The helical structure of cholesterics leads to unusual optical properties as noted earlier and the inclusion of polymer networks can provide important advantages. We discuss here two different device applications which made use of some of these properties: The reflective cholesteric liquid crystal display and the light shutter.
When the pitch of the cholesteric material lies in the range of visible wavelengths, the property of selective reflection from the planar cholesteric texture in contrast to the partial or full transmission through the focal conic and homeotropic textures provides opportunities for a variety of display applications. For example, in the planar state, a polymer stabilized cholesteric cell reflects incident light, and appears bright. In the focal-conic state, the incident light is transmitted through the cell and reveals the color of the coating on the rear of the window.
Fortunately, both of these states are stable at E=0. This means that the textures are "locked in" and will remain intact until acted upon again (i.e. the device is bistable). Switching from planar to focal conic requires a low voltage pulse while the return from focal conic to planar requires a higher voltage pulse to drive the device into a homeotropic state which then relaxes through a transient planar texture to the final planar state (Chap 5 &12, Crawford and Zumer, 1996; Huang, 1997).
Bistable reflective cholesteric cells are prepared (as described earlier, in connection with the formation of networks in Polymer Stabilized Cholesterics) to have an appropriately short pitch and then photopolymerized in the initial planar state. As discussed there, the polymer network breaks up the planar texture into small domains which are only slightly misoriented from each other. These polydomains respond more rapidly to the switching waveform than would bulk material, an important factor for many display purposes. In addition, a good gray scale operation can be achieved through choice of polydomain sizes and switching waveforms. Even more exciting is the fact that color displays are now possible through the use of either layered cholesteric cells, or the formation of pixels. For the layered cholesterics, each layer is set to a different optical pitch to reflect different colors. The pixel cells work much like in other system by having each pixel contain one specific color ready to be turned on or off.
Among the other advantages in devices employing bistable reflective cholesteric material is their relatively low power requirement, due to the ability to hold pixels in either reflective or transmitting states at no field, in addition to their use of reflected light (in comparison with the traditional LCD which requires backlighting and polarizers). They also offer the possibility of highly multiplexed passive displays and of the use of plastic substrates which have non-uniform birefringence but, in this case, do not enter the optical path. The ease of polymer network bonding to plastic substrates offers mechanical stability, as well. Finally, while video rates are not yet available, and reflective brightness still needs improvements, research in materials and in drive methods continues to show promising developments. In the meantime, some improvements in current technology should yield several possible applications including electronic books and newspapers.
By lengthening the cholesteric liquid crystal pitch to the order of infra-red wavelengths, the bistable effect at zero applied electric field is avoided. Instead, the choice of either planar or focal conic stable state is determined by initial conditions at the time of polymer network formation, as discussed below. The starting materials are the same as those used in the bistable reflecting cholesteric except for a reduction in concentration of the chiral agent to lengthen the cholesteric pitch. We treat the reverse mode shutter first since more polymer network studies have been carried out in this configuration, then move on to the normal mode shutter and illustrate its characterization with a simulation.
In preparation of a reverse mode cell, a solution of longer pitch cholesteric material containing a few percent of reactive monomers is placed between two glass plates whose inner surfaces have been coated with a transparent conducting material, such as indium-tin oxide (ITO), to form electrodes and then prepared to yield a planar texture. Photopolymerization in the absence of an applied field yields a polymer network which stabilizes this planar texture. The network is predominantly oriented parallel to the glass plates. As illustrated in the figure below, visible light is transmitted by the cell in the planar texture because the pitch of the helices lies in the infra-red wavelength range. However, under the application of a moderate electric field perpendicular to the windows, the cholesteric material switches to a focal conic texture. This strongly scatters visible light because of the index of refraction changes at the polydomain boundaries between focal conic regions.
|Figure (a) : A reverse mode cell with no applied electric field (off-state). The liquid crystals are aligned in the planar texture and transmit visible light due to their long wavelength helical pitch. The blue wedges represent layers of planes of liquid crystals and indicate their average director orientation. The schematic polymer network is green.||Figure (b) : A reverse mode cell with an applied electric field (on-state). The liquid crystals are in the focal conic texture and scatter light. The black lines represent the light rays entering and being scattered throughout the cell.|
When the electric field is turned off, the cholesteric material relaxes back to the original planar texture. Such a cell can be reliably cycled between these two states many times.
