cholesteric lcd display quotation

Yuan H (1996) Bistable reflective cholesteric displays. In: Crawford GP, Zumer S (eds) Liquid crystals in complex geometries. Taylor & Francis, London, pp 265–280

Commonly, liquid crystal material may be modulated to produce a display. Conventional liquid crystal displays commonly use twisted nematic (tn) liquid crystal materials having a pair of states that may differentially pass or reflect incident light. While twisted nematic displays may be reflective or transmissive, cholesteric displays are usually reflective (but they may also be transmissive).

In cholesteric displays, the cholesteric material has very high optical activity. Such liquid crystal material switches between a reflective texture called the planar cholesteric texture and the transparent configuration with the focal conic texture. The cholesteric molecules assume a helical configuration with the helical axis perpendicular to the surface of the substrates.

The cholesteric liquid crystal molecules, in response to an electric field, align as planar texture with the optical axis, reflecting light of a particular wavelength. Generally, the maximum reflection in the planar cholesteric texture is at a wavelength directly proportional to the material"s pitch distance.

Conventionally, an electric field is applied in the direction of the optical axis in order to change the phase and the texture of the cholesteric material. However, these changes are generally in the form of the material either being reflective to the spectrum of light of a given wavelength or not reflecting light at all.

Thus, a given completed cholesteric liquid crystal cell may produce reflected light with a specific color, such as red, green or blue, but not any combination of them. Therefore, the conventional approach is to provide separate cholesteric display elements for each of the three primary colors (e.g., red, green and blue). These separate display elements may be stacked up one on top of the other in order to generate the desired full color reflected light output. Alternatively, the three elements may be placed side by side each displaying the same color. The three different colors may be achieved using color filter material.

The use of color filter material substantially reduces the display brightness and increases the overall cost of the display. Similarly, the use of three separate cells in a stack effectively triples the cost of the display. Stacked elements may even reduce the optical brightness of each display pixel.

Bistable reflective cholesteric displays are particularly advantageous for many portable applications. The bistable material is advantageous because it may be placed in one of the two states that have different optical properties. Once placed in either state, the material stays in that state even when power is removed. Thus, a given displayed pixel may remain, without refresh, in a given state until it is desired to change the optical information that is displayed. Being reflective in nature, and hence avoiding the need of backlight plus avoiding the need for refresh will substantially reduce power consumption of the display subsystem.

FIG. 4 is a diagram showing a bistable cholesteric display in an active matrix display arrangement, in accordance with one embodiment of the present invention; and

Referring to FIG. 1, a cholesteric display may include a bistable cholesteric material 16 in one embodiment of the present invention. The material 16 is sandwiched between two substrates 12 and 24. The substrate 24 is advantageously substantially transparent and may conventionally be glass with an absorbing undercoating such as carbon black. The substrate 24 may or may not be transparent. The substrate 24 may be made of a variety of materials. The substrates 12 and 24, in one embodiment, may include transparent electrodes 14 and 22. The transparent electrodes 14 and 22, in one embodiment, may be made of indium tin oxide (ITO).

Sandwiched between the substrates 12 and 24 is a sideways electrode 26 band an opposed sideways electrode 26 cwhich, in turn, is also opposed to a sideways electrode 26 d. Between the electrode 26 and the substrate 24 is a material 20. In an active matrix embodiment, the material 20 may be a thin film transistor or other active element to drive the actual display. In a passive matrix embodiment, the material 20 may be a row or column contact or electrode.

A display 10, shown in FIG. 2, may be formed out of a plurality of pixels 40 arranged in a grid work array. Each pixel 40, such as a pixel 42, may be divided into three or more subpixels 42 a, 42 b, and 42 c. In one embodiment, each subpixel 42 may be responsible for generating light of a different wavelength. Thus, each pixel 40 may produce three different wavelengths of light, such as red, green, and blue wavelengths.

Referring to FIG. 3, the electrodes 14 and 22 apply an electric field along the optical axis O of the display 10. The optical axis O is aligned with the direction of incident light “L”. The light L, directed toward the upper surface of the substrate 12, passes through the upper surface and the electrode 14 and is reflected (or not) by the cholesteric material 16, as indicated by the light beam R, to produce the perceived image. Since the light arrives at and is reflected from the top upper surface, the optical axis O is oriented generally transversely to the substrates 12 and 24.

