Bistable liquid crystal devices
View Patent ↗In a liquid crystal device a substrate ( 1 ) favors at least two stable or metastable differently directed liquid crystal alignments, and switching means for causing the liquid crystal material to switch between the alignments includes means arranged for optically irradiating said device. The latter may provide linearly polarized light ( 3 ) for inducing a torque on the liquid crystal to determine the alignment direction, and may optionally cooperate with a second energy supplying means such as an electric field (V) for assisting and switching. Alternatively, the alignment of the liquid crystal may be switched by a second energy supplying means such as an electric field, the light serving to produce heat to aid the switching. Either or both energy sources may be applied locally for switching of selected areas or pixels. Energy levels at the bistable substrate may be adjusted by the use of an oligomeric additive (slippery surface). As shown, the alignment at the surface of an opposed substrate ( 2 ) optionally follows the alignment at substrate ( 1 ).
1. A liquid crystal device comprising liquid crystal material in contact with the surface of a substrate, said surface favouring at least first and second stable or metastable liquid crystal alignments thereat with respective first and second different directions, and switching means for causing the liquid crystal material to switch between said alignments, wherein said switching means includes first energy supplying means arranged for applying optical radiation to said liquid crystal material, characterized in that said liquid crystal material and said optical radiation are selected for significant absorption of the radiation by the liquid crystal material.
2. A device according to claim 1 wherein said first energy supplying means is arranged to provide linearly polarised light of a polarisation direction and spectral composition selected or selectable for effectively rendering one of the first and second alignments less energetically favourable than the other of said the fire and second alignments.
3. A device according to claim 1 wherein said first energy supplying means is arranged to provide linearly polarised light of a polarisation direction and spectral composition selected or selectable for effectively inducing a torque on the liquid crystal molecules to alter the liquid crystal alignment between the first and second alignments.
4. A device according to claim 2 wherein the liquid crystal material contains no dichroic additive, and said spectral composition includes an absorption band of the liquid crystal material.
5. A device according to claim 1 wherein the liquid crystal material comprises a dichroic additive in a liquid crystal host.
6. A device according to claim 5 wherein the said spectral composition includes an absorption band of the dichroic additive.
7. A device according to claim 5 wherein the said spectral composition includes an absorption band of the liquid crystal host.
8. A device according to claim 2 wherein said switching means further comprises second energy supplying means to assist in the switching of the liquid crystal material between said alignments.
9. A device according to claim 8 wherein the second energy supplying means is arranged to apply energy to the liquid crystal material to destabilise the existing liquid crystal alignment.
10. A device according to claim 8 wherein the energy from said second energy supplying means induces a homeotropic alignment at the surface of the substrate.
11. A device according to claim 8 wherein the energy from said second energy supplying means induces a planar alignment at the surface of the substrate.
12. A device according to claim 8 wherein the said second energy is provided by an electric field.
13. A device according to claim 8 wherein said second energy supplying means is arranged to promote said switching of the liquid crystal material between said alignments, but is insufficient of itself to cause said switching.
14. A device according to claim 1 and further comprising second energy supplying means to assist in the switching of the liquid crystal material between said alignments.
15. A device according to claim 14 wherein said second energy supplying means is arranged to determine which of said alignments is adopted.
16. A device according to claim 15 wherein said first energy supplying means cooperates with the device to produce heat by light absorption, to cause said switching in cooperation with said second energy supplying means.
17. A device according to claim 15 wherein the energy from said second energy supplying means favours a homeotropic alignment of the LC material at the substrate.
18. A device according to claim 15 wherein the energy from said second energy supplying means induces a planar alignment of the LC material at the substrate.
19. A device according to claim 14 wherein the said second energy is provided by an electric field.
20. A device according to claim 1 wherein the liquid crystal material comprises an oligomer for reducing the energy between the first and second alignments.
21. A device according to claim 1 wherein the first energy supplying means is arranged for local irradiation of the device.
22. A device according to claim 1 wherein the first alignment is planar.
23. A device according to claim 1 wherein the second alignment is planar.
24. A device according to claim 1 wherein the second alignment is homeotropic.
25. A device according to claim 1 wherein the alignment is comprised of a grating structure.
26. A display comprising a device according to claim 1 .
27. An optical system or display comprising a plurality of optical devices, at least one said device being a device according to claim 1 .
28. An optical system comprising a plurality of said devices each according to claim 1 .
29. A system according to claim 27 wherein said plurality is tiled in a common plane.
30. A method of controlling the alignment of liquid crystal material in contact with a substrate surface which favours at least first and second stable or metastable liquid crystal alignments thereat with respective first and second different directions, including the step of optically irradiating said material with radiation, a significant portion of said radiation absorbed by said liquid crystal material.
31. The method according to claim 30 wherein the step of optically irradiating provides radiation selected for significant absorption by the liquid crystal material.
32. The method according to claim 30 wherein said step of optically irradiating includes the provision of linearly polarised light.
33. The method according to claim 32 wherein said linearly polarised light is such as to effectively exert a torque on the liquid crystal molecules or to effectively rotate the liquid crystal molecules.
34. The method according to claim 33 wherein said step of optically irradiating includes the provision of unpolarised light.
35. The method according to claim 34 wherein the unpolarised light produces heating in the liquid crystal material.
36. The method according to claim 30 including the additional step of providing a further energy input to the device for controlling the alignment.
37. The method according to claim 36 wherein the further energy input is an electric field.
38. The method according to claim 36 wherein the optical irradiation continues after the further energy input has ceased.
39. The method according to claim 36 wherein the further energy input continues after the optical irradiation has ceased.
40. The method according to claim 36 wherein the further energy input is applied locally.
41. The method according to claim 30 wherein the optical irradiation is applied locally.
42. The method according to claim 30 wherein one said alignment is planar.
43. The method according to claim 42 wherein another said alignment is planar.
44. The method according to claim 42 wherein another said alignment is homeotropic.
45. The method according to claim 30 including the step of providing an oligomer in the liquid crystal phase of the material.
46. The method according to claim 30 wherein said substrate surface is provided as a grating structure.