Photorefractive materials exhibit photoconductive and electro-optic behavior, and have the ability to detect and store spatial distributions of optical intensity in the form of spatial patterns of altered refractive index. Photoinduced charges create a space-charge
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چکیده
Photorefractive materials exhibit photoconductive and electro-optic behavior, and have the ability to detect and store spatial distributions of optical intensity in the form of spatial patterns of altered refractive index. Photoinduced charges create a space-charge distribution that produces an internal electric field, which, in turn, alters the refractive index by means of the electro-optic effect. Ordinary photoconductiue materials are often good insulators in the dark. Upon illumination, photons are absorbed, free charge carriers (electron-hole pairs) are generated, and the conductivity of the material increases. When the light is removed, the process of charge photogeneration ceases, and the conductivity returns to its dark value as the excess electrons and holes recombine. Photoconductors are used as photon detectors (see Sec. 17.2). When a photorefractive material is exposed to light, free charge carriers (electrons or holes) are generated by excitation from impurity energy levels to an energy band, at a rate proportional to the optical power. This process is much like that in an extrinsic photoconductor (see Sec. 17.2). These carriers then diffuse away from the positions of high intensity where they were generated, leaving behind fixed charges of the opposite sign (associated with the impurity ions). The free carriers can be trapped by ionized impurities at other locations, depositing their charge there as they recombine. The result is the creation of an inhomogeneous space-charge distribution that can remain in place for a period of time after the light is removed. This charge distribution creates an internal electric field pattern that modulates the local refractive index of the material by virtue of the (Pockels) electro-optic effect. The image may be accessed optically by monitoring the spatial pattern of the refractive index using a probe optical wave. The material can be brought back to its original state (erased) by illumination with uniform light, or by heating. Thus the material can be used to record and store images, much like a photographic emulsion stores an image. The process is illustrated in Fig. 18.4-1 for doped lithium niobate (LiNbO,). Important photorefractive materials include barium titanate (BaTiO,), bismuth silicon oxide (Bi,,SiO,,), lithium niobate (LiNbO,), potassium niobate (KNbO,), gallium arsenide (GaAs), and strontium barium niobate (SBN).
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