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The light-harvesting protein in the purple membrane of halobacterium halobium, called bacteriorhodopsin, has been proposed as an erasable holographic storage medium. It has demonstrated recording properties superior to those of conventional materials, with switching times on the order of picoseconds. It has been shown capable of storing both amplitude and phase holograms, or a combination thereof, with diffraction efficiencies on the order of 7%. While this is lower than in some silver halide applications, other properties, such as its virtually infinite shelf life and its diffraction limited resolution of 5000 l/mm, are outstanding compared to conventional materials. There are many holographic applications of bacteriorhodopsin, such as spatial light modulation, optical associative memory, and high speed interferometry. Each of these applications requires the temporary storage of an image during processing, with rapid erasure and re-exposure. Through its change in absorption spectrum, its large variation in refractive index, and its response to external voltage, it is capable of modulating both phase and amplitude. This is important in pattern recognition architectures, for example. Also, because it is self-developing and recyclable, the use of bR in aero-optic applications allows the recording of high speed interferometric `movies' of short- lived events, superseding the archaic one-shot silver halide plates which are still used in such applications today.
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