687, ordinary index no ≈ 1.508, form HCCH).These 100 wt.% PSLC mixtures are further homogeneously blended with 0.5 wt.% of a photoinitiator (IRG651; form HCCH).
The twisted nematic (TN) cell was made of indium tin oxide glass substrates covered with a homogeneous polyimide alignment layer (PI-5291, Nissan Chemicals). The substrate was coated with an alignment layer and subsequently treated using the conventional rubbing method to form the alignment direction. The alignment direction of the liquid crystal was parallel to the rubbing direction. Two rubbed substrates were assembled orthogonally, and the cell gap (3, 7, 12, and 20 µm) was controlled using ball spacers. Next, the photocurable PSLC mixture was filled into the TN cell through a capillary. Photopolymerization was performed under ultraviolet (UV) light irradiation with an intensity of 88 mW/cm2 at room temperature for 20 min, eventually forming a TN mode PSLC. We attached the crossed polarizers to the TN mode PSLC cell in a normally transparent mode (the transmitting axis of each polarizer was parallel to the alignment direction) such that the cell exhibits a transparent state, before applying a voltage.
3. Operating principle
The proposed TN mode PSLC integrated with crossed polarizers as a transparent waveguide display is illustrated in Fig. 2. When no voltage is applied, the light from the edge light source encounters the well-aligned liquid crystal and the polymer network that a single refractive index and does not scatter. This light undergoes total internal reflection at the glass substrate and air interface and propagates to the end then completely absorbed, indicating an OFF state. In contrast, the light from the background after passing through the polarizer satisfies the Mauguin condition and is rotated by 90° by the liquid crystal then passing through the analyzer [22]. Therefore, the OFF state is also in a transparent state. After the voltage is applied (applied voltage at fixed frequency of 1 kHz square wave), the liquid crystal aligns in a direction parallel to the applied electric field (or vertical to the substrate). Owing to the nonuniform polymer network, the liquid crystal forms multiple regions with different tilt angles, resulting in a refractive-index mismatch. Consequently, the light from the edge light source is scattered, indicating an ON state, and this scattered light exhibits polarization dependence such that the light polarized in a direction parallel to that of the liquid crystal alignment shows the highest intensity [21]. Thus, the light from the background is scattered, as well as the linearly polarized light, whose polarization is not rotated owing to the liquid crystal arrangement not meeting the Mauguin condition, is absorbed by the analyzer to reduce the transmittance of the background light [22]. Therefore, the ON state is also a background blocking state. This mode can simultaneously display the image and block the background information in a single area to optimize the image quality, without requiring additional cells.
Fig. 2. Schematic of the TN mode PSLC transparent waveguide display. (a) OFF state, also as a transparent state. (b) ON state, also as a block state.
This transparent waveguide display can be operated using the color-sequence method. The RGB (R: red, G: green, B: blue) color of the edge light source can be quickly switched to achieve the full color in the visual field. If the frame rate of a single color is 60 Hz, then the full color requires 180 Hz, with a display response time of 5 ms. The EO performance of the device under different processes and curing conditions is discussed later.
4. Results and discussion
The EO performance of the TN mode PSLC device, integrated with crossed polarizers, under different cell gaps and curing conditions was investigated. To evaluate the light-emission mechanism and EO performance of the device, we analyzed the brightness contrast between the OFF state (transparent state) and the ON state (block state), various noise blocking performances (parallel transmittance), operating voltage (i.e., the voltage at the highest brightness), and response times of this device.
First, we discuss the influence of the cell gap on this device. The monomer (RM257) concentration was set to 7% and was photopolymerized under UV light irradiation, with an intensity of 88 mW/cm2, at room temperature for 20 min. For different cell gaps, the outgoing light intensities at different applied voltages are shown in Fig. 3(a). The contrast ratios of the outgoing light intensity along with the driving voltages and response times for different cell gaps are summarized in Table 1. In terms of outgoing light intensity, a thicker cell gap causes more light to scatter and destroy the total internal reflection, resulting in higher outgoing light intensity. However, owing to the increase in the cell gap, the liquid crystals are not well aligned and scattered in the OFF state, resulting in a poor contrast. The driving voltage increases as the cell gap increases. At any cell gap, the liquid crystals are arranged gradually to yield a uniform refractive index with the increasing applied voltage, thereby reducing the scattering. As summarized Table 1, as the cell gap increases, the force induced by the boundary on the interlayer liquid crystal also becomes weak, leading to an increase in the response time.