Application of a very high electric field will switch the cholesteric material to an untwisted homeotropic texture. This can deform the polymer network sufficiently that the material no longer returns to the original transparent planar texture when the field is turned off. Hence, caution must be exercised in such tests. The following movie schematically demonstrates the electric field response of cholesteric material in the presence of a polymer network formed in the reverse mode configuration. In the case of this movie, for simplicity, the red triangles represent a full or at least a partial helix but illustrate an average director orientation of a few liquid crystal layers only near the center of the helix. There is no significance to the fact that they all point in the same direction in the planar state shown with the field off.
Dierking, et al (Dierking, 1997) have studied factors affecting the polymer network and its morphology as well as their influence on the electro-optic properties of reverse mode polymer stabilized cholesteric cells. Among other things, they have discovered a two stage reorientation effect in a study of the switching process between the planar and focal conic textures. For sufficiently low polymer concentration, the cholesteric material in the vicinity of the polymer network experiences elastic interactions which accelerate its relaxation and reorientation while the cholesteric material in the network voids behaves in more of a bulk-like manner, exhibiting a more extended relaxation.
Normal mode cells are photopolymerized in the homeotropic texture produced by a strong electric field. When the electric field is removed the cell settles into the focal conic texture. In the absence of an electric field the cell scatters light and is opaque. With the application of an electric field the cell is once again transparent.
|Figure (c) : A normal cell with an electric field applied (on-state). The liquid crystals are aligned in the homeotropic texture. The red wedges represent the average director orientation of layers of planes of the liquid crystals and the blue schematically shows the polymer network.||Figure (d) : A normal cell without an electric field (off-state). The liquid crystals are in the focal conic texture and scatter light. The black lines represent the light rays passing through the cell.|
Much of the polymer network is oriented perpendicular to the cell surface as illustrated here.
With regard to electro-optic properties, transmittance for both the normal and reverse mode cells depends on the voltage applied to the cell. As the voltage across a normal mode cell increases, the transmittance increases. A low voltage yields a low transmittance, whereas a high voltage produces the transparent homeotropic texture, with high transmittance because the index of refraction is quite uniform throughout the liquid crystal material. This is in contrast with the focal conic case (off-state) where the index variation across focal-conic boundaries yields strong light scattering. The normal mode cell exhibits significant hysteresis in the dependence of its transmittance with voltage, as illustrated in the following simulation, while the reverse mode cell does not.
The transmittance of a reverse mode cell increases as the voltage decreases; maximum transmittance occurs at zero voltage. With a steady increase in voltage, there is initially no change until a transition stage is reached where the liquid crystal material begins to enter the focal conic texture and the transmittance decreases. The reverse mode cells minimum transmittance occurs at high voltage.
In light shutter applications, the choice of mode depends upon the desired operations, but there is relatively little variation in transmission across the visible wavelength spectrum exhibited in either mode and they both have a wide viewing angle. Yang, et al (chap 5, Crawford & Zumer, eds, 1996) discuss characteristics in much greater detail as well as the factors which influence application.
To view representative electro-optical properties of normal mode cells and how these measurements are made, please see the simulation linked above.
As discussed in the Polymer Stabilized Cholesteric Liquid Crystals section, photocurable monomers dispersed in a cholesteric liquid crystal mixture form a bistable, polymer stabilized cholesteric texture (PSCT). Initially, low concentrations of the monomer were used. This was done because high concentrations yielded dense polymer networks in the liquid crystal. This would result in significant light scattering in the focal conic state, which should only weakly scatter the light. Thus, there is less contrast between this state and the planar state, which reflects the light. Color purity and brightness is also reduced by the light scattering. However, high concentrations are of interest for their structural benefit. For example, they provide a self-adhering and self-sustaining structure necessary for flexible devices of large area on polymer substrate.
