In the efforts to improve the temporal response of liquid crystal displays (LCDs) in order to obtain a better display of moving images with, for example, frame rates of 100-120 Hz and higher, an LC display mode has been proposed in 2007[1] that uses highly twisted cholesteric phases in a special configuration called Blue Phase. This new operational mode for LCDs also does not require anisotropic alignment layers (e.g., rubbed polyimide) and thus simplifies the LCD manufacturing process.
History
In Reinitzer's reports from 1888 on the melting behaviour of cholesteryl benzoate, there is a note that the substance briefly turned blue as it changed from clear to cloudy upon cooling. This subtle effect however remained unexplored for more than eighty years until some experimental results were published during the late 1960s and early seventies that indicated that the blue color was due to at least two new and very different liquid crystalline phases.[2]
For almost one hundred years, scientists assumed that the most stable cholesteric helical structure could be described by a single helical axis about which the director rotates. It turned out that in the new structure the director rotates in a helical fashion about any axis perpendicular to a line as illustrated in fig. 1. Although an unlimited number of helical axes are actually present, this structure was named double twist structure.
This double twist structure is more stable than the single twist structure (i.e., the normal helical structure of chiral nematics) only up to a certain distance from the line at the center. Since this distance is on the order of the pitch of the chiral nematic liquid crystal (typically 100 nm) and since the geometries of usual liquid crystal samples are much larger, the double twist structure occurs only rarely.
Blue phases are special cases when double twist structures fill up large volumes. When double twist structures are limited in all directions to the distance from the center line where the twist amounts to 45° a double twist cylinder results. Because of its small radius, such a cylinder is more stable than the same volume filled with a single twist chiral nematic liquid crystal.
A large structure can be composed from these double twist cylinders, but defects occur at the points where the cylinders are in contact as illustrated in fig. 5.[3] These defects occur at regular distances and tend to make the structure less stable, but it is still slightly more stable than the single twist structure without defects, at least within a temperature range of about 1 K below the transition from the chiral nematic phase to an isotropic liquid.
The defects that occur at regular distances in three spatial dimensions form a cubic lattice just as we know it from solid crystals. Blue phases are thus formed by a regular three-dimensional lattice of defects within a chiral liquid crystal. Since the spacings between the defects of a blue phase are in the range of the wavelength of light (several hundred nanometers), for certain wavelength ranges of the light reflected from the lattice constructive interference occurs (Bragg reflection) and the blue phase reflects colored light (note that only some of the blue phases actually reflect blue light).[4]
Wide temperature range blue phases
In 2005, researchers from the Centre of Molecular Materials for Photonics and Electronics at the University of Cambridge reported their discovery of a class of blue-phase liquid crystals that remain stable over a range of temperatures as wide as 16-60 °C.[5] The researchers showed that their ultrastable blue phases could be used to switch the color of the reflected light by applying an electric field to the material, and that this could eventually be used to produce three-color (red, green, and blue) pixels for full-color displays.[6] The new blue phases are made from molecules in which two stiff, rod-like segments are linked by a flexible chain.
Furthermore, electro-optical switching with response times of the order of 10-4 s for the stabilized blue phases at room temperature has been shown.[7]
First blue phase LC-display
In May, 2008 Samsung Electronics announced that it has developed the world’s first Blue Phase LCD panel which can be operated at an unprecedented refresh rate of 240 Hz. Samsung unveiled a 15” model of its Blue Phase LCD panel at the SID (Society for Information Display) 2008 international Symposium, Seminar and Exhibition, which had been held in Los Angeles from May 18 to 23, 2008.
Developed with a look at cost-efficiency, Samsung’s Blue Phase mode does not require liquid crystal alignment layers, unlike today’s most widely used TFT LCD modes such as Twisted Nematic (TN), In-Plane Switching (IPS) or Vertical Alignment (VA). The Blue Phase mode can make its own alignment layers, eliminating the need for any mechanical alignment and rubbing processes. This reduces the number of required manufacturing steps, resulting in savings on production costs. Additionally is has been claimed that Blue Phase panels will reduce the sensitivity of the liquid crystal layer to mechanical pressure which can impair the lateral uniformity of display (e.g., luminance).
Response Time Compensation through Overdrive circuits that are currently applied to many LCD panels with 120 Hz frame frequency for improvement of the display of moving images in premium LCD TVs will become obsolete since the Blue Phase mode features a superior response speed, allowing images to be reproduced at 240 Hz frame rate or higher without the need for any overdrive circuit.
In such a blue phase based LC-display for TV applications, it is not the selective reflection of light according the the lattice pitch (Bragg reflection), but an electric field deforms the lattice which results in anisotropy of the refractive indices of the layer, followed by a change of transmission between crossed polarizers.
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