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The work demonstrates a polarizer-free bistable reflective electro-optical switch that is based on dichroic dye-doped cholesteric liquid crystal (DDCLC). Bistable opaque and transparent states can be achieved using the planar and ULH textures of CLC, respectively. In the planar texture, the liquid crystal and dye molecules have a periodic helical structure with axes perpendicular to the substrate surface. They therefore absorb arbitrarily polarized light. In the ULH texture, the liquid crystal and dye molecules twist along the helical axes parallel to the substrate, and allow most of the arbitrarily polarized light to pass. Both the planar and ULH textures of CLC are stable states, and each can be switched to the other by applying a low-frequency (30Hz) electrical field, owing to the electro-hydrodynamic effect. A bistable electro-optical switch has the advantages of being polarizer-free and consuming low power. It can therefore potentially be used in portable information systems.
©2011 Optical Society of America
1. Introduction
Guest-host liquid crystal devices (GH-LCDs) have been developed and attracted much attention because of their wide viewing angle, high brightness, and lack of need for a polarizer [1–6]. Most GH-LCDs prepared by doping dichroic dyes into a liquid crystal (LC) host; the dye molecules are aligned by the liquid crystal molecules. Because of the dichroism, the dye molecules strongly absorb incident light that is polarized parallel to the principal axes of the dichroic dyes. They weakly absorb incident light that is polarized perpendicular to the dichroic principal axes. Therefore, the absorption properties of dichroic dye-doped LC cells vary with their alignment, and the absorption is easily modulated by controlling the LC directors.
Like general non-bistable LC devices, GH-LCDs can be maintained in a constant state only by applying an electric field. Their power consumption limits their application in portable displays. Bistable reflective cholesteric liquid crystal (CLC) displays, which do not require polarizers, compensation films, or backlights are ideal for use in update-on-demand electronic equipment [7–11]. However, a guest-host absorption display cannot be constructed from typical bistable CLC. Recently, we demonstrated an electrically switchable multi-stable cholesteric liquid crystal (CLC) that can adopt stable planar, focal conic, and uniformly-lying helical states (ULH) [12]. The three stable states have different optical properties because each has a unique structure. Therefore, when dichroic dyes are doped into the CLC, the dye molecules are aligned along the liquid crystal molecules and the different states of CLC absorb differently.
This investigation studies the optical properties of three stable states of dye-doped CLC (DDCLC) and further demonstrates a bistable electro-optical switch in reflective mode using only planar and ULH states. Each of the two stable states, the planar state (dark) and the ULH state (bright), can be switched to the other by applying a low-frequency (30Hz) electrical field. The bistable reflective electro-optical switch, requiring no polarizer and consuming relatively little power, is highly suited to portable information systems.
2. Experiment
The subject system was prepared by adding 6wt% chiral material (LC756, BASF) with a high helical twisting power (> 76 μm-1) and 3wt% dichroic dye (S-428, Mitsui Chemical Inc.) into nematic liquid crystal (E7, Merck). The mixing ratio of the chiral-nematic LC system was selected to reflect ultraviolet light (pitch = 220nm), and the mixed system was stirred for ~4 hr to ensure complete dissolution and form a homogenous mixture. It was then injected into an empty cell that was separated by a 7.5 μm-thick spacer. The cell was constructed from two glass plates that were coated with polyimide to form homogeneously aligned DDCLC films.