Fig. 3. (a) Outgoing light intensity versus applied voltage for different cell gaps. (b) PT versus applied voltage for different cell gaps.
Table 1. Contrast ratios of the outgoing light intensity, driving voltages, and response times for different cell gaps.
Next, we discuss the ability of the different cell gaps to block the background information. The parallel transmittance (PT) exhibited by the different cell gaps at various applied voltages is shown in Fig. 3(b). The thin cell gap of 3 µm exhibits a larger electric field per unit thickness of the liquid crystal layer, and the liquid crystal is easily aligned with the electric field, resulting in fewer refractive-index mismatch areas; thus, the scattering effect is poor. However, the PT was significantly reduced owing to the light absorption by the analyzer. In the 7-µm cell gap, an increased scattering effect, between 20 and 30 V of applied voltage, is observed because of this increase in the scattering layer thickness. Thus, the PT is low in this case. As the voltage increases, the scattering decreases, and the PT increases when the liquid crystal is aligned with the same refractive index. As the cell gap is increased to 12 and 20 µm, the haze increases to an extremely high level and saturates after 40 V because of this increase in the thickness of the scattering layer. When the applied voltage exceeds 40 V, the PT becomes nearly zero. The increase in the cell gap results in a stronger scattering and lower PT, and the overall blocking effect is improved. However, as the thickness increases, the operating voltage also increases. Thus, compared to the 3- and 7-µm cell gaps, a 12-µm cell gap can result in a higher brightness and a strong blocking effect. Moreover, compared to the 20-µm cell gap, the 12-µm gap resulted in a higher contrast ratio of the outgoing light intensity, an optimal operating voltage, and a relatively low response time. Therefore, in this study, a thickness of 12 µm was selected as the optimal cell gap for the experiments.
In addition to the thickness of the cell gap, we also studied the EO properties of the device at different monomer (RM257) concentrations. We used the samples with a 12-µm cell gap and photopolymerized under UV light of intensity of 88 mW/cm2 at room temperature for 20 min. The outgoing light intensity versus applied voltage for different concentrations of RM257 is shown in Fig. 4(a). The low monomer concentration is due to the small number of RM257 molecules per unit volume, indicating that the formed polymer network density is low. When a voltage is applied, because of the low-density polymer network, there are fewer regions with refractive-index mismatch. Thus, the scattering of the edge light is reduced, resulting in a low-brightness outgoing light. When the concentration of RM257 is increased to 7%, the polymer network density increases, thereby increasing the interface between the liquid crystal and the polymer network, causing an enhancement in the light scattering. Further increasing the concentration of RM257 results in a thick and dense polymer network, which is sufficiently strong to confine the liquid crystal, rendering it difficult for the liquid crystal to align with the applied electric field. The refractive-index mismatch between different regions of the polymer network decreases, thereby reducing the outgoing light intensity in the ON state. The contrast ratios of the outgoing light intensity, driving voltages, and response times for different concentrations of RM257 are summarized in Table 2. The outgoing light intensity in the OFF state increases because of the increase in the concentration of RM257, resulting in a low contrast. Furthermore, the increase in the density of the polymer network impedes the alignment of the liquid crystal parallel to the applied electric field, resulting in a large operating voltage. However, the speed at which the liquid crystal returns to the original alignment is relatively high, which shortens the response time.
Fig. 4. (a) Outgoing light intensity versus applied voltage for different RM257 concentrations. (b) PT versus applied voltage for different RM257 concentrations.
Table 2. Contrast ratio of outgoing light intensity, driving voltage, and response time for different RM257 concentrations.
