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Abstract
A comparative study of the optical and electro-optic properties of polymer-stabilized blue phase liquid crystal (PS-BPLC) cells with photoalignment layers and rubbing alignment layers is performed. The optical microscope (OM) images of the photoaligned and rubbing-aligned cells depict that they have uniform PS-BPLC textures, while the post-processed OM images of the two cells reveal that the former has a more uniform PS-BPLC texture than the latter. The relatively uniform PS-BPLC texture in the photoaligned cell is verified by the reflective spectra of the two cells. The voltage-dependent transmittance curves and time-dependent transmittance curves of another two photoaligned and rubbing-aligned cells with in-plane-switching (IPS) electrodes indicate that the contrast ratio of the former is 41% larger than that of the latter, and the operation voltage (response time) of the former is 11 (10) % smaller than that of the latter. This result arises from the fact that the photoaligned cell with the IPS electrodes exhibits a relatively uniform PS-BPLC texture and relatively weak voltage-shielding effect. Therefore, the photoalignment can improve the electro-optic properties of PS-BPLC cells with alignment layers. The photoaligned and rubbing-aligned cells have no thermal hysteresis in low voltage regions due to the uniform surface alignment layers, and exhibit very low voltage-induced hysteresis in high voltage regions owing to no thermal hysteresis. Briefly, uniform surface alignment layers can eliminate the thermal hysteresis of PS-BPLC cells, suppressing their voltage-induced hysteresis. In other words, the hysteresis of PS-BPLC cells with alignment layers is independent of alignment methods as long as the alignment layers have strong surface anchoring energy.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Polymer-stabilized blue phase liquid crystals (PS-BPLCs) have attracted considerable attention in advanced displays and electrically controllable optical devices due to their submillisecond optical response [1–12]. Monodomain PS-BPLCs have many advantages such as free hysteresis, low operating voltage, and high contrast ratio. The monodomain PS-BPLCs can be achieved by thermal cycles [13], external applied voltage [14], linearly polarized UV light [15], and surface alignment [16]. Chen et al. depicted that the application of thermal cycles to BPLC cells can enlarge the domain sizes of the BPLC platelets and improve the uniformity of the orientations of the BPLC lattices, so the monodomain PS-BPLC textures are fabricated after UV curing. However, such a method is time-consuming due to the thermal cycle process [13]. Therefore, rapid methods for fabricating monodomain PS-BPLC cells must be developed. Chen et al. depicted that the application of voltages to BPLC cells with vertical field switching (VFS) electrodes can reorient the BPLC lattices, so the monodomain PS-BPLC textures are fabricated after UV curing [14]. However, such a method is impracticable for PS-BPLC cells with in-plane switching (IPS) electrodes due to their non-uniform fringing fields. This drawback hinders the usage of the PS-BPLC cells in practical applications. Xu et al. revealed that irradiation of linearly polarized UV light on BPLC cells with IPS electrodes can induce anisotropic polymer networks in a direction parallel to the electrical field of the UV light, so the monodomain PS-BPLC textures are formed after UV curing [15]. However, such a method is impracticable for PS-BPLC textures which include tri-functional monomers [e.g., 1,1,1-Trimethylolpropane Triacrylate (TMPTA)] because linearly-polarized UV light cannot cause anisotropic polymer networks in the textures. The PS-BPLC cells which include tri-functional monomers have faster optical response than those which include mono-functional monomers [e.g., dodecyl acrylate (C12A)] [17]. Therefore, methods for fabricating monodomain PS-BPLC cells which include tri-functional monomers must be developed. Yan et al. demonstrated that the application of rubbing alignment to polyimide-coated substrates can generate monodomain PS-BPLC cells which include tri-functional monomers, and this method is feasible for PS-BPLC cells with IPS electrodes and those with VFS electrodes [16]. However, the contact surface treatment should produce scratches on the rubbing alignment layers of the PS-BPLC cells, hindering the growth of the BPLC lattices, and deteriorating the optical and electro-optic properties of the cells.
