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A method of erasable/rewritable optical data storage is provided in which a laser beam from a laser (10) operated in either a pulsed or continuous wave mode is focussed by an optical system (11-18) onto a photorefractive polymeric material (32) to cause two photon excitation of the material to recor

A method of erasable/rewritable optical data storage is provided in which a laser beam from a laser (10) operated in either a pulsed or continuous wave mode is focussed by an optical system (11-18) onto a photorefractive polymeric material (32) to cause two photon excitation of the material to record data which may subsequently be erased by illuminating the material with radiation having a wavelength in the ultraviolet or visible spectrum to erase the recorded data. New photorefractive polymeric materials having a relatively narrow absorption band for use in the method are also disclosed.

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1. A method of writing and erasing optical data comprising:providing a photorefractive polymeric material having a single isomeric state with a refractive index; focusing light on the photorefractive polymeric material to cause two-photon excitation of the material at the focal point to produce a re

1. A method of writing and erasing optical data comprising:providing a photorefractive polymeric material having a single isomeric state with a refractive index; focusing light on the photorefractive polymeric material to cause two-photon excitation of the material at the focal point to produce a refractive index inhomogeneity resulting from a non-uniform space-charge distribution within the region of excitation within the photorefractive polymeric material, thereby modulating the refractive index of the material at the focal point to record data; and illuminating the material with radiation to erase the recorded data. 2. A method of writing and re-writing optical data in a photorefractive polymeric material having a single isomeric state and a refractive index comprising:focusing light on the photorefractive polymeric material to cause two-photon excitation of the material at the focal point of the beam to produce a refractive index inhomogeneity resulting from a non-uniform space-charge distribution within the region of excitation within the photorefractive polymeric material, thereby modulating the refractive index at the focal point to write data; illuminating the material with radiation to erase the recorded data; focusing light on the photorefractive polymeric material to cause two-photon excitation of the material at the focal point to produce a refractive index inhomogeneity resulting from a non-uniform space-charge distribution within the region of excitation within the photorefractive polymeric material, thereby modulating the refractive index at the focal point to re-write data in the photorefractive polymeric material. 3. A method according to claim 1 wherein the photorefractive material is illuminated with electro-magnetic radiation having a wavelength in the ultraviolet (UV) or visible spectrum to produce a redistribution of the spatial distribution of the electric charges forming bits of the data to erase the recorded data.4. A method according to claim 3 wherein the photorefractive polymeric material is such that it absorbs radiation in only a narrow band in the UV to visible region of the electromagnetic spectrum.5. A method according to claim 3 wherein the maximum of the absorption band of the photorefractive polymeric material falls substantially within the range from about 380 nm to about 600 nm.6. A method according to claim 3 wherein the photorefractive polymeric material is such that is absorbs substantially no radiation above a wavelength of about 630 nm.7. A method according to claim 1 wherein the data recorded in the photorefractive polymeric material is read by illuminating the photorefractive polymeric material with coherent light of a wavelength falling substantially within the range from about 630 nm to about 1200 nm.8. A method according to claim 1 wherein the light used to record data in the photorefractive material has a wavelength falling substantially within the range from about 750 nm to about 1200 nm to cause two-photon excitation.9. A method according to claim 1 wherein a pulsed laser beam is used to record data in the photorefractive polymeric material.10. A method according to claim 1 wherein a continuous wave laser beam is used to record data in the photorefractive polymeric material.11. A method according to claim 1 wherein polarized coherent light is used to record polarized bits of data in the photorefractive polymeric material.12. A method according to claim 11 wherein different polarization states of the recording beam are used to record multiple bits of data at the same position having different polarization states in the photorefractive polymeric material.13. A method according to claim 11 wherein bits of recorded data are read by using a reading beam having an appropriate polarization state.14. A method according to claim 11 wherein individual bits of data are erasable by changing the polarization state of the individual bits.15. A method according to claim 1 wherein the photorefractive polymeric material includes at least about 25% of a polymer by percentage weight of the total weight of the photorefractive material.16. A method according to claim 1 wherein the photorefractive polymeric material includes a chromophore which provides absorption in the UV to visible region of the electromagnetic spectrum.17. A method according to claim 1 wherein the photorefractive polymeric material includes a photosensitive material which provides absorption in the UV to visible region of the electromagnetic spectrum.18. A method according to claim 1 wherein the photorefractive polymeric material includes a plasticizer to reduce the glass transition temperature of the material.19. A method according to claim 1 wherein the photorefractive material includes at least some of the following materials in quantities falling substantially within the following ranges by percentage of the total weight of the photorefractive material:25%-99.5% of a polymer; 0.5%-60% of a chromophore; 0.5%-5% of a photosensitive material; and 0%-40% of a plasticizer. 