3. Results and discussions
Figure 1(a) schematically depicts the structures and absorption properties of the DDCLC cell in three stable states, planar, focal conic, and ULH state. When a DDCLC film is in the planar state, the liquid crystal and dye molecules twist along the helical axis, which is perpendicular to the cell surface. For display applications in this study, Bragg reflection by CLC can be ignored owing to its ultraviolet reflection band. Accordingly, dye molecules can absorb arbitrarily polarized light if it is within the absorption spectrum of the dye. When the DDCLC film is in the focal conic state, incident light cannot pass through the cell because it is absorbed by dye molecules and scattered by randomly oriented liquid crystal molecules. The CLC and dye molecules in the ULH state twist along the helical axes with uniformly oriented parallel helixes in the plane of the glass layer. Therefore, only light whose polarization is perpendicular to the helical axis of the ULH texture and parallel to the principal axis of the dichroic dye is absorbed; the other 75% passes through the cell. Restated, the ULH state of the DDCLC film is bright, while the planar and focal conic states are opaque. To understand the absorption in the three states, the transmittance spectra from 400 to 700nm were measured in these states, and shown in Fig. 1(b). When the DDCLC is switched into the homeotropic state by applying 120V, a high transmittance of ~70% is obtained. The approximately 30% loss is caused by the low order parameter of the dye and the fact that the dye molecules are not entirely perpendicular to the glass substrate at 120V. The dye-doped vertically aligned liquid crystal still weakly absorbs incident light. When the DDCLC film is in the ULH state, it allows specifically polarized light to pass through and its transmittance is ~75% of that in the homeotropic state. According to Beer’s law [13], the normalized transmittance can be expressed as T = exp(-αcd), where α, c and d are the effective absorption coefficient, concentration of dye molecules, and absorptive path, respectively. The transmittance of the DDCLC is proportional to the effective absorption coefficient α. Although both the planar and the focal conic states have low transmittance, the planar state is opaque because the averaged absorption coefficient (α∥ + α⊥)/2 exceeds the absorption coefficient, (α∥ + 2α⊥)/3, of the opaque focal conic state. Hence, the absorption in the planar state is better than that in the focal conic state and is more effective as a dark (off) state.
Fig. 1 (a) Schematic structure of DDCLC film in planar, focal conic, and ULH states. (b) Transmission spectra of four states of CLC planar, focal conic, ULH, and homeotropic.
Figures 2(a) –2(d) presents the textures of the planar, focal conic, and ULH states in a zero field, observed under transmission-polarizing optical microscope. All transitions were induced by applying low-frequency voltages (30 Hz). A low-frequency voltage (30Hz, 60V) can be applied to yield the ULH state caused by the electro-hydrodynamical effect [14]. When a low-frequency voltage is applied to the cell, an electro-hydrodynamic effect induces a perturbation and disturbs the original planar and focal conic states. This effect realigns the CLCs with their helical axes perpendicular to the electrical field and parallel to the substrate in the direction of rubbing. Some ionic impurities can also be doped into typical CLC materials to strengthen the electro-hydrodynamic effect. When the driving voltage is turned off, given a suitable cell gap and pitch, the ULH texture can be obtained, and the material will not then relax back to a planar or focal conic state. Notably, the voltage that is applied to achieve the ULH state cannot exceed 60V. As the applied voltage is increased from 60 to 115 V, a strong perturbation causes the CLC to adopt the focal conic state after the electrical field is removed. When a large voltage (> 120V) is applied to the cell, the liquid crystal molecules are aligned in a homeotropic state. After the voltage is removed, the liquid crystal molecules relax to the planar state. Figure 2(a) shows the image of planar texture with a short pitch of 220nm under cross polarizers. A uniform dark state can be observed because of the ultraviolet reflection band of CLC [15,16]. Besides, some oily streaks exist in the planar texture due to the formation of defects. The focal conic texture is a multi-domain structure, as displayed in Fig. 2(b). Figures 2(c) and 2(d) present the textures of a ULH state in which the optical axes were at 45 degrees (bright) and 0 degrees (dark), respectively, to the polarizer. CLC with the ULH texture behaves as a uniaxial crystal whose optical axis is parallel to the rubbing direction.
Fig. 2 Textures in three stable states: (a) planar state, (b) focal conic state, [(c) and (d)] ULH state with optical axis at 45° and 0° with respect to polarizer.
