We demonstrate a vertical-field-driven polymer-stabilized blue phase liquid crystal (PS-BPLC) mode for solving low transmittance and high driving voltage problems in conventional in-plane-switching (IPS) PS-BPLC modes. By controlling the ray directions of incident beams by means of two prism sheets attached to the top and bottom substrates, continuous grayscale properties can be achieved with a vertical field, where the transmittance of the proposed structure can be increased to become twice as high as that of a IPS PS-BPLC cell, and its driving voltage can also be lowered by about 20 V. With the vertical-field-driven PS-BPLC mode, the hysteresis problem of the IPS PS-BPLC mode can also be solved due to a reduction of the electric field required to achieve sufficient field-induced retardation.
©2011 Optical Society of America
Recently, blue phase liquid crystals (BPLCs) have attracted much interest as promising next-generation liquid crystal display (LCD) materials due to the advantages they offer in terms of display performance and fabrication process [1–7]. The BPLC mode can show a fast gray-to-gray response time of sub-milliseconds, which is over ten times faster than that of nematic LCDs [1,3,5]. The BPLC mode can be designed without an alignment layer, which means that the conventional rubbing process can be also eliminated. In the field-off state, the initial BPLC cell acts as an optically isotropic film due to its nano-sized double twist cylinder structures. It can thus display an ideal dark state and a high contrast ratio (CR) as well as wide-viewing angle properties under crossed polarizers . However, in order to be used practically, current BPLC modes present critical liabilities: a comparatively low light efficiency and resulting low degree of brightness, and too high a driving voltage to be operated by conventional a-Si thin film transistors.
Generally, BPLC modes are operated by in-plane-switching (IPS) schemes that use patterned electrode structures because the field-induced retardation increases along the field direction due to the electro-optic Kerr effect. If the BPLC is driven by a vertical electric field with non-patterned top and bottom electrodes, the BPLC cell under crossed polarizers will remain in a dark state and will not show gray-scale properties, because the BPLC layer acts as a c-plate optically under any applied voltage. However, the current IPS-driving scheme of BPLC modes causes low optical transmittance due to the low aperture ratio of the pixel area  because the intensity of the transversal electric fields is very low and the field-induced retardation is also low in the BPLC volume regions on the IPS electrode structures [9–12]. In addition, when conventional IPS electrode structures are used, the intensity of the transversal electric field near the top substrate remains low even at very high operating voltages, which means that it is difficult to obtain high field-induced retardation and high transmittance at low driving voltages. Currently, for the use of the BPLC phase in a wide temperature range, the self-organized BPLC nanostructures are stabilized by polymer networks, that are formed within BPLC defect regions from a mixture of BPLC molecules and monomers via a phase-separation method [1–3]. In the case of polymer-stabilized BPLC (PS-BPLC), the dielectric loss and the distributed LC anchoring from the polymer networks also necessarily increase the driving voltage. The severe hysteresis problem of the PS-BPLC mode, which exhibits quite different gray-scale curves for increasing and decreasing applied voltages, partly originates also from the IPS driving schemes because non-uniform electric fields are induced and extremely high transversal electric fields are formed near the bottom substrate with increasing driving voltages in order to obtain increased brightness [13–15].
To solve these problems, improvements in the BPLC materials itself for a higher Kerr constant are most important. However, improvements in the device structures also need be developed. There have been a number of approaches to reducing the operation voltage by increasing the effective transversal electric field near the top substrate by means of modified IPS PS-BPLC structures. These approaches have utilized electrodes on protrusions [15,16], periodic corrugated electrodes with saw-like shapes , corrugated electrodes for double-penetrating fringe fields , wall-shaped electrodes , and patterned electrodes for fringe field switching . These various electrode patterns, however, require fabrication processes that are too complex for practical production. In addition, most previous reports have been limited to the proposal of new electrode schemes and have investigated these only using simulation results.
Here, we demonstrate a vertical-field-driven PS-BPLC mode that resolves the conventional problems of low transmittance and high operating voltage in IPS PS-BPLC modes. By controlling the ray directions of the incident beams from the backlight with two prism sheets attached to the top and the bottom substrates, continuous grayscale properties can be achieved with a vertical field . Since the proposed structure does not need patterned electrode structures, the steps involved in fabrication can be simple, and the problems of the low aperture ratio as well as the low transmittance in conventional IPS PS-BPLC modes  can be resolved. The uniform electric field distribution by the non-patterned top and bottom electrodes through the vertical direction of the cell provides effective and comparatively large field-induced retardation that produces higher optical efficiency as well as lower hysteresis voltage shifts.