Figure 4(b) depicts the background information blocking performance of the device for different RM257 concentrations. When the concentration is low, the polymer network shows a weak ability to confine the liquid crystal; thus, the driving voltage is low in this case, and the PT gradually increases after reaching a minimum. As the concentration increases, the polymer network becomes thicker and denser, which enhances its ability to confine the liquid crystal more strongly; thus, in this case, the driving voltage is higher, and the PT under the applied voltage is extremely low. At a concentration of 7%, RM257 exhibits relatively better operating conditions, the best background-blocking performance, and the highest outgoing-light brightness. Based on the presented results of the different process parameters, we selected the manufacturing parameters that yielded the highest brightness and contrast ratio when displaying the image, and exhibited good performance in both the transparent and blocking states: 12 µm cell gap, 7% concentration of RM257, and 88 mW/cm2 curing intensity. The combination of these parameters was used as the optimal condition for the transparent waveguide display fabricated in this study.
The aforementioned parameters were used to measure the outgoing light intensity and PT of the samples both with and without integrated polarizers, as shown in Fig. 5(a) and Fig. 5(b). The TN mode PSLC was integrated with crossed polarizers such that the transmitting axes of the polarizers were parallel to the rubbing direction of the substrate. When no voltage was applied, the device was in the OFF state (transparent state). In this case, the light from the edge light source underwent total internal reflection at the substrate and air interface to propagate in the display. After the voltage was applied, the liquid crystal aligned parallel to the electric field, and was divided into multiple refractive-index mismatch regions because of the restriction of the polymer network, and scattering of light occurred. The scattered outgoing light was linearly polarized, and the degree of polarization along the direction parallel to the alignment direction was relatively large. In the ON state of the sample without the polarizer, the intensity of the scattered outgoing light included 83.02% of the linearly polarized light parallel to the alignment direction and 16.98% of the linearly polarized light perpendicular to the alignment direction. Therefore, after integrating the polarizer, the intensity emitted in the ON state decreased by approximately 17%, whereas the contrast ratio increased from 19.48 (without polarizer) to 26.15 (with polarizer), as shown in Table 3.
Fig. 5. (a) Outgoing light intensity versus applied voltage for the TN mode PSLC with and without crossed polarizer. (b) PT versus applied voltage for the TN mode PSLC with and without crossed polarizer.
Table 3. Contrast ratio of the outgoing light intensity and that of PT for the TN mode PSLC.
After the integration of the polarizer, the polarized light perpendicular to the transmitting axis of the polarizer was absorbed, and the PT of the transparent state was reduced by approximately half (from 84.78% (without polarizer) to 35.34% (with polarizer)). After the voltage was applied, the PT decreased from 33.51% (without polarizer) to 0.38% (with polarizer) in the blocking state. This is because the TN mode PSLC strongly scatters the linearly polarized light parallel to the alignment direction, and a fraction of the liquid crystals driven by the electric field caused the polarized light to not meet the Mauguin condition and be absorbed by the analyzer. As summarized in Table 3, the contrast ratio of the PT in the blocking state increased from 2.53 (without polarizer) to 93 (with polarizer), which significantly enhanced the background-blocking ability.
A schematic of the experimental setup is shown in Fig. 6, and the photographs of the TN mode PSLC display are shown in Fig. 7. The graphic of “LCP” was another screen set up behind the sample to represent the background information. Evidently, the ability to block the background information after the integration of crossed polarizers was significantly increased, and only a small amount of outgoing light brightness was sacrificed. By integrating the TN mode PSLC with the orthogonally aligned polarizer and analyzer, we successfully achieved image optimization of the displayed image and simultaneously blocked the background information in a single pixel.
Fig. 7. Photos of the TN mode PSLC: (a) OFF state (transparent state) without polarizer. (b) ON state (block state) without polarizer. (c) OFF state (transparent state) with polarizer. (d) ON state (block state) with polarizer.
The previous mentioned combination of parameters was implemented to produce a driving voltage of 60 V, an outgoing light contrast ratio of 26, and an excellent background blocking performance (the PT was reduced from 34% to 0.4%) in the TN mode PSLC waveguide display with a response time of 3.2 ms.