Photoalignment is a contactless process, which eliminates the mechanical damage introduced by rubbing alignment. Therefore, photoalignment is an ideal candidate for the fabrication of monodomain PS-BPLC cells. Photoalignment can be achieved by doping methyl red dyes into LCs [18, 19] or coating SD1 dyes onto the substrates of LC cells [20]. However, the pumped methyl red dyes will aggregate on the irradiated surfaces of BPLC cells, causing the non-uniform BPLC textures in the cells [19]. Therefore, SD1 dyes are an ideal candidate for fabricating highly uniform PS-BPLC cells.
Dye-adsorbed layers [19], obliquely evaporated SiO2 layers [21] and vertical polyimide layers [10] have been used to study the effect of contactless surface alignments on the optical properties of PS-BPLC and BPLC cells. However, the electro-optic properties of the PS-BPLC and BPLC cells with the contactless alignment layers are not completely reported. In addition, a comparative study of the electro-optic properties of PS-BPLC and BPLC cells with contact and contactless surface alignments has not yet been performed. This study presents the optical and electro-optic properties of PS-BPLC cells with photoalignment layers and rubbing alignment layers in detail, including their optical microscope (OM) images, post-processed OM images, reflective spectra, voltage-dependent transmittance curves, and time-dependent transmittance curves. These experiments set this study apart from the previous studies [21, 22].
2. Sample preparation
Two PS-BPLC cells on which inner surfaces are individually deposited with photoalignment layers and rubbing alignment layers are used to study the effect of the contactless and contact alignments on the optical properties of PS-BPLC textures. Each cell comprises two glass substrates, which are separated by 14.9μm-thick spacers. Such thick spacers in each cell will prevent the Fabry–Pérot interference of the glass substrates from affecting the reflective spectrum of the PS-BPLC texture in that cell. 8nm-thick SD1 dye layers (DIC Corp.) are deposited on the glass substrates in the photoaligned cell, and then they are irradiated with linearly polarized UV light (λ = 365 nm) with an intensity of 0.15 mW/cm2 for 200 sec. The SD1 dye layers in the photoaligned cell cause the homogeneous alignment of the LC directors near the glass substrates. 500nm-thick polyimide layers (AL-1426CA, Daily Polymer) are coated on the glass substrates in the rubbing-aligned cell, and then they are rubbed by a nylon roller. The polyimide layers in the rubbing-aligned cell cause the homogeneous alignment of the LC directors near the glass substrates. Zhang et al. have depicted that the azimuthal anchoring energy of an irradiated SD1 layer with a thickness of 10 nm exceeds 4 × 10−4 J/m2, which is comparable with that of a rubbed polyimide layer [23]. They have also presented that the polar anchoring energy of an irradiated SD1 layer with a thickness of 10 nm equals to 5 × 10−4 J/m2. This value is smaller than that of a rubbed polyimide layer, which is typically 1 × 10−3 J/m2, but it is large enough for LCD applications, for which (1−10) × 10−4 J/m2 is suggested [24].
The BPLC precursor in each cell consists of a nematic LC HTW114200-100 (57.7 wt. %, Fusol Material), major chiral dopant S811 (33 wt. %, Fusol Material), minor chiral dopant S1011 (0.7 wt. %, Merck), monomer RM257 (4.5 wt. %, Merck), monomer TMPTA (3.5 wt. %, Sigma-Aldrih), and photoinitiator DMPAP (0.6 wt. %, Acros Organics). The physical properties of the LC are listed as follows: Δn = 0.266 at 589 nm, Δε = 10.6 at 1 kHz, and clearing temperature Tc = 103 °C. The BPLC precursor in each cell is heated to its isotropic phase (33.0 °C) and then cooled to its blue phase (24.5 °C) at a cooling rate of 0.1 °C/min. Following the cooling process, the BPLC texture in each cell is irradiated with a randomly-polarized UV light with an intensity of 0.5 mW/cm2 for 60 min. Each cell exhibits the PS-BPLC texture after the UV irradiation, and its temperature range lies between 0 °C and 50 °C. A PS-BPLC cell, which has the same configuration as the photoaligned and rubbing-aligned cells but no alignment layers on its inner surfaces, is used to evaluate the optical properties of the two cells.