In conventional fashion, the electric field developed by the electrodes 14 and 22 may cause the bistable cholesteric material 16 to transition between the reflective planar cholesteric texture and the transparent, focal conic texture. The procedures for applying potentials for causing these transitions to occur are well known in the art.

In general, an electric field may be applied by an alternating current voltage source 30 that is electrically coupled to the electrodes 14 and 22. When the cholesteric material 16 is in its transparent texture, in some embodiments, the lower substrate 24 becomes visible. When the material 16 is in its planar cholesteric texture, light of a given wavelength is reflected. That given wavelength is generally determined by the helical pitch of the material 16. In conventional cholesteric displays, this pitch is defined and is fixed. Thus, in conventional cholesteric displays each display element either provides one reflected color or is transparent, displaying the color of the substrate 22.

The electrodes 26 allow the pitch set by the electrodes 14 and 22 to be varied. In one embodiment of the present invention, the electrodes 26 enable the fixed pitch to be varied between three different pitches. Each of the different pitches, associated with a given potential on the electrodes 26, may produce one of three different light colors. In one embodiment, for example, red, green and blue light may be selectively produced from a single display element 10.

In order to generate light for a black and white display, for example, the helix of the material within each of the subpixels 42 may be appropriately altered to separately produce red, green, and blue light at the same time. The complementary reflectance of these three colors renders a pixel 40 white in color as a whole. Thus, any color may be produced by operating one of the three subpixels 42 and the color white may be produced by operating all of the subpixels 42. Conversely, in one embodiment, when none of the subpixels are reflective, the pixel 40 appears to be dark or black.

Through the use of electrodes 26, a multi-colored display pixel may be produced with only a single cholesteric display element. As a result, substantial cost savings may be achieved by avoiding the need for three different display elements that are either laterally displaced from one another or stacked one atop the other. Moreover, when three display elements are utilized in a laterally displaced arrangement, color filter arrays are generally needed and color filter arrays would significantly increase the cost of the display.

Referring to FIG. 4, in one embodiment of the present invention, an active matrix display may be implemented. In such case, the material 20 may constitute a thin film transistor or other active element. In one embodiment, the gate 22′ of the thin film transistor may be coupled to a line 36, that is in turn coupled to the electrode 22. At the same time, the source of the transistor 20 is coupled via line 38 to the electrode 14. The drain 22″ may be coupled via a line 40 to an appropriate ground connection in one embodiment of the present invention. An external storage capacitor 34 may be provided in some embodiments.

Similarly, in a passive matrix display embodiment, shown in FIG. 5, the electrode 14 may be coupled to a column potential and the electrode 22 may be coupled to a row potential. In such passive matrix addressing case, a thin film transistor is not needed to provide electrical addressing with row and column potentials on the material 20.

Metasurfaces can enable polarization multiplexing of light so as to carry more information. Specific polarized light necessitates bulk polarizers and waveplates, which significantly increases the form size of metasurface devices. We propose an electrically programmable metasurface enabled by dual-frequency cholesteric liquid crystals (DF-CLCs) for simultaneous near- and far-field displays. Moreover, the integrated device can be electrically programmed to demonstrate 6 different optical images by engineering the DF-CLCs with frequency-modulated voltage pulses. Such programmable metasurfaces are potentially useful for many applications including information storage, displays, anti-counterfeiting, and so on.

Liquid crystals have great potential for developing photonic devices that control the optical behaviors of liquid crystals in smart devices with external stimulation. In this study, we have demonstrated a series of bistable cholesteric liquid crystal devices using the dual-frequency nematic liquid crystal HEF951 assisted by a predesigned chiral ferroelectric liquid crystal. The synthesized chiral ferroelectric liquid crystal was used to induce the formation of the cholesteric liquid crystal phase and decrease the driving voltages and response times of the fabricated bistable liquid crystal devices. The fabricated ferroelectric liquid crystal-assisted bistable cholesteric liquid crystal devices showed a stable opaque focal conic state and stable transparent planar state without any energy consumption once they were turned “from focal-conic to planar” and “from planar to focal-conic”. To enhance the reliability, the fabricated bistable cholesteric liquid crystal devices were further stabilized by polymer matrixes. Switching of the bistable liquid crystal cells from one state to another was achieved by a one-step voltage bias with various frequencies. Furthermore, the response time of the sample cell was calculated as 1.7 ms. These results suggest that fabricated ferroelectric liquid crystal-assisted bistable cholesteric liquid crystal devices can be applied to produce energy-saving green liquid crystal displays and other related smart devices.