Bistable liquid crystal devices

Abstract

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 ).

What is claimed is: 
     
       1. A liquid crystal device comprising liquid crystal material in contact with the surface of a substrate, said surface favouring favoring 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 favorable than the other of said the fire first 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 molecules of the liquid crystal molecules material 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 a spectral composition associated with the optical radiation includes an absorption band of the dichroic additive. 
     
     
       7. A device according to  claim 5  wherein the said a spectral composition associated with the optical radiation 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 favors a homeotropic alignment of the LC liquid crystal 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 liquid crystal 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 surface comprises 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 of optical devices 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, comprising of: 
 optically irradiating said a liquid crystal material with radiation, wherein a significant portion of said radiation is absorbed by said liquid crystal material; and 
 controlling, based at least in part on said optically irradiating, an alignment of said liquid crystal material in contact with a substrate surface which favors both a first stable or metastable alignment associated with a first direction and a second stable or metastable alignment associated with a second direction, wherein the second direction is different than the first direction. 
 
     
     
       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 configured to exert a torque on molecules of the liquid crystal molecules material or to effectively rotate the liquid crystal molecules. 
     
     
       34. The method according to claim  33  30 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 of the liquid crystal alignments is planar. 
     
     
       43. The method according to  claim 42  wherein another said alignment of the liquid crystal alignments is planar. 
     
     
       44. The method according to  claim 42  wherein another said alignment of the liquid crystal alignments is homeotropic. 
     
     
       45. The method according to  claim 30  including the step of providing an oligomer in the a liquid crystal phase of the material. 
     
     
       46. The method according to  claim 30  wherein said substrate surface is provided as a grating structure. 
     
     
       47. A device, comprising:
 a substrate configured to promote a phase alignment in a first direction and a second direction; and   a liquid crystal material located adjacent the substrate, wherein different portions of the liquid crystal material are configured to be coincidentally aligned with the first and second directions of alignment when localized areas of the device are optically addressed based, at least in part, on a significant absorption of optical radiation by the liquid crystal material.    
     
     
       48. The device of claim 47, wherein the first direction of alignment is a favored alignment of the substrate, and wherein metastable portions of the liquid crystal material that correspond to the localized areas of the device are configured to be aligned with the favored alignment when the device is optically addressed.  
     
     
       49. The device of claim 48, wherein the favored alignment is more stable than a state of the metastable portions.  
     
     
       50. The device of claim 47, wherein the localized areas of the device are both optically addressed and electrically addressed.  
     
     
       51. The device of claim 47, wherein an electric field is configured to be applied universally to the device while the localized areas of the device are optically addressed.  
     
     
       52. The device of claim 47, wherein energy states associated with the first and second directions of alignment are equal.  
     
     
       53. The device of claim 52, further comprising another substrate, wherein the liquid crystal material is located between the two substrates, and wherein a direction of alignment of the liquid crystal material at the other substrate is midway between the first and second directions of alignment.  
     
     
       54. The device of claim 47, wherein the different portions of the liquid crystal material are configured to switch between the first and second directions of alignment whereas other portions of the liquid crystal material do not switch.  
     
     
       55. The device of claim 54, wherein the other portions of the liquid crystal material are not optically addressed.  
     
     
       56. The device of claim 54, wherein at least some of the different portions are aligned in the first direction and at least some of the other portions are aligned in the second direction.  
     
     
       57. A method, comprising:
 optically addressing a liquid crystal material located adjacent a substrate, wherein the substrate is configured to promote a phase alignment in both a first direction of alignment and a second direction of alignment; and   switching different portions of the liquid crystal material coincidentally between the first direction of alignment and the second direction of alignment based, at least in part, on a significant absorption of optical radiation by the liquid crystal material.    
     
     
       58. The method of claim 57, wherein the different portions of the liquid crystal material are configured to switch according to a localized absorption of the optical radiation in the liquid crystal material.  
     
     
       59. The method of claim 57, wherein the different portions of the liquid crystal material are selectively addressed by the optical radiation.  
     
     
       60. The method of claim 59, wherein certain portions of the liquid crystal material are not optically addressed.  
     
     
       61. The method of claim 60, wherein a first portion of the liquid crystal material is aligned differently than a second portion of the liquid crystal material after the liquid crystal material is optically addressed.  
     
     
       62. The method of claim 57, further comprising supplying a secondary source of energy to the liquid crystal material to assist said switching.  
     
     
       63. The method of claim 62, wherein the secondary source of energy is provided by an electric field.