3. Results and discussion
3.1 Optical microscope images
Figures 1(a), 1(b), and 1(c) present the OM images of the photoaligned, rubbing-aligned, and unaligned PS-BPLC cells, respectively. The photoaligned and rubbing-aligned cells exhibit more uniform PS-BPLC textures than the unaligned cell because the PS-BPLC textures in the former have no grain boundaries, while the PS-BPLC texture in the latter exhibits many grain boundaries. This result is expected because the surface anchoring energies of the photoalignment and rubbing alignment layers cause the uniform PS-BPLC textures in the two cells [16].
Fig. 1 OM images of (a) photoaligned, (b) rubbing-aligned, and (c) unaligned PS-BPLC cells. Post-processed OM images of (d) photoaligned and (e) rubbing-aligned cells. (f) Kossel diagram of photoaligned cell.
Figures 1(d) and 1(e) are the photos that are post-processed by changing the color levels of Figs. 1(a) and 1(b) using Photoimpact software, respectively. Figures 1(d) and 1(e) reveal the detailed morphologies of the PS-BPLC textures in the photoaligned and rubbing-aligned cells, respectively. The PS-BPLC texture in each of the cells comprises bright and dark green platelets. The bright green platelets in each cell refer to the platelets that reflect the light, the wavelength of which equals the peak wavelength of the reflection spectrum of that cell. The dark green platelets in each cell are the platelets that reflect the light, the wavelengths of which appear in the reflection spectrum of that cell but exclude the peak wavelength of that cell. The bright green platelets in the photoaligned cell are uniformly dispersed into the dark green platelets, while the bright and dark green platelets in the rubbing-aligned cell are individually merged into irregular stripes. This result arises from the fact that the contactless surface treatment causes the smooth alignment layers in the photoaligned cell, while the contact surface treatment produces scratches on the alignment layers in the rubbing-aligned cell. The post-processed OM images reveal that the PS-BPLC texture in the photoaligned cell is an optically isotropic film, while that in the rubbing-aligned cell is an optically anisotropic film. Therefore, the photoaligned cell has a more uniform PS-BPLC texture than the rubbing-aligned cell.
Rubbing can result in very smooth surfaces on a macroscale level (naked eye) while it causes bumpy surfaces on a microscale level [25]. The bumpy surfaces will restrain the growth of PS-BPLC and BPLC lattices because surface treatment greatly affects the orientations of the lattices [26]. Photoalignment uses light to align LC molecules while rubbing alignment utilizes mechanical drums to align them. In addition, the distance between the highest hill and lowest valley in the atomic force microscope image of the SD1 (unrubbed polyimide) surface is close to 2 (40) nm [27, 28]. Therefore, photoalignment layers are smoother than rubbing alignment layers.
Chen et al. have depicted that the cell with the parallel alignment layers exhibits a more uniform PS-BPLC texture than that with the vertical alignment layers because the adhesion energy between the PS-BPLC precursor and a parallel alignment layer exceeds that between the PS-BPLC precursor and a vertical alignment layer [22]. SD1 and polyimide are both planar aligning materials, so the adhesion energy between the PS-BPLC precursor and SD1 layer may be close to that between the PS-BPLC precursor and polyimide layer. Efforts are being made at the authors’ laboratory to study the wetting effect of the PS-BPLC cells with SD1 and polyimide alignment layers.
A broadband white light that is filtered with a bandpass filter with a central wavelength of 456 nm is used to generate the Kossel diagram of the photoaligned cell. Figure 1(f) shows the Kossel diagram of the photoaligned cell. The Kossel diagram is used to identify the phase of the PS-BPLC texture in the photoaligned cell and to determine the orientations of the PS-BPLC lattices in the cell. The experimental result in Fig. 1(f) displays that the PS-BPLC texture in the photoaligned cell is in the BPI phase, and the crystal plane parallel to the substrates is (1, 1, 0) in the cell. Because the rubbing-aligned cell has the same Kossel diagram as the photoaligned cell, it is not presented in this study.