Figure 3(a) schematically depicts the structures and operating principle of the bistable electro-optical switch that exploits the planar and ULH states of dye-doped CLC. We choose the planar state as the opaque state because the absorption in the planar state is better than that in the focal conic state. When a large voltage (> 120V) is applied to the cell, the liquid crystal and dye molecules are aligned homeotropically. When the voltage is rapidly removed, the liquid crystal and dye molecules relax to the planar state. The reflective design causes ambient light to move along the absorptive path twice, causing most of it to be absorbed. The planar state of DDCLC is regarded as the dark (off) state. When a 30Hz voltage (60V) is applied to the DDCLC film, an electro-hydrodynamic effect causes a perturbation in the original state. This effect realigns the DDCLC with its helical axes perpendicular to the electrical field and parallel to the substrate in the direction of rubbing. When the driving voltage is turned off, the ULH texture is obtained. The ULH state of DDCLC allows incident light to pass through, and is bright (on). Figure 3(b) presents the reflective spectra of the DDCLC bistable electro-optical switch from 400 to 700nm. Since the optical path in the reflective (R) mode is twice as long as that in the transmission (T) mode, absorption in the R mode is stronger than that in the T mode. The reflectance in the ULH and planar state of DDCLC is 35% and 6%, respectively (at 550nm).
Fig. 3 (a) Structures and operating mechanisms of the DDCLC bistable electro-optical switch. (b) Reflection spectra of bistable DDCLC in planar and ULH states.
Figures 4(a) and 4(b) show the dependence of the reflective intensity on the incident angle of light in the planar and ULH state.(ELDIM) The incident angle varies from θ = 0° to 50° and φ = 0° to 360°, whereθandφare the viewing angle and the azimuthal angle. In the planar state, the reflective intensity decreases as the viewing angle increase due to the increase of optical path, and the planar state exhibits an approximate circular viewing symmetry. In the ULH state, the reflective intensity equally decreases as the viewing angle increases; however the ULH state represents slightly elliptic symmetry. The reflective intensity along the horizontal viewing (φ = 0°) is weaker than that along the vertical viewing (φ = 90°) because the absorption parallel the helical axes of the ULH state is stronger than that perpendicular the helical axes. The viewing diagrams was measured by commercial instrument (ELDIM), so the reflectance is slightly different from spectrum data due to the different integral algorithm.
Figure 5(a) shows photographs of the bistable device. In the dark planar state, the DDCLC film absorbs arbitrarily polarized light. The bright ULH state allows most light to pass through the DDCLC film. Figure 5(b) plots the measured contrast ratio as a function of wavelength. The contrast ratio in the experiment is defined as the ratio of the light intensity in the bright state (ULH state) to that in the dark state (planar state). The contrast ratio at a wavelength of 550nm is 6:1. The absorption of various wavelengths depends on the dichroic dye. The contrast ratio can be improved by increasing the order parameter of the dye, its absorption or its concentration.
Fig. 5 (a) Photographs of the DDCLC bistable electro-optical switch. (b) Contrast ratio of device as a function of wavelength.
In this study, to ignore the Bragg reflection of the planar state, we use a short-pitch CLC, resulting in a high operating voltage of 120V. Actually, if we replace planar state with focal conic state for the opaque state, the operating voltage can be reduced by using a longer-pitch CLC. For example, when the CLC with a pitch of 380nm was utilized in the device, the operating voltage can be reduced to 35V. However, the tradeoff is the reduction of contrast ratio because the absorption of dye in focal conic state is not as good as in planar state. Although some problems such as poor CR and high operating voltage existed in the device, these parameters can be adjusted and optimized for specific applications.
4. Conclusion
In conclusion, the optical absorption properties of three stable states of dichroic dye-doped CLC were studied. A polarizer-free bistable electro-optical switch using planar and ULH states was demonstrated. The planar and ULH states of CLC were dark (off) and bright (on) states, respectively. The planar state absorbed arbitrarily polarized light and the ULH state allowed most light to pass through. Moreover, the planar and ULH states were stable, and each could be switched to the other by applying a low-frequency (30Hz) electrical field. A bistable electro-optical switch has the advantages of requiring no polarizer and consuming little power; it therefore has potential for use in portable information systems.
Acknowledgments
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC NSC 99-2119-M-110-006-MY3. Ted Knoy is appreciated for his editorial assistance.
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