Fast progress in information technologies triggers the need for new materials for high-density optical memory storage (see for example Albota, M. et al., Science 1998 281, 1653; C. E. Olson, M. J. R. Previte, J. T. Fourkas. Nature Materials 1, 2002, 225; S, Kawata, Y. Kawata, Chem. Rev. 2000, 100, 1777; Photoreactive Materials for Ultrahigh-Density Optical Memory, Ed. M. Irie, Elsevier, Amsterdam, 1994; A. Renn,; U. P. Wild, A. Rebane. Chem. Rev. 2000, 100, 1741; D. S. Tyson, C. A. Bignozzi, F. N. Castellano, J. Am. Chem. Soc. 2002, 124, 4562; and J. R. Sheats, P. F. Barbara, Acc. Chem. Res. 1999, 32, 191).
An increase in storage capacity can be achieved by shifting from two-dimensional to three-dimensional (3D) optical data storage: holographic recording with photorefractive media (Photorefarctive materials and Their Applications I, edited by P. Gunter and J.-P. Huignard (Springer, Berlin, 1988)), spectral hole burning (Persistent Spectral Hole Burning: Science and Applications, edited by W. E. Moerner (Springer, Berlin, 1987)) and photon echo (R. Kachru and M. K. Kim, Opt. Lett 28, 2186 (1989)).
An increase in storage density has been further accomplished by the use of two-photon processes introduced by Rentzepis (D. A. Parthenopoulos and P. M. Rentzepis, Science 245, 843 (1989); A. S. Dvornikov and P. M. Rentzepis, Opt. Commun. 119, 341 (1995)) and Webb, (W. Denk, J. H. Strickler, W. W. Webb, Science 248, 73 (1990)). On the materials side, new modes in data storage have been examined by employing polymer photonic crystals (B. Siwick, O. Kalinina, E. Kumacheva, R. J. D. Miller, J. Noolandi, J. Appl. Phys. 90 5328 (2001); N. I. Koroteev, S. A. Krikunov, S. A. Magnitskii, D. V. Malakhov, V. V. Shubin, Jpn. J. Appl. Phys. Part 1 37, 2279-2280 (1998), and D. Kraemer, B. Siwick, Miller R. J. D. J. Chem. Phys. 285, 73 (2002). These crystals can be prepared via “bottom-to-top” approach from the core-shell latex particles with fluorescent cores and optically inert shells (Kalinina, O., Kumacheva, E. Macromolecules 32, 4122-4129 (1999); Kumacheva, E., Kalinina, O., Lilge, L. Adv. Mater. 11, 231-234 (1999)). Encryption or recoding of information in 3D polymer photonic crystals was achieved by local one-photon or two-photon photobleaching of the fluorescent dye localized in periodic domains (bits). The existence of the “dead” space between the bits, acting as a barrier to cross-talk, led to a two-fold increase of signal-to-noise ratio compared to the material with a uniform dye distribution.
Moreover, information retrieval with an order of magnitude beyond the Rayleigh limit was recently predicted due to photobleaching of domains with well-defined Fourier components (Kraemer D. Kraemer, B. Siwick, Miller R. J. D. J. Chem. Phys. 285, 73 (2002)).
The use of bicolored (or multicolored) multiphase periodic medium for bit-like optical data storage has several advantages. First, in the bit-like binary memory storage, reading is achieved by distinguishing between “1” s (photobleached domains) and “0” (fluorescent domains) (B. Siwick, O. Kalinina, E. Kumacheva, R. J. D. Miller, J. Noolandi, J. Appl. Phys. 90 5328 (2001). Thus reading of optically-inert (non-fluorescent) matrix is similar to reading of “1” s; this uncertainty may induce errors in retrieving information. The incorporation of the second dye in the matrix of the photonic crystal resolves this problem. More important, the use of several dyes with distinct excitation and emission peaks increases the number of modes in data storage from 2 (binary encryption mode, one-dye system) to 2n (binary code, multi-dye system), where n is the number of dyes. The incorporation of different dyes in different phases of the material reduces energy transfer between them. Data recording achieved by photo-induced changes in different dyes accompanied by information retrieval at a particular well-defined wavelength tailors additional security features to the recorded information.
Patent Publication WO 03/044276 discloses a procedure for preparation of a paper containing security elements (photoluminescent objects such as fiber, thread, rod, tape, film, windows and combination of thereof) by their incorporation into paper structure. Authenticity of the paper is then determined using polarized light.
JP 2002317134 discloses a fluorescent ink composition comprising a non-fluorescent dye absorbing in a visible range and fluorescent dye with no absorption in the visible range and fluorescence in the visible range. Authenticity of the document is determined by irradiating it with a light source capable of exciting fluorescent dye and observing an emission.
JP 2002088688 discloses a composition for laser printing with toner particles mixed with or coated by fluorescent dye or pigment absorbing in UV range. Authenticity of the document is checked by exciting the UV dye and observing fluorescence.
WO 01/09435 discloses a security paper comprising at least two of the following security features: a water insoluble dye which is incorporated in the paper and bleeds when placed in contact with organic solvent; a dye that becomes fluorescent in daylight when brought in contact with alkaline substance; a dye which becomes fluorescent when exposed to UV light.
None of above-mentioned patents teaches or suggests information encryption using multiple imaging and selective photobleaching. All features are used to prove authenticity of the document or paper whereas the material produced in accordance with the present invention can be used to identify a document holder and demonstrate the authenticity of the document.
US2003/0183695 discloses a method for making a secure ID card with multiple images. The multicolored images are printed on the information-bearing layer in such a way as to provide multiple images when printed information is viewed at different predetermined angles through the lenticular lens. This method does not use selective photobleaching of fluorescent dyes. There is a high probability that the images can be viewed all together or one by one when different angles are used.
Patent publication WO 03101755 discloses an article for authentication (e.g. security document or sheet made from a transparent polymeric film which comprises a first luminescent material in a first region and a second luminescent material in a second region. The second region contains a transparent window and the rest of the material is optionally opacified. The two materials luminesce at different visible wavelengths when excited (e.g. the coating fluoresces red/orange when excited with UV-C at 254 nm whereas the window fluoresces blue when excited with UV-A at 365 nm). However when the article is folded the first region is viewed through the transparent window and when excited the combined luminescence of both materials is seen as a colour change which may be used to authenticate the article. This method provides only one authentication feature: the color of fluorescence which is not easy to identify without expensive equipment.
It would be particularly advantageous to provide an optical data storage device and a method of writing or storing data in the optical storage device using selective single-photon or two-photon photobleaching.