A theoretical analysis is performed to verify the crystal plane of the PS-BPLC textures of the photoaligned and rubbing-aligned cells. The calculated peak wavelength of the reflection spectrum of a PS-BPLC cell for normal incidence can be expressed as [29]
where n, a, and (h, k, l) are the average refractive index, lattice constant and Miller indices of the PS-BPLC texture, respectively. n is given by [30]where ne and no are the extraordinary and ordinary refractive indices of the LC. a equals to the pitch length of the PS-BPLC precursor for BPI, and it can be determined from the reflective spectrum of the PS-BPLC precursor. Substituting ne = 1.78, no = 1.51, a = 242 nm, h = 1, k = 1, and l = 0 yields λ = 548 nm. The experimental peak wavelengths (538 nm and 544 nm) of the photoaligned and rubbing-aligned cells are close to their calculated peak wavelength (548 nm). The slight difference between the calculated wavelength and experimental wavelength in each cell may arise from the fact that the PS-BPLC lattices with a crystal plane of (1, 1, 0) oblique with respect to the normal of the substrates of that cell.3.2 Reflective spectra
Figure 2 shows the reflective spectra of the photoaligned, rubbing-aligned, and unaligned cells. The transmittance of each cell is defined as the ratio of the intensity of the light that is reflected from that cell to the intensity of the light that is reflected from a mirror. The baselines of the reflective spectra are at a reflectance of 10%, and mainly arise from the light reflection from the air/glass interfaces [14, 16]. The unaligned cell exhibits a broadband spectrum and appears colorful, indicating that the PS-BPLC lattices are oriented randomly. The peak reflectance (63%) of the photoaligned cell is larger than that (59%) of the rubbing-aligned cell, and the FWHM (18 nm) of the reflection band of the former is smaller than that (23 nm) of the reflection band of the latter. The relatively large peak reflectance and relatively small FWHM display that the orientations of the PS-BPLC lattices are more uniform in the photoaligned cell than in the rubbing-aligned cell. Therefore, the photoaligned cell has a more uniform PS-BPLC texture than the rubbing-aligned cell. This result verifies that of the post-processed OM images. The experimental results in Fig. 2 demonstrate that the photoalignment is ideal for fabrication of highly uniform PS-BPLC cells. The peak wavelength of the photoaligned cell is shifted from that of the rubbing-aligned cell. This result arises from the fact that the surface anchoring energies of the photoalignment and rubbing alignment layers orient their lattice planes towards different directions. As a result, the photoaligned and rubbing-aligned cells have different peak wavelengths. This work uses the surface anchoring energies of the photoalignment and rubbing alignment layers to tune the Bragg reflection peaks of the PS-BPLC cells. Application of external voltages to BPLC cells can also shift their Bragg reflection peaks [31, 32].
Three duplicate cells, which have the same configurations and fabrication processes as the photoaligned, rubbing-aligned and unaligned cells, are used to verify the reliability of the optical properties of the cells. Table 1 presents the peak wavelengths, peak transmittances and FWHM of the duplicate photoaligned, rubbing-aligned and unaligned cells. Each of the photoaligned, rubbing and unaligned cells and its duplicate cell have the similar optical properties. Therefore, the experimental data in Fig. 2 are repeatable in the duplicate experiment. The rubbing-aligned cell and its duplicate cell have relatively large differences between their peak wavelengths, peak transmittances and FWHM due to the contact surface treatment.
