Ferroelectric liquid crystal display
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The FLCD did not make many inroads as a direct view display device. Manufacturing of larger FLCDs was problematic making them unable to compete against direct view LCDs based on nematic liquid crystals using the Twisted nematic field effect or In-Plane Switching. Today, the FLCD is used in reflective microdisplays based on Liquid Crystal on Silicon technology. Using ferroelectric liquid crystal (FLC) in FLCoS technology allows a much smaller display area which eliminates the problems of manufacturing larger area FLC displays. Additionally, the dot pitch or pixel pitch of such displays can be as low as 6 µm giving a very high resolution display in a small area. To produce color and grey-scale, time multiplexing is used, exploiting the sub-millisecond switching time of the ferroelectric liquid crystal. These microdisplays find applications in 3D head mounted displays (HMD), image insertion in surgical microscopes and electronic viewfinders where direct-view LCDs fail to provide more than 600 ppi resolution.
Working of Ferroelectric Liquid Crystals
Ferroelectric liquid crystals are chiral smectic liquid crystals that have a layered order. Within the layer the liquid crystal molecules (called mesogenes) are tilted away from the layer normal (90°), forming a so-called smectic C liquid crystal. Chiral behavior is introduced by inserting asymmetric carbon atom into the mesogenic molecule, termed now smectic C* (the asterix denotes the chirality). The chirality causes a smectic layer to exhibit a permanent spontaneous polarization at right angle to the tilt plane, giving rise to the term ferroelectric. In an unconstrained system a helical twist in the structure lowers the energy of the structure, i.e. the tilt direction changes from layer to layer by some degree. In other words, the azimuthal direction in which the molecules tilt away from the layer normal will differ slightly from one layer to the next. Therefore, the overall polarization of an unconstrained smectic C* phase will be zero.
Typically, the FLCDs are built with cell gaps less than 2 µm for stable molecular alignment. In this constraint system the interaction between the alignment layers and the smectic C* liquid crystal suppress the helical superstructure. Proper ferroelectricity now forms in domains. The spontaneous polarization of the smectic C* layer interacts with the electric field applied to the electrodes. Depending on the direction of the electric field the mesogenes are titled either to left or the right side of the layer normal. This in turn results in opaque or transparent state when used in combination with crossed polarizers as in LCD.
FLCos microdisplays work as light amplitude modulators. Consider the display as a mirror with an electrically switchable quarter wave plate formed by the ferroelectric liquid crystal (FLC) layer as shown in the figure found here: http://www.forthdd.com/technology/amplitude-modulation/
If a light source, a mirror, and your eye are arranged at right angles from one another with a polarizing beam splitter (PBS) in the middle, and a quarter waveplate (λ/4) is inserted between the mirror and the beam splitter, and the PBS is orientated at an angle of 45 degrees relative to the S-polarized light, then the following sequence of events occurs:
- The illuminator produces S- and P-polarized light which hits the PBS.
- The P-polarized light passes through the PBS and leaves the system.
- The S-polarized light is reflected by the PBS onto the mirror, this time via the quarter-wave plate, so it is circularly polarized when it reaches the mirror.
- The reflected light is circularly polarized in the opposite direction and having passed through the quarter wave plate a second time, it is once again linearly polarized, but rotated by 90 degrees, ie. P-polarized.
- The PBS passes the P-polarized light through to the observer.
Now, replace the mirror and quarter wave plate with a FLCoS microdisplay. The same sequence of events occurs when the panel is switched to its "on" state. That is, it behaves just like the mirror and quarter wave plate. However, when the panel is switched to its "off" state, the S-polarized light incident on the microdisplay does not have its polarization state changed. As a result, the light reflects back to the PBS and then to the light source. The observer then does not see any light coming from the microdisplay, and it appears black. Each pixel of the FLCoS is operating in this manner, therefore allowing the display of patterns of dark and light. Using time multiplexing, color and grayscales can be generated from this "on" and "off" modulation of light.
Properties and uses
- Very thin layers of ferroelectric liquid crystal (less than 2 µm thick) produce a 90° polarisation twist.
- High density FLCoS microdisplays with small display areas can be manufactured.
- Switching time is less than 50 µs
- High frame rate video displays are possible.
- Polarization effect is bistable.
- Can be used for low frame rate displays that can run on very low power
- This property can help build displays with non-volatile memory with the advantage that the memory can be changed easily.
- In-plane Switching provides reduced viewing angle dependence of contrast and color.