2. Principle and experiments
2.1 Device structure and operation principles
Figure 1 shows a cross-section view of our vertical-field-driven PS-BPLC cell structure, in which the non-patterned electrodes are used as the pixel and the common electrode at the bottom and top glass substrates, respectively. In our structure, the field in the BPLC layer is formed uniformly in a vertical direction. In the field-off state, the BPLC layer is optically isotropic, so an ideal dark state is obtained along any viewing direction under the crossed polarizers, if the effects of light leakage by the polarizers are not considered [22,23]. As an applied voltage is increased, field-induced birefringence is induced along the field direction as follows:Fig. 1, the prism sheet attached to the bottom glass substrate refracts the normal incident beams into the oblique beams with an incident angle of θi in the BPLC layer, following Snell’s law, and thus continuous grayscale properties can be obtained by varying the applied voltage as follows:22,23]. The two prism sheets were attached to the top and the bottom glass substrates such that the groove vector direction of the prism structure is 45° to the transmission axis of the crossed polarizers. For practical use of the presented scheme for display applications, additional optical film layers like diffusing sheets may be required to diminish diffraction or moiré effects which can be produced by the periodic structure of the top and bottom prism sheets.
As the BPLC material, a PS-BPLC was used by the photo-induced phase separation method with a precursor of a 65 wt% host nematic LC (NLC), a 25 wt% chiral dopant (S811, Merck), and 10 wt% prepolymers (a mixture of trimethylopropane triacrylate (TMPTA, Tokyo Kasei Kogyo) and RM257 (Merck)) with a small amount of photoinitiator. The host NLC (BYLC53XX, BaYi Space Co.) used here is a eutectic mixture consisting of several mesogen units, whose birefringence is 0.119. Phase sequence of the blended precursor was as follows; Iso-(48 °C)-BPI-(47 °C)-Ch. The cell filled with the blended precursor was irradiated with UV light (metal halide lamp) of 10 mW/cm2 (measured at 365 nm) for 3 min at BPI phase, resulting in the PS-BPLC. The PS-BPLC showed thermally stable BPI from 45 °C to 0 °C including room temperature (RT). Thus, we could conduct all the experiments at RT. Figure 2(a) shows the plot of Δn induced/λ for the PS-BPLC fabricated in this work as a function of the square of electric field at RT. The evaluated Kerr constant from Eq. (1) was about 0.09 nm/V2, which was lower than previously reported values [6,7].
To ascertain the cell gap and the incident angle conditions required for sufficient brightness with our PS-BPLC before attachment of the prism sheets, the transmittance of the vertical-field-driven PS-BPLC cell was measured by tilting the cell toward the incident beam (Ar laser, λ = 488 nm), as shown in Fig. 2(b). Since the incident beam to the PS-BPLC layer is refracted on an air/glass interface, the actual θi is different from the tilting angle (θt), as shown in Fig. 2(c). Figure 2(d) shows the voltage-transmittance (V-T) curves of the PS-BPLC cells according to the variation in θi, where the cell gap conditions of the three samples were d = 4 μm, 6 μm, and 10 μm. To determine the field-induced effective retardation in the cells, depending on the cell gap and the θi conditions, the amounts of the reflective optical loss at the glass/air surface were measured, and then the transmittance, as determined by the cell condition itself, was obtained by scaling the reflective optical loss. The V-T curves were measured with increasing applied voltages. Hereafter, the voltage sweeping condition was the same way with increasing an applied voltage, except in the hysteresis experiment. As we can see in Eq. (2), the transmittances in all the samples increased with increasing θi, due to the increased effective retardation under the same applied voltage condition. However, the saturated transmittances of the samples with cell gaps of d = 4 μm and 6 μm were still low - under 50% transmittance - until θi was increased to 38°, due to the low Kerr constant of our PS-BPLC material. By increasing the cell gap to 10 μm, we could obtain a transmittance value that exceeded over 90%. In the conventional IPS PS-BPLC mode, the level of transmittance cannot be effectively improved by increasing the cell gap, because the transversal electric field decreases steeply as it recedes from the bottom surface. Thus, its available transmittance is highly limited by the Kerr constant of the material itself. In our structure, however, the field-induced effective retardation can be effectively improved by increasing the cell gap, due to the uniform field distribution in the entire volume area of the PS-BPLC. Of course, the voltage required for saturated transmittance linearly increases as the cell gap increases. For this reason, the cell gap must be optimized, depending on the PS-BPLC.