Table 1. Optical Properties of Photoaligned Cell, Rubbing-aligned Cell, Unaligned Cell and Their Duplicate Cells
3.3 Electro-optical properties
A comparative study of the electro-optic properties of another two photoaligned and rubbing-aligned PS-BPLC cells with IPS electrodes is performed. Figures 3(a) and 3(b) present the configurations of the photoaligned and rubbing-aligned cells with the IPS electrodes, respectively. These two cells have the same configurations as the photoaligned and rubbing-aligned cells of Fig. 2, but each of the former has IPS electrodes on one of the glass substrates. The line width of the IPS electrodes is 10 μm, and the gap between each adjacent pair of the electrodes is also 10 μm. The SD1 dye layers in the photoaligned cell with the IPS electrodes are irradiated with UV light (λ = 365 nm) with linear polarization parallel to the IPS stripes, so the LC directors near the glass substrates of the cell are reoriented perpendicular to the stripes before a BPLC texture is formed [20]. The horizontal polyimide layers in the rubbing-aligned cell with the IPS electrodes are rubbed perpendicular to the IPS stripes, so the LC directors near the glass substrates of the cell are reoriented perpendicular to the stripes before a BPLC texture is formed. The gap of each cell is held by 5.1μm-thick spacers.
Fig. 3 Configurations of (a) photoaligned and (b) rubbing-aligned PS-BPLC cells with IPS electrodes.
Figure 4 presents the experimental setup for measuring the electro-optical properties of the photoaligned and rubbing-aligned cells with the IPS electrodes. A probe light that is emitted from a red laser (λ = 633 nm) is normally incident to each cell, and that cell is placed between two crossed polarizers with an extinction ratio of 104:1. The transmission axis of one of the polarizers is at an angle of 45° with the IPS stripes. A function generator and voltage amplifier are used to drive the PS-BPLC texture in each cell. The PS-BPLC cells with the IPS electrodes can be treated as phase gratings when the cells are applied with voltage, so the linearly polarized light that passes through each cell will be diffracted into a set of the diffracted beams [33]. The intensities of the diffracted beams of each cell vary with the applied voltage. Therefore, a lens that is placed between the analyzer and a detector collects the diffracted beams of each cell [14, 15, 29]. The detector measures the intensities of the diffracted beams of each cell, and an oscilloscope that is connected with the detector records the voltage-dependent light intensity of that cell. The voltage-dependent transmittance of each cell is normalized using a ratio of I/I0, where I is the voltage-dependent intensity of the transmitted light of that cell, and I0 refers to the intensity of the light which passes through that cell in an isotropic state and under parallel polarizers [34, 35].
Fig. 4 Experimental setup for measuring electro-optical properties of photoaligned and rubbing-aligned cells with IPS electrodes.
Figure 5 displays the hysteresis loops of the photoaligned and rubbing-aligned cells with the IPS electrodes. Thermal (voltage-induced) hysteresis occurs in PS-BPLC cells as they are applied with the electric fields that are smaller (larger) than the critical electric fields of the cells [34]. The thermal hysteresis arises from the misalignment of the BPLC lattices, and the voltage-induced hysteresis is caused by the electrostriction of the polymer network. The thermal hysteresis of a PS-BPLC cell will affect its voltage-induced hysteresis because the structural strength of the polymer network in the cell is related to the uniformity of the orientations of the BPLC lattices [36, 37]. The SD1 and polyimide layers align the BPLC lattices during the cooling processes of the photoaligned and rubbing-aligned cells. In addition, many groups depict that the thermal hysteresis of PS-BPLC cells can be suppressed as the PS-BPLC platelets have sufficiently large grain sizes or no grain boundaries [13, 38]. The PS-BPLC platelets in the photoaligned and rubbing-aligned cells have no grain boundaries, as displayed in Figs. 1(a) and 1(b). Therefore, the photoaligned (rubbing-aligned) cell has no thermal hysteresis in a low voltage region between 0 V and 130 (140) V. The photoaligned (rubbing-aligned) cell exhibits very low voltage-induced hysteresis in a high voltage region between 130 (140) V and 155 (180) V due to no thermal hysteresis. Briefly, uniform surface alignment layers can eliminate the thermal hysteresis of PS-BPLC cells, suppressing their voltage-induced hysteresis. In other words, the hysteresis of PS-BPLC cells with alignment layers is independent of alignment methods as long as the alignment layers have strong surface anchoring energy.
