Holographic-Data-Storage Materials
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re than 30 years ago. 15 The lack of suitable spatial light modulators (SLMs) for encoding the data pages and detector arrays for the readout of stored data limited this storage mechanism to academic studies. In recent years many of the enabling technologies, such as projection televisions incorporating liquid crystal SLMs and video camcorders with chargecoupled-device (CCD) detector chips, became readily available, affordably priced consumer products. Today it is reasonable to anticipate the availability of devices that will modulate or detect 1024 X 1024 pixel arrays 1000 times per second for a data rate of 1 Gbit/s, a rate that is on the leading edge in comparison to other storage technologies. Storage density is another area where holographic storage shines. The ultimate limit of any optical-storage scheme is limited by A, the wavelength of light. In the case of holographic data storage, the maximum storage density is theoretically 1/A3—that is, of the order of several Tbytes/cm3. Whether this limit is likely to be approached in actual devices depends on properties of the specific storage materials used and the creativity of scientists and engineers in designing systems that optimally utilize the strengths of a given material. An ideal material for holographic data storage would be very sensitive to the light from a cheap, commercially available solid-state laser. It would rapidly convert the absorbed light into a change of its local optical properties. It would be advantageous if this change were reversible, allowing data to be erased. In a reversible medium, each hologram that is written partially erases previously written holograms, requiring a compensation scheme known as "scheduling" in which the initial holograms are written longer
than later ones.6 8 The diffraction efficiency of the resulting holograms scales as the inverse square of the number of exposures. A similar phenomenon occurs in the case of irreversible, write-once materials due to "bias build-up"—that is, the exhaustion of the material response due to the portions of the object and reference beams that do not contribute to the interference pattern being recorded. For efficient readout of each hologram, the change of optical properties should reproduce the reconstructed data page with high diffraction efficiency. To match the grating vector of the stored pattern the readout utilizes light of the same wavelength that was used during writing—that is, it will erase the stored information in a reversible material or saturate an irreversible material unless the stored information has been somehow fixed. One solution would be a material that uses two photons of different wavelengths for writing and only requires one of the wavelengths for readout of the data. Meeting this formidable requirement however is a problem that has not yet been solved. The holographically reconstructed wave front will be at best equal in quality to an image transmitted through the storage medium onto the camera. It is therefore imperative that the medium have outstanding opt
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