2.2 Methods of prism sheet fabrication
Figure 3 shows the steps involved in the fabrication of the prism sheets used in our experiments. The prism sheets were fabricated by using an imprinting process from the shape of an anisotropic wet-etched Si (100) surface. Before etching, the Si surface was modified to become a SiO2 surface (with an oxidation depth of 1 μm) by means of thermal oxidation, and then a periodic SiO2 line pattern (line width = 20 μm, spacing between lines = 100 μm, line length = 2 cm) was made to provide periodic protection layers for the anisotropic wet etching process. Following the anisotropic wet etching with a tetramethyl ammonium hydroxide (TMAH) solution diluted by 5 wt%, a saw-like periodic pattern (periodicity = 120 μm) was formed in order for the etching angle to be 54.7°, due to the crystal structure of the Si. To obtain an ideal prism structure, the etching rate (0.8 μm/min) and the etching depth were carefully controlled. The etched Si surface was modified to become a hydrophobic surface, and then the Si surface structure was replicated by the polymerized polydimethlysiloxane (PDMS) film. Due to the hydrophobic surface pretreatment on the Si, the replicated PDMS mold could be easily detached from the patterned Si surface without generating any defects. Finally, the transparent prism sheet was formed onto the glass by an imprinting process, using the PDMS mold and a UV curable polymer (NOA89, Norland). The prism angle (θp = 54.7°) of the final prism structure obtained with NOA89 was identical to the etching angle of the anisotropic wet-etched Si surface as shown in the SEM image of Fig. 3(a). The refractive index (nprism) of the NOA89 is about 1.45 at the wavelength of the probe beam used in our experiment.
When we consider the refraction of the normal incident beam at the prism surface, the incident angle (θi) to the BPLC layer is about 24°, where the additional refraction effects between the prism/glass interface are assumed to be negligible because the nprism is similar to the refractive index of the glass (nglass~1.5). As we can see in the V-T curves obtained by the sample rotation methods shown in Fig. 2(d), an incident angle of 24° is not sufficient to enhance the optical efficiency where the transmittance remains below 40%, even for the cell with a cell gap of 10 μm. One of merits of using PDMS for the mold structure in the imprinting process is that the periodicity of the PDMS surface pattern can be simply varied by compressive force, due to its elastic property [24,25]. Figure 3(b) shows the fabrication process used to produce a higher prism angle of the prism film. As shown in the SEM image in Fig. 3(b), we were able to increase the prism angle from θp = 54.7° to θp = 64°, which resulted in the enhancement of θi from 24° to 28°. In this case, the results in Fig. 2(d) show that a level of transmittance that exceeds over 60% can be achieved for the vertical-field-driven PS-BPLC cell (d = 10 μm). From Fig. 2(d), we can see that θi over 38° will show best transmittance, where θp of 78.5° is required. However, the reflection loss at the air/prism interface of the bottom prism sheet increases as θp increases.
3. Results and discussion
3.1 Grayscale properties in vertical-field-driven PS-BPLC mode
Figure 4 shows the transmittance of the vertical-field-driven PS-BPLC cell (d = 10 μm) with two prism sheets (θp = 54.7°) on the top and the bottom substrates, which was measured with increases in an applied voltage under the normal incidence condition. For the PS-BPLC cell with two prism sheets, align markers were also patterned at the edge of the prism films by the photolithographic process and the imprinting process. The top and the bottom prism sheets were aligned through the optical microscope. To obtain transmittance, the transmitted light intensity under the parallel polarizers was first measured without an applied voltage, and the transmitted light intensities under the crossed polarizers were then measured by varying an applied voltage. For comparison, the V-T curve (d = 10 μm, θi = 24°) obtained using the sample rotation method in Fig. 2(d) is co-plotted in Fig. 4. That figure shows that a continuous grayscale property can be achieved in a vertical-field-driven PS-BPLC cell by refracting the normal incident rays into the oblique ones in the PS-BPLC layer through the prism sheets. The saturated transmittance was about 35%, where the optical efficiency enhancement effect was not yet good, due to the low θp and the resulting low θi. We can see, however, that the two V-T curves agree well with each other, which means that the additional refraction effects, except for the refraction at the air/prism interface, can be assumed to be negligible.
To see the effect of the top prism sheet on brightness under a normal viewing condition, two types of samples were prepared. One (Sample I) had only one prism sheet on the bottom substrate, and the other (Sample II) had two prism sheets in order to redirect the beams transmitted through the PS-BPLC cell. The other cell conditions were identical for the two samples (d = 10 μm, θp = 54.7°). Figure 5(a) shows the results of a brightness comparison for the two samples, depending on the viewing angle. Without a top prism sheet, the degree of brightness at an oblique viewing angle became even higher than that of normal brightness as shown in Fig. 5(a). For Sample I, the viewing angle showing maximum brightness is theoretically about 37.5° considering θi = 24°, which agrees well with the experimental result of Fig. 5(b). With the sample with two prism sheets, on the other hand, the degree of the brightness decreased monotonically from the maximum brightness as the viewing angle was increased from the normal direction. Polarizing optical microscopic (POM) images in Fig. 5(a) also show that the brightness of Sample II is much greater than that of Sample I under the same backlight conditions.
3.2 Optical efficiency and operation voltage
To enhance the optical efficiency of a vertical-field-driven PS-BPLC cell, two prism sheets with a higher prism angle (θp = 64°) were attached to the cell (d = 10 μm), and the V-T curve was then measured, as shown in Fig. 6 . Note that the incident angle into the PS-BPLC layer becomes 28° in this case. For comparison, the V-T curve of a vertical-field-driven PS-BPLC cell (d = 10 μm, θp = 54.7°) with two prism sheets having a lower prism angle, and the V-T curve of a PS-BPLC cell (d = 3 μm) driven by conventional IPS electrode structures were co-plotted together. In the IPS PS-BPLC cell, the spacing and the line width of the in-plane-patterned ITO electrodes were 3 μm and 4 μm, respectively.
When we compare two V-T curves of the vertical-field-driven PS-BPLC cells with two prism sheets, the saturated transmittance of the cell with two prism sheets with a higher prism angle was twice higher than that of the cell with two prism sheets with a lower prism angle as shown in Fig. 6(a). Considering that the incident angle was increased from 24° to 28° due to the increased θp, d(θi) in Eq. (2) was increased by about 3% from d(θi = 24°) = 10.95 μm to d(θi = 28°) = 11.27 μm. On the other hand, Fig. 6(c) shows that the field-induced Δneff(θi) was increased by about 48% from 0.0089 to 0.0133, which means that the improvement in optical efficiency originated in the increased Δneff(θi) in Eq. (2) by utilizing the increased θi.
When we compare the V-T curves of the IPS PS-BPLC cell and the vertical-field-driven PS-BPLC cell with a lower prism angle, we found that the saturated transmittances were similar, at about 35%. In the low operating voltage ranges, however, the transmittance of the vertical-field-driven PS-BPLC cell increased faster than that of the IPS PS-BPLC cell. The reason for this is that the field-induced birefringence was uniformly induced in the entire volume area of the vertical-field-driven PS-BPLC cell even at comparatively low operating voltages, and the total retardation accumulated as the oblique incident beam traveled through the cell. In the IPS BPLC cell, on the other hand, the Kerr effect near the top surface was insignificant at low operating voltages.
For the vertical-field-driven PS-BPLC cell with a higher prism angle, the saturated transmittance was enhanced to become almost 68%, which was twice the transmittance value of the IPS PS-BPLC cell. Note that the maximum available transmittance of the IPS PS-BPLC cell was limited to be below 43% due to the low aperture ratio from the IPS electrode itself as shown in Fig. 6(b), or due to the negligible field-induced effective retardation on the IPS electrode, regardless of the PS-BPLC materials. In conventional IPS PS-BPLC modes, the relationship between the IPS electrode spacing and the operating voltage inevitably involves a trade-off. In order to increase the aperture ratio under the same electrode spacing condition, the width of the electrodes must be narrower, a factor that is strongly related to the production yield. When the operating voltage denotes the applied voltage required for 90% transmittance to the saturated transmittance, the operating voltage (~43 V) of the vertical-field-driven PS-BPLC cell with two prism sheets (θp = 64°) was about 20 V lower than that (~64 V) of the IPS PS-BPLC cell. For a PS-BPLC material with a higher Kerr constant, it has been reported that the developed Kerr constant is almost 15 times higher than that which we used in our experiment . This means that the operating voltage can be lowered close to 10 V (a quarter of the operating voltage presented here) with a high-Kerr material. To obtain the same degree of brightness with the IPS PS-BPLC cell, the voltage required for the vertical-field-driven PS-BPLC cell was just 20 V, which was over 40 V lower than that of the IPS PS-BPLC cell. The field-on and the field-off response times of the vertical-field-driven PS-BPLC cell were about 0.2 ms and 0.6 ms, respectively, at a driving voltage of 70 V.
