Photorefractive Nonlinear Optics
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Photorefractive Nonlinear Optics JACK FEINBERG Departments of Physics and Electrical Engineering, University of Southern California, Los Angeles, CA
90089-0484
ABSTRACT Photorefractive crystals are great for image processing, beam amplification, and phase conjugation. I will describe these and a number of other new applications of photorefractive crystals. I will also show how light beams in these crystals are like politicians: if given the chance they will always choose the path that maximizes their own gain. