A cholesteric liquid crystal will adopt a helical structure with the director rotating around an axis perpendicular to the substrate surfaces in an electro-optical cell with homogeneous alignment. Because of the self-assembled helical structure of cholesteric liquid crystal, in the planar texture where cholesteric helix is aligned vertically, the incident light is decomposed into its right and left circular components with one component reflected and the other transmitted. The unique ability of a cholesteric liquid crystal to reflect light comes from their helical superstructure. The central reflected wavelength (λo) in a direction normal to the surface can be described as λ0= n·p= n·(C·HTP)−1, where p is the helical pitch, in which the director rotates 360 degree, n is the average refractive index of the liquid crystal, C is the concentration of chiral dopant and HTP is the helical twisting power of the chiral material. The bandwidth (Δλ) of the reflected light equals Δnλ/ n, where Δn is the birefringence of liquid crystal and n is average of refractive index. A continuous tunable and electrically programmable optical filter based on cholesteric liquid crystal can be fabricated for filtering different spatial wavelength. The bandpass filters can achieve 100% transmission or reflection when a combination of two cholesteric filters with the same reflection wavelength and opposite handedness are stacked.
When the helical pitch of a cholesteric liquid crystal is adjusted to Bragg reflect in the visible spectrum, it reflects an iridescent color. Depending on the magnitude of an applied voltage, the cholesteric liquid crystal in an electro-optical cell can be switched to different optical states such as the planar to focal conic and planar to homeotropic in which the incident light is weakly scatted or totally transmitted, respectively. The cholesteric cell displays an image which can remain on a display permanently without an applied voltage. This memory phenomenon can be achieved either by using surface treatment or polymer stabilization, as detailed, e.g. in U.S. Pat. Nos. 5,437,811, 5,691,795 and 5,695,682
For example, in responding to an applied low voltage, a liquid crystal with positive dielectric anisotropy initially with a planar texture is transformed into the focal conic texture. The focal conic state is stable at zero voltage. Even the gray levels can be stable such that a display which has a combination of planar and focal conic will maintain that particular combination and hence level of reflectivity over an indefinite period of time. When the applied voltage is above the threshold necessary for unwinding the helix, the cholesteric liquid crystal is transformed into one with a homeotropic texture where ambient light is totally transmitted and the cells appears transparent. The homeotropic state reverts back to the initial planar state upon the quick removal of the applied voltage. When the surface of the back panel is painted black, both the focal conic texture and homeotropic states appear black. The color reflective planar texture and the transparent focal conic texture can be stable over a sufficiently long period of time such that an image can be addressed on a high resolution matrix display and the image will remain on the display after the voltage is removed.
A multicolor cholesteric display was first introduced by using a color pixelation technique with a combination of photo illumination tuned chiral material to adjust the helical pitch in the exposed regions to produce red, green and blue colors as seen in U.S. Pat. No. 5,668,614. While the feasibility has been demonstrated, there is a loss in reflective brightness. Another color reflective display technology was introduced shortly using vertical stacked RGB panels to achieve the multicolor and enhance the reflectivity. The brightness of the color panel is maximized by using a combination of left and right-handed circularly polarized cholesteric material in different panels. A full color cholesteric display with reflectivity exceeds 50% of the ambient incident light was reported in U.S. Pat. No. 6,654,080. The bottleneck for the full color cholesteric displays to be realized commercially resides in the production yield and cost. For example, to display a full color image it requires three color cholesteric films and electronic drivers which increase the cost of the display. Furthermore, the shift register of pixels from separate panels causes parallax problem. Parallax demands that the thickness of the stacked layers be thinner than the pixel size. As a result, the yield in producing full color displays is low because of complexity in manufacturing process.
An alternative method to produce full color reflective cholesteric display involves the use of electrically tunable color technology. It is not anticipated that the focal conic state be used in which the switched color requires the voltage to remain on to display the desired color. The electric-field induced color change in cholesteric liquid crystals color can be traced back to the 1960's. Because the relationship of λo= n p cos θ, the increase in tilt angle of cholesteric helix observed 15° from normal to the surface in response to applied voltage results in a smaller cholesteric pitch and thus, the spectral wavelength is blue shifted. Pitch dilation in cholesteric liquid crystal in which the color changes from blue to red with an increase in an applied field has been noted. In general, the cholesteric liquid crystal response to applied voltage by the rotation of the cholesteric helix away from normal direction of substrate surface. Without strong surface anchoring, there are insufficient cholesteric pitches to reflect incoming light in the normal direction. Consequently, these methods yield low reflectivity and short spectrum tuning range.
Another electrically-tuned color technology utilizes a display with in-plane inter-digitized electrodes configured on one surface and only LC alignment layer on the other surface without electrode as described in U.S. Pat. No. 6,630,982. The device enables an inhomogeneous distribution of electric field across the cell thickness and unwinds and elongates the cholesteric helix when an appropriate voltage is applied to the inter-digitized electrodes. Using a positive dielectric anisotropy cholesteric, the cholesteric pitch is extended with the increase in applied voltage. To achieve high reflectivity for each switched color, this display requires a thick cell. As a consequence, high switching voltage and slow response time are major challenges associated with the in-plane switched color technique.
The use of gel to preserve the polymer structure and uniform distribution of polymer within the cell has been reported. With a polymer consisting of mesogenic diacrylate and monoacrylate, the gel enables a shift in reflection band to low wavelength with increasing voltage, which was associated with the tilting of the cholesteric helix. In a second case, with a chiral monoacrylate additive, the reflection band is not shifted but reduces the reflectivity with increasing voltage, which is associated with Helfrich deformation following unwinding the helices. The negative aspects include broadening of reflective bandwidth and low reflectivity. The use electrical-field induced color change in cholesteric liquid crystal has been described using Helfrich deformation. The field-induced change in optical property of the cholesteric reactive mesogen is photopolymerized and fixed on a polymer film by masked curing the cholesteric reactive mesogen at different voltages. The negative aspects of this approach are a multicolor static film with loss of reflectivity at normal direction and broadening of spectral wavelength.
It would therefore desirable to provide a technique for fabricating cholesteric liquid crystal light modulating devices in which the spectral wavelength can be electrically switched, and for displays with all of the mentioned benefits, which can be practically implemented.