Previous studies reported that the platelets in the PS-PBLC cells with the rubbing alignment layers exhibit two or more two distinct colors [22, 29]. Therefore, the PS-PBLC platelets have significant grain boundaries in the cells. The significant grain boundaries imply that the anchoring forces of the rubbing alignment layers are too weak. Therefore, the PS-PBLC cells with the rubbing alignment layers exhibit the thermal hysteresis. Because the voltage-induced hysteresis of a PS-BPLC cell will be affected by its thermal hysteresis, these cells have significant hysteresis in the whole voltage regions.
The transmittance T0V (0.008) of the photoaligned cell is ~38% smaller than that (0.011) of the rubbing-aligned cell as they are applied with zero voltage, as displayed in the inset of Fig. 5. This result arises from the uniformity of the orientations of the PS-BPLC lattices in the two cells. The experimental results in Figs. 1(d) and 1(e) depict the fact that the uniformly distributed PS-BPLC lattices in the photoaligned cell can be treated as an optically isotropic film, while the stripy PS-BPLC lattices in the rubbing-aligned cell can be treated as an optically anisotropic film. This fact reveals that the PS-BPLC lattices near the photoalignment layers exhibit a smaller residual birefringence than those near the rubbing-aligned layers. Therefore, the photoaligned cell has a smaller transmittance at zero voltage than the rubbing-aligned cell.
The peak transmittance Tpeak (0.55) of the photoaligned cell is close to that (0.54) of the rubbing-aligned cell, and the two values are slightly smaller than the simulated peak transmittance (~0.57) of a BPLC cell with 10/10 IPS electrodes [39]. This result arises from the two cells have no grain boundaries [Figs. 1(a) and 1(b)]. The contrast ratio ( = Tpeak/T0V: 1) of each cell is also discussed in this study. The contrast ratio (69:1) of the photoaligned cell is 41% larger than that (49:1) of the rubbing-aligned cell. The contrast ratios of both cells can be increased by compensating the polarization rotation of the PS-BPLC cells [15].
The operation voltage (155 V) of the photoaligned cell is 11% smaller than that (175 V) of the rubbing-aligned cell, indicating that the photoalignment can reduce the operation voltages of PS-BPLC cells with alignment layers. This result arises from the voltage-shielding effect of the photoalignment and rubbing-aligned layers. The explanation for the voltage-shielding effect is discussed as the following. The IPS electrodes are used to drive the PS-BPLC cells with the photoalignment and rubbing alignment layers. Because the fringe field is non-uniform in each cell, it is difficult to study the voltage-shielding effect of the alignment layers in the photoaligned and rubbing-aligned cells. Therefore, two PS-BPLC cells with vertical-field-switching (VFS) electrodes, instead of the IPS cells, are used to study the voltage-shielding effect [40]. The structure of each of the VFS cells is glass/ ITO/ alignment layer/ PC-BPLC/ alignment layer/ ITO/ glass. One of the VFS cells uses SD1 films as the alignment layers, and the other utilizes polyimide films as the alignment layers. Suppose that the SD1 (polyimide) films in the photoaligned (rubbing-aligned) cell with the VFS electrodes have the same thickness as those in the photoaligned (rubbing-aligned) cell with the IPS electrodes, and that the gap of the VFS cells equals to that of the IPS electrodes. The effective parallel dielectric constant (ε//_eff) of the LC in each of the VFS cells is given by [40]
where da and εa are the thickness and dielectric constant of the alignment layers; d refers to the cell gap, and ε// presents the intrinsic parallel dielectric constant of the LC. The dielectric constants of the SD1 and polyimide films can be measured in a separated experiment using a digital LCR meter (Agilent 4284 A) [41], and the measured values for the former and latter are 2.9 and 3.2 at 1 kHz, respectively. This method can also be used to measure ε//, and the measured value is 15.1 at 1 kHz. Substituting da = 8 (500) nm, εa = 2.9 (3.2), d = 10 μm, and ε// = 15.1 into Eq. (3) yields ε//_eff = 15.0 (11.0) for the VFS photoaligned (rubbing-aligned) cell. The effective vertical dielectric constant (ε⊥_eff) of the LC in each of the VFS cells is given by [40]where ε⊥ presents the intrinsic vertical dielectric constant of the LC. The measured value of ε⊥ is 4.5 at 1 kHz. Substituting da = 8 (500) nm, εa = 2.9 (3.2), d = 10 μm, and ε⊥ = 4.5 into Eq. (4) yields ε⊥_eff = 4.5 (4.3) for the VFS photoaligned (rubbing-aligned) cell. The effective dielectric anisotropy Δεeff ( = ε//_eff −ε⊥_eff) of the LC equals to 10.5 (6.7) in the VFS photoaligned (rubbing-aligned) cell. The LC has a larger effective dielectric anisotropy in the VFS photoaligned cell than in the VFS rubbing-aligned cell, so the voltage that is shielded by the SD1 films is smaller than the voltage that is shielded by the polyimide films as the two cells are applied with an identical external voltage. In other words, the VFS photoaligned cell has a larger voltage on the PS-BPLC film than the VFS rubbing-aligned cell as they are applied with an identical external voltage. Therefore, the voltage-shielding effect of the SD1 and polyimide layers causes the fact that the IPS photoaligned cell has a smaller operating voltage than the IPS rubbing-aligned cell.The exponential convergence model that Yan et al. proposed is used to fit the voltage-dependent transmittance curve of each cell [30]. The saturated refractive index change δns and saturation electric field Es of the photoaligned (rubbing-aligned) cell obtained from the fitting are 0.068 (0.064) and 11.3 (11.9) V/μm, respectively. The induced birefringence Δni of a PS-BPLC texture at an applied electric field (E) is given by
where λ is the wavelength of the light that is incident to the texture, and K refers to the Kerr constant. Substituting Δni = δns = 0.068 (0.064), E = Es = 11.3 (11.9) V/μm and λ = 0.633 μm into Eq. (5) yields K = 0.84 (0.76) nm/V2 for the photoaligned (rubbing-aligned) cell. The photoaligned cell has a larger Kerr constant than the rubbing-aligned cell, so the former has a smaller operating voltage than the latter.The response times of the photoaligned and rubbing-aligned cells are investigated in this section. Figures 6(a) and 6(b) present the screen captures of the oscilloscope for the photoaligned and rubbing-aligned cells with the IPS electrodes, respectively. The response time (890 μs) of the photoaligned cell is 10% smaller than that (993 μs) of the rubbing-aligned cell due to the relatively uniform PS-BPLC texture and relatively weak voltage-shielding effect of the photoaligned cell, and the detailed discussion is presented in Section 3.4. Therefore, the SD1 dye can accelerate the optical response of PS-BPLC cells with alignment layers for the development of color sequential displays. The experimental results in Figs. 5 and 6 are repeatable in duplicate experiments; however, the repeatable data are not presented herein.
Fig. 6 Response times of (a) photoaligned and (b) rubbing-aligned PS-BPLC cells with IPS electrodes.
3.4 Comprehensive discussion of electro-optical properties
The decay time of a PS-BPLC cell is given by [36, 42]
where p refers to the pitch length of the cholesteric LC; γ1 is the rotational viscosity of the LC in the chiral dopant, and k is the averaged elastic constant of the LC and polymer network. P and γ1 depend on the chiral concentration and ambient temperature, respectively, so they can be treated as constants in this study [36]. k is proportional to the structural strength (S) of the polymer network [36], and approximately equals to the square of the order parameter (O) of the LC that is in the chiral dopant and polymer network [37]. Because of S∝O2, the polymer network will be strong and stable as most of the BPLC lattices orient along an identical direction. The strong and stable polymer network will assist the chiral molecules in twisting the LC molecules following removal of a voltage that is applied to the PS-BPLC cell, so the decay time and voltage-induced hysteresis of the cell are decreased [36]. Briefly, the high uniformity of the orientations of the BPLC lattices in a PS-BPLC cell with alignment layers can cause fast decay time and low voltage-induced hysteresis of the cell. Because the orientations of the PS-BPLC lattices are more uniform in the photoaligned cell than in the rubbing-aligned cell, so the photoaligned cell has a faster decay time and lower voltage-induced hysteresis than the rubbing-aligned cell.The rise time of a PS-BPLC cell is given by [36, 42]
