User:Panjasan/Blue Phase Mode LCD

Blue Phase Mode LCD

Introduction

Figure 1: Schematic of the twist of a chiral helix rotating (twisting) about half of the chiral pitch (rotation of 180°) with the helical axis, h, in the plane of the figure. The director points out of the plane of the figure and is shown as a projection on that plane.

In Reinitzer's reports from 1888 on the melting behaviour of cholesteryl benzoate we find a note that the substance briefly turned blue as it changed from clear to cloudy. This subtle effect however remained unexplored for more than eighty years until some experimental results were published during the late nineteenhundredsixties and early seventies that indicated that the blue color was due to at least two new and very different liquid crystalline phases .

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. Although an unlimited number of helical axes are thus actually present, this structure was named double twist structure.

Figure 2: Top view on a double twist structure. The plane containing the various helical axes, h, is the plane of the figure. The director points out of the plane of the figure in the center and it rotates as you move away from the center.

This double twist structure is more stable than the single twist structure (i.e. 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 0,5 μm) and since the geometries of usual LC-samples are much larger, the double twist structure occurs only rarely.

Figure 3: Perspective view on the double twist cylinder with the lines on the outside are supposed to indicate a 45° rotation of the director at that distance from the center line.

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 structure 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.

Figure 4: Illustration of a cubic lattice formed by double twist cylinders. All angles are supposed to be right angles.

A large structure can be composed from these double twist cylinders, but defects occur at the points where the cylinders are in contact. These defects that occur at regular distances tend to make the structure less stable, but still slightly more stable than the single twist structure without defects, at least within a temperature range of about 1° just 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 crystals. Blue phases are thus formed by a regular three-dimensional lattice of defects within a 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 reflections) and the blue phase reflects colored light (note that only some of the blue phases reflect blue light).

Figure 5: Disclinations form where the double twist cylinders are in contact.

Wide temperature range blue phases

In 2005 researchers from the University of Cambridge have discovered a class of blue-phase liquid crystals that remain stable over a range of temperatures as wide as from 16 to 60 degrees Celsius . The researchers showed that their ultrastable blue phases could be used to switch the colour of the reflected light by applying an electric field to the material, and that this could eventually be used to produce three-colour (red-green-blue) pixels for full-colour displays . The new blue phases are made from molecules in which two stiff, rod-like segments are linked by a flexible chain.

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 frame frequency of 240 Hertz. 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 was 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 LCD modes such as Twisted Nematic (TN), In-Plane Switching (IPS) or Vertical Alignment (VA) modes. 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 LC-layer to mechanical pressure which can impair the lateral uniformity of display e.g. luminance.

Overdrive circuits that are currently applied to many LCD panels with 120Hz 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 240Hz 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.

External Links

Cambridge University, Department of Engineering [1]

World’s First 'Blue Phase' Technology LC TV [2]