5. Conclusion
In this study, we proposed and fabricated a TN mode PSLC transparent waveguide display. We verified the polarization characteristics of the scattered outgoing light, and the degree of polarization of the linearly polarized light parallel to the alignment direction was found to be relatively large. Using this outgoing light polarization characteristic and integrating the device with crossed polarizers, it was possible to maintain a specific brightness. Furthermore, the polarization of the background light was modulated to cause scattering and blurring of the background information. Moreover, the conditions were set such that the background light was absorbed by the analyzer to reduce the background brightness. Subsequently, the EO properties of this TN mode PSLC containing different cell gaps and polymer concentrations were evaluated, and the effects of the aforementioned parameters on the outgoing light intensity and background blocking were analyzed. When the cell gap was increased, the blocking effect was better, and the outgoing light brightness was higher. In contrast, when the concentration of RM257 was excessively high or low, the refractive-index mismatch effect was worse, and the blocking performance and outgoing light brightness degraded significantly. Based on these results, an optimal combination of parameters, viz. 12-µm cell gap, 7% RM257 concentration, and 88 mW/cm2 curing intensity, were selected for evaluating the display performance of the TN mode PSLC transparent waveguide display. When integrated with a polarizer, the device showed an outgoing light brightness contrast ratio of 26, PT of only 0.4%, and response time of 3.2 ms, which meet the requirements of the color sequence method. Image optimization was successfully realized by simultaneously displaying the image and blocking the background information in the same area. In the future, we will attempt to create a stable state mode to make this device more energy saving. The fabricated device can also be integrated white light-emitting diodes for develop smart windows that can switch between lighting and shading.
Funding
Ministry of Science and Technology, Taiwan (MOST 109-2112-M-110-013-MY3, MOST 110-2223-E289 110-001-).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. R. T. Azuma, “A survey of augmented reality,” Presence: Teleoperators & Virtual Environments 6(4), 355–385 (1997). [CrossRef]
2. B. Furht, Handbook of Augmented Reality (Springer Science and Business Media, 2011). [CrossRef]
3. Z. Feng, Y. Wu, B. Surigalatu, X. Zhang, and K. Chang, “Large transparent display based on liquid crystal technology,” Appl. Opt. 59(16), 4915–4920 (2020). [CrossRef]
4. C. H. Lin, W. B. Lo, K. H. Liu, C. Y. Liu, J. K. Lu, and N. Sugiura, “P-144L: Late-news poster: Novel transparent LCD with tunable transparency,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library2012), 1159–1162.
5. S. K. Park, M. Ryu, S. Yang, C. Byun, C. Hwang, K. I. Cho, W. B. Im, Y. E. Kim, T. S. Kim, Y. B. Ha, and K. Kim, “18.1: Invited Paper: Oxide TFT driving transparent AM-OLED,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library 2010) 41(1), 245–248 (2010).
6. J. Chung, J. Lee, J. Choi, C. Park, J. Ha, H. Chung, and S. S. Kim, “11.4: Transparent AMOLED display based on bottom emission structure,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library 2010) 41(1), 148–151 (2010).
7. S. Ju, J. Li, J. Liu, P. C. Chen, Y. G. Ha, F. Ishikawa, H. Chang, C. Zhou, A. Facchetti, D. B. Janes, and T. J. Marks, “Transparent active matrix organic light-emitting diode displays driven by nanowire transistor circuitry,” Nano Lett. 8(4), 997–1004 (2008). [CrossRef]
8. K.-T. Chen, Y.-H. Huang, Y.-H. Tsai, W.-C. Chen, L.-H. Wang, H.-Y. Chen, G. Chen, J.-C. Ho, and C.-C. Lee, “60-1: Invited Paper: Highly transparent AMOLED display with interactive system,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library 2019) 50(1), 842–845 (2019).
9. Y.-T. Liu, K.-Y. Liao, C.-L. Lin, and Y.-L. Li, “66-2: Invited Paper: PixeLED display for transparent applications,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library 2018) 49(1), 874–875 (2018).