where ε0 is the dielectric constant in vacuum; Δε is the dielectric anisotropy of the LC, and E is the external electric field that is applied to the cell. A competition between Δε, E and k determines τr because P and γ1 can be treated as constants in this study. The LC has a larger dielectric anisotropy in the photoaligned cell than in the rubbing-aligned cell due to the voltage-shielding effect of the alignment layers [see the discussion that involves Eqs. (3) and (4)]; the former is applied with a smaller external voltage than the latter (Fig. 6), and the LC has a larger elastic constant in the former than in the latter due to the relatively strong polymer network of the photoaligned cell [see the discussion that involves Eq. (6)]. Relatively large Δε is favorable for the rise time of the photoaligned cell, but relatively small E and relatively large k are unfavorable for the rise time of the cell. Because the photoaligned cell has a faster rise time than the rubbing-aligned cell (Fig. 6), the voltage-shielding effect of the alignment layers dominates the rise times of the photoaligned and rubbing-aligned cells.The Kerr constant of a PS-BPLC cell is given by [36]
where Δn is the birefringence of the LC, and λ is the light that is incident to the cell. The photoaligned and rubbing-aligned cells use the same PS-BPLC precursor, so Δn and P can be treated as constants in our study. A competition between Δε and k determines K. The LC has a larger dielectric anisotropy in the photoaligned cell than in the rubbing-aligned cell due to the voltage-shielding effect of the alignment layers [see the discussion that involves Eqs. (3) and (4)], and the LC has a larger elastic constant in the former than in the latter due to the relatively strong polymer network of the photoaligned cell (see the first paragraph in the reply of this comment). Relatively large Δε is favorable for the Kerr constant of the photoaligned cell, but relatively large k is unfavorable for the Kerr constant of the cell. Because the photoaligned cell has a larger Kerr constant than the rubbing-aligned cell [see the discussion that involves Eq. (6)], the voltage-shielding effect of the alignment layers dominates the Kerr constants of the photoaligned and rubbing-aligned cells. Because the Kerr constant of a PS-BPLC cell is inversely proportional to its operating voltage, the photoaligned cell has a smaller operating voltage than the rubbing-aligned cell.In summary, the fast decay time and low voltage-induced hysteresis of the photoaligned cell result from the high uniformity of the orientations of the PS-BPLC lattices; the fast rise time and small operating voltage of the cell arise from the weak voltage-shielding effect of the photoalignment layers..
4. Conclusion
This study examines the effect of the photoalignment and rubbing alignment on the optical and electro-optic properties of the PS-BPLC cells. Experimental results depict that the photoaligned cell has a more uniform PS-BPLC texture than the rubbing-aligned cell since the PS-BPLC platelets in the former are uniformly dispersed, while those in the latter are merged into irregular stripes. This result is verified by the reflective spectra of the two cells. The relatively uniform PS-BPLC texture and relatively weak voltage-shielding effect cause the contrast ratio, operation voltage, and response time of the photoaligned cell with the IPS electrodes to be at least 10% smaller or larger than those of the rubbing-aligned cell with the IPS electrodes. Therefore, the photoalignment plays a key role in improving the optical and electro-optic properties of PS-BPLC cells with alignment layers. The photoaligned and rubbing-aligned cells with the IPS electrodes have no thermal hysteresis in the low voltage regions due to the uniform surface alignment layers, and exhibit very low voltage-induced hysteresis in the high voltage regions due to no thermal hysteresis. Briefly, uniform surface alignment layers can eliminate the thermal hysteresis of PS-BPLC cells, suppressing their voltage-induced hysteresis. Therefore, the hysteresis of a PS-BPLC cell depends on the surface anchoring energy of its alignment layers rather than on the alignment methods.
Acknowledgments
The authors would like to thank Prof. I. K. Yang for the support of a temperature controlled stage. This research was financially supported by the Ministry of Science and Technology (MOST) of Taiwan under contract no. MOST 104-2112-M-029-004-MY3.
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