20. A method according to claim 15 wherein the polymer comprises poly (N-vinylcarbazole) (PVK).21. A method according to claim 15 wherein the polymer comprises poly (methyl methacrylate) (PMMA).22. A method according to claim 16 wherein the chromophore comprises 2,5-dimethyl-4-(p-nitro-phenylazo) anisole (DMNPAA).23. A method according to claim 17 wherein the photosensitive material comprises 2,4,7-trinitro-9-fluorenone (TNF).24. A method according to claim 18 wherein the plasticizer comprises N-ethylcarbazole (ECZ).25. A method according to claim 19 wherein the polymer comprises poly (N-vinylcarbazole) (PVK).26. A method according to claim 19 wherein the polymer comprises poly (methyl methacrylate) (PMMA).27. A method according to claim 19 wherein the chromophore comprises 2,5-dimethyl-4-(p-nitro-phenylazo)anisole (DMNPAA).28. A method according to claim 19 wherein the photosensitive material comprises 2,4,7-trinitro-9-fluorenone (TNF).29. A method according to claim 19 wherein the plasticizer comprises N-ethylcarbazole (ECZ).30. A method according to claim 1 wherein the photorefractive polymeric material used in the method includes the following materials:poly(N-vinylcarbazole) (PVK); 2,5, dimethyl-4-(p-nitrophenylazo) anisole (DMNPAA) 2,4,7-trinitro-9-fluorenone (TNF); and N-ethylcarbazole (ECZ). 31. A method according to claim 30 wherein the PVK;DMNPAA;TNF and ECZ are present in approximately the following concentrations by percentage weight of the total weight of the photorefractive material 33:50:1:16.32. A method according to claim 1 wherein the photorefractive polymeric material used in the method includes the following materials:poly (methyl methacrylate) (PMMA); 2,5, dimethyl-4-(p-nitrophenylazo) anisole (DMNPAA); 2,4,7-trinitro-9-fluorenone (TNF); and N-ethylcarbazole (ECZ). 33. A photorefractive polymeric material according to claim 32 wherein the PMMA: DMNPAA; TNF and ECZ are present in approximately the following concentrations by percentage weight of the total weight of the photorefractive polymeric material 73:10:1:16.34. A method according to claim 2 wherein the photorefractive material is illuminated with electro-magnetic radiation having a wavelength in the ultraviolet (UV) or visible spectrum to produce a redistribution of the special distribution of the electric charges forming bits of the data to erase the recorded data.35. A method according to claim 34 wherein the photorefractive polymeric material is such that it absorbs radiation in only a narrow band in the UV to visible region of the electromagnetic spectrum.36. A method according to claim 34 wherein the maximum of the absorption band of the photorefractive polymeric material falls substantially within the range from about 380 nm to about 600 nm.37. A method according to claim 34 wherein the photorefractive polymeric material is such that is absorbs substantially no radiation above a wavelength of about 630 nm.38. A method according to claim 2 wherein the data recorded in the photorefractive polymeric material is read by illuminating the photorefractive polymeric material with coherent light of a wavelength falling substantially within the range from about 630 nm to about 1200 nm.39. A method according to claim 2 wherein the light used to record data in the photorefractive material has a wavelength falling substantially within the range from about 750 nm to about 1200 nm to cause two-photon excitation.40. A method according to claim 2 wherein a pulsed laser beam is used to record data in the photorefractive polymeric material.41. A method according to claim 2 wherein a continuous wave laser beam is used to record data in the photorefractive polymeric material.42. A method according to claim 2 wherein polarized coherent light is used to record polarized bits of data in the photorefractive polymeric material.43. A method according to claim 42 wherein different polarization states of the recording beam are used to record multiple bits of data at the same position having different polarization states in the photorefractive polymeric material.44. A method according to claim 43 wherein bits of recorded data are read by using a reading beam having an appropriate polarization state.45. A method according to claim 44 wherein individual bits of data are erasable by changing the polarization state of the individual bits.46. A method of optical data storage comprising:providing a photorefractive polymeric material having a single isomeric state with a refractive index, the photorefractive polymeric material including the following materials: poly(N-vinylcarbazole) (PVK); 2,5, dimethyl-4-(p-nitrophenylazo) anisole (DMNPAA); 2,4,7-trinitro-9-fluorenone (TNF); and N-ethylcarbazole (ECZ); and focusing light on the photorefractive polymeric material to cause two-photon excitation of the material at the focal point to produce a refractive index inhomogeneity resulting from a non-uniform space charge distribution within the region of excitation within the photorefractive material, thereby modulating the refractive index of the material at the focal point to record data. 47. A method of optical data storage according to claim 46 further including the steps of illuminating the material with radiation to erase the recorded data, and of re-writing data by focusing light on the photorefractive polymeric material after the original recorded data has been erased to cause two-photon excitation of the material at the focal point to produce a refractive index inhomogeneity resulting from a non-uniform space-charge distribution within the region of excitation within the photorefractive polymeric material thereby modulating the refractive index at the focal point to re-write the data.48. A method of optical data storage comprising:providing a photorefractive polymeric material having a single isomeric state with a refractive index, the photorefractive polymeric material including the following materials: poly (methyl methacrylate) (PMMA); 2,5, dimethyl-4-(p-nitrophenylazo) anisole (DMNPAA); 2,4,7-trinitro-9-fluorenone (TNF); and N-ethylcarbazole (ECZ); and focusing light on the photorefractive polymeric material to cause two photon excitation of the material at the focal point to produce a refractive index inhomogeneity resulting from a non-uniform space-charge distribution within the region of excitation within the photorefractive polymeric material, thereby modulating the refractive index of the material at the focal point to record data. 49. A method of optical data storage comprising:providing a photorefractive material having a single isomeric state with a refractive index; and focusing light on the photorefractive polymeric material to cause two-photon excitation of the material at the focal point to produce a refractive index inhomogeneity resulting from a non-uniform space-charge distribution within the region of excitation within the photorefractive polymeric material, thereby modulating the refractive index of the material at the focal point to record data.