3.3 Hysteresis effects
In PS-BPLC technologies, the problem of hysteresis is also one of the important technical issues that need to be resolved. Hysteresis indicates the voltage difference required to obtain the same gray-level for increases and decreases in the voltage sweeps, where the hysteresis voltage (ΔV) is generally defined by the voltage difference at half the maximum transmittance. It is known that the hysteresis effect originates from the destabilization effects of the BP lattice structures or the supporting polymer networks, and the process of their structural restoration due to the high helical power of BPLC molecules . To minimize this hysteresis, improvement of the device structure in order to reduce the electric field required to obtain sufficient brightness, as well as material optimization, need to be developed [13–15]. However, to reduce the hysteresis through material development alone, the polymer networks of the PS-BPLC need to be a fine network structure with a uniform distribution, which generally results in an increase in the operation voltage.
Figure 7 presents the experimental results for a comparison between the hysteresis effects of our vertical-field-driven PS-BPLC cell (d = 10 μm) with two prism sheets (θp = 64°) and those of a conventional IPS PS-BPLC cell (d = 3 μm, electrode spacing = 3 μm, electrode width = 4 μm). In the V-T curves of the IPS PS-BPLC cell, a typical hysteresis property could be seen with ΔV~8 V on sweeping the voltage (63 V) to 90% maximum transmittance, where the ratio of ΔV to the peak sweep voltage (Vp) was ΔV/Vp~12.7%. At the same peak sweep voltage of 63 V, our cell also showed hysteresis effects of ΔV~3 V and ΔV/Vp~4.7%, though the hysteresis was effectively diminished. However, when the peak sweep voltage was 40 V, an appreciable difference between our vertical-field-driven PS-BPLC cell and the IPS PS-BPLC cell could be observed. The V-T curves of the IPS PS-BPLC cell still showed ΔV~3.5 V, while the vertical-field-driven PS-BPLC cell showed nearly identical V-T curves at increasing and decreasing voltage sweeps.
We can attribute these results to the field distribution. In the vertical-field-driven cell, uniform electric fields are formed, whereas the electric fields formed by the IPS electrodes are highly non-uniform. More importantly, in the conventional IPS PS-BPLC modes, extremely high electric fields are required near the bottom substrate, even for a much lower degree of brightness. For our structure, the intensity of the vertical electric field was about 4 V/μm at an operating voltage of 40 V, and it exhibited nearly 60% transmittance. On the other hand, the transversal electric field, near the bottom substrate in the IPS PS-BPLC cell, was about 13.3 V/μm at the same voltage of 40 V, and the transmittance was under 25%. To operate without hysteresis using the IPS scheme with our material, the operating voltage should be under 12 V, which could be easily expected with the maximum field condition without showing hysteresis in the vertical-field-driven PS-BPLC cell. With the IPS PS-BPLC cell, the critical voltage for a hysteresis-free operation was also experimentally confirmed, that was about 12 V. Under the applied voltage of 12 V, the transmittance of the IPS PS-BPLC cell was just about 3%. Figure 7 shows that the proposed structure is suitable for reducing the hysteresis effects by effectively utilizing the field-induced retardation as well as by providing the benefits of uniform electric field distribution.
In this paper, we demonstrated that the serious problems of low optical efficiency, high operation voltages, and high hysteresis properties that can be found in conventional IPS PS-BPLC modes could be improved by utilizing a vertical-field-driven structure. By using two prism sheets attached to the top and the bottom substrates, optically efficient field-induced effective retardation as well as a high degree of normal brightness could be achieved, which resulted in a higher degree of brightness and a lower driving voltage. In the proposed structure, due to the reduction of the required operating voltage and uniform electric field distribution, the hysteresis problem could also be effectively diminished. Due to one dimensional periodic prism structure, the effect of utilization of oblique beams is limited to rays along the grating vector direction of the prism film when the presented PS-BPLC structure is combined with a backlight with an angular light distribution. To resolve such problem, two dimensional periodic prism structures or micro-lens array structures can be proposed, which are under development for the next work. However, our results obviously show the value of discovering a device structure for the utilization of uniformly field-induced effective retardation using oblique rays. By optimizing the material and the cell structure, it is expected that an operating voltage under 10 V can be achieved with our switching scheme, and that our approach can then be applied to several areas of application that require a faster response than can be obtained from current NLCs, such as LCDs for 3D displays or color sequential displays, spatial light modulator panels, 3D shutter glasses, etc.
This research was financially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2011-0001083) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0023306).
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