Polymer walls make use of high polymer content without adversely affecting the electro-optic characteristics as reported by ALCOM researchers (Kim, 1998). In a pixel array, these walls are formed in the interpixel region such that the polymer network is not very dense in the liquid crystal region. There are two ways to produce the polymer wall. One involves irradiating only selective areas of a cell containing ultra-violet (UV) curable monomers and liquid crystal with UV light through a photomask. This causes the phase separation by photopolymerization such that the polymer only exists in those regions where the UV light was allowed to pass through. The reason this occurs is that the monomer polymerizes in this region, thereby reducing the concentration of monomer. Due to the concentration gradient, more monomers disperse into this region and polymerize as well until most of the monomers have been polymerized in what becomes the interpixel region.
The second method uses the same materials but the polymer is attracted to the interpixel region by a patterned electric field. This is done by etching a cross pattern of indium tin oxide (ITO) on the substrate. The temperature is then decreased to phase separate the sample while an electric field is applied by means of the ITO. This field causes the monomer to segregate into the low field region, i.e. areas where the ITO was not applied. The reason this occurs is that the liquid crystal mixture has a larger dielectric constant than the monomer and experiences a greater force from the fringing fields in the interpixel. This forces the liquid crystal molecules to the high-field regions, leaving the monomer in the low-field region. The monomer is then cured by blanket UV exposure. This forms the polymer walls that define the interpixel region and leaves the pixels rich in liquid crystal material, i.e. low monomer concentration. Further research into polymer walls has allowed the use of UV-cured monomers in both Twisted nematic (TN) and electrically controlled birefringence (ECB) displays. Although the choice of monomers in these curing processes is more restricted, the results are displays with an increased mechanical strength.
The planar state of PSCTs reflects light of a certain wavelength. For light at normal incidence, the wavelength is given by Bragg's reflection law, l = p * n, where p is the pitch and n is the average refractive index. Thereby, a change in the pitch length of the material would lead to a different wavelength of light being reflected. Alcom researchers (Chien, 1998) have shown that a tunable chiral material (TCM) can be used for such a purpose. When added to the cholesteric formulation, it increases the chirality and thereby decreases the pitch (initially the cholesteric formulation reflects red light and after the TCM is added it reflects blue). Ultraviolet radiation polymerizes the tunable chiral dopant such that its concentration decreases. As the concentration decreases, the chirality of the mixture decreases and the mixture's pitch increases. Thus, longer UV exposure means the pitch of the mixture increases a greater amount and the reflected wavelength increases proportionately according to Bragg's reflection law (the reflected wavelength can be increased back to the original wavelength, that of red light). However, bleeding or diffusion of color may occur over time because of the concentration gradient due to the varying concentration of the TCM. Linking the TCM to a polymer network can be used to hold the TCM in place and prevent color diffusion. The downside to this is the same as discussed above; more polymer content means poorer reflectance and color brightness. Using TCMs with higher helical twisting power would allow less to be used. Despite some problems still to be overcome, TCMs allow a sequential array of the three primary colors to be made from the same cholesteric mixture by exposing pixels to varying amounts of UV light. As illustrated below, selective masking and repeated UV exposure accomplishes the dual purpose of color patterning of pixels through modifying the chirality of the TCM and photopolymerizing the remaining monomer to produce the desired polymer network for device stabilization. In the illustration longitudinal stripes rather than pixels are shown and the requisite increase of UV exposure time is achieved by a series of 2 masks and exposures.
Colored bistable reflectives possess the same primary advantage as the monochrome devices - very low power requirement. However, the rather complicated fabrication process needed for this TCM type in combination with uncertainty over the long term stability of the colors underscores the need for further development work in this area.