– A second substantial improvement could be to start from ho- mogeneous weak anchoring instead of a Schlieren texture. This could be obtained by applying an external horizontal magnetic field while filling the cell and cooling to the nematic phase.
Less influence of the interconnection electrodes on the electric field in the liquid crystal would increase the equivalence between the three driving configurations:
– If the thickness of the dielectric layer between the interconnection electrodes and the hexagonal electrodes is increased, their influ- ence on the electric field in the liquid crystal is lower. On the other hand, this complicates the technology for the vias through the layer.
– A new design of the interconnection electrodes using thin lines in- stead of covering the whole surface would reduce their influence.
But, in this case less light would be reflected by the device.
Finally a last improvement would be to use another metal for the electrode with a higher reflectivity such as gold, aluminum or silver.
This is not critical for the working of the device, but would improve the optical characteristics.
5.6 Applications for the new liquid crystal device
The device acts as a rotating birefringent wave plate. Other device con- figurations that exhibit this characteristic have been proposed [49, 103, 124–129], but the advantage of the device with hexagonal electrodes is the distribution of the electrodes over the whole surface of the device.
In this way, the required driving voltages can in principle be very low.
Furthermore, there is no need for extra space next to the actual pixel to place the electrodes. A true azimuthally degenerated anchoring sur- face would enable the multistability of the device as explained above, but unfortunately such materials have not been reported yet.
The proposed operating principle has a number of interesting ca- pabilities. Not only can it act as a wave plate, also other applications are possible for slightly modified parameters of the device. Possible extensions are briefly discussed below.
5.6.1 Hexagonal device with rubbed alignment layers
If the azimuthally degenerated planar anchoring surfaces at top and bottom are replaced by anti-parallel rubbed alignment layers with the preferential director along the average horizontal electric field of one driving configuration, a director rotation over an angle of 360◦ is no longer possible and the multistability disappears. On the other hand, the azimuthal angle now becomes continuously adjustable between
−60◦ and 60◦ about the rubbing direction by applying one of the other two driving configurations. The amount of rotation can be controlled by the strength of the applied electric field.
When the electric field is turned off, the elastic force stemming from the alignment at the boundaries will align the director in the liquid crystal homogeneously. By applying the driving configuration with average horizontal field parallel to the anchoring direction for a short time, the director is forced back into the off-state and thus the switching off time is decreased.
5.6.2 Electric field driven alignment direction
Some recent types of liquid crystal devices are based on switching be- tween parallel and twisted states [131, 132]. Our new device can han- dle this type of switching. Decreasing the dimensions of the electrodes and the spacing between them below the thickness of the liquid crystal layer, lowers the effect of the driving field at the top substrate. The electrodes create an electric field at the bottom of the liquid crystal layer which forms a switchable alignment direction. A strong anchor- ing alignment layer on the top substrate in combination with such a switchable alignment at the bottom of the liquid crystal makes it possi- ble to switch between the parallel and twisted state.
5.6.3 Intermediate director alignment
The liquid crystal director aligns along the average electric field. In the device proposed above, the average electric field and thus the director can be aligned along three different directions.
In order to align the director along an intermediate direction, two solutions are possible. If the device is really multistable, it is enough to switch off the applied voltage before the equilibrium state is achieved.
5.6 Applications for the new liquid crystal device 145
After switching off, the director will align along the average twist at that time.
If the surface suffers from too strong memory alignment and contin- uous application of the potential is required, another method might be possible. By alternating between two driving configurations at a high frequency, the time averaged electric field is then somewhere between the two driving configurations. So by varying the time ratio in which the two driving configurations are applied, any direction in between can be achieved.
The results of the director simulations have been published in the SCI JournalJournal of applied physics D: applied physics[133]. Abstract on the simulations and experiments have been submitted to the Interna- tional Workshop for Liquid Crystals on Photonics(26–28 April 2006, Gent, Belgium) [134] and the21st International Liquid Crystal Conference (2–7 July, 2006, Keystone, Colorado, USA) [135] and are both accepted for oral presentation.
Chapter 6
Conclusion
In the preceding chapters, I have tried to explain the most important findings of my PhD research. The described work deals with many aspects of liquid crystals: modeling of the director distribution, optical transmission, surface anchoring and technology. In this chapter I want to conclude with a brief summary of the main achievements and an outlook to the future.