10. J. Fan, C.-Y. Lee, S.-j. Chen, L. M. Gang, Z. L. Jun, S. Yang, L. M. Cai, X. H. Fei, and L. Nian, “30.2: Invited Paper: A RGB chip Full Color Active Matrix Micro-LEDs Transparent Display with IGZO TFT Backplane,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library 2019) 50(S1), 326–328 (2019).
11. K. L. Lo, Y. H. Tsai, W. Y. Cheng, J. W. Shiu, and J. L. Chen, “12.4: Recent development of transparent electrowetting display,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library 2013) 44(1), 123–126 (2013).
12. Y. S. Ku, S. W. Kuo, Y. H. Tsai, P. P. Cheng, J. L. Chen, K. W. Lan, K. L. Lo, K. C. Lee, and W. Y. Cheng, “62.4: The structure and manufacturing process of large area transparent electrowetting display,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library 2012) 43(1), 850–852 (2012).
13. R. Massard, J. Mans, A. Adityaputra, R. Leguijt, C. Staats, and A. Giraldo, “Colored oil for electrowetting displays,” J. Inf. Disp. 14(1), 1–6 (2013). [CrossRef]
14. G. W. Kim, R. Lampande, D. C. Choe, I. J. Ko, J. H. Park, R. Pode, and J. H. Kwon, “Next generation smart window display using transparent organic display and light blocking screen,” Opt. Express 26(7), 8493–8502 (2018). [CrossRef]
15. C.-C. Li, H.-Y. Tseng, H.-C. Liao, H.-M. Chen, T. Hsieh, S.-A. Lin, H.-C. Jau, Y.-C. Wu, Y.-L. Hsu, W.-H. Hsu, and T. Lin, “Enhanced image quality of OLED transparent display by cholesteric liquid crystal back-panel,” Opt. Express 25(23), 29199–29206 (2017). [CrossRef]
16. Y. Numata, K. Okuyama, T. Nakahara, T. Nakamura, M. Mizuno, H. Sugiyama, S. Nomura, S. Takeuchi, Y. Oue, H. Kato, S. Ito, A. Hasegawa, T. Ozaki, M. Douyou, T. Imai, K. Takizawa, and S. Matsushima, “Highly transparent LCD using new scattering-type liquid crystal with field sequential color edge light,” in 24th International Workshop on Active-Matrix Flatpanel Displays and Devices (AM-FPD), 2017, pp. 1–4.
17. Y. Shin, J. Jiang, G. Qin, Q. Wang, Z. Zhou, and D.-K. Yang, “Patterned waveguide liquid crystal displays,” RSC Adv. 10(68), 41693–41702 (2020). [CrossRef]
18. C. Meng, E. Chen, L. Wang, S. Tang, M. Tseng, J. Guo, Y. Ye, Q. F. Yan, and H. Kwok, “Color-switchable liquid crystal smart window with multi-layered light guiding structures,” Opt. Express 27(9), 13098–13107 (2019). [CrossRef]
19. X. Zhou, G. Qin, L. Wang, Z. Chen, X. Xu, Y. Dong, A. Moheghi, and D. K. Yang, “Full color waveguide liquid crystal display,” Opt. Lett. 42(18), 3706–3709 (2017). [CrossRef]
20. L. Wang, N. Jia, L. Duan, T. Sun, Z. Liu, J. Fang, M. Hou, X. Liu, Y. Shi, Y. Li, X. Zhou, Y. Shin, G. Qin, S. Kim, X. Li, Y. Peng, S. Zhang, F. Yang, J. Sun, Q. Liu, B. Kristal, and D. Yang, “P-89: Development of waveguide liquid crystal display for transparent display applications,” in SID Symp. Dig. Tech. Pap. (Wiley Online Library 2019) 50(1), 1573–1575 (2019).
21. A. Moheghi, H. Nemati, and D.-K. Yang, “Polarizing light waveguide plate from polymer stabilized liquid crystals,” Opt. Mater. Express 5(5), 1217–1223 (2015). [CrossRef]
22. G. W. Gray and S. M. Kelly, “Liquid crystals for twisted nematic display devices,” J. Mater. Chem. 9(9), 2037–2050 (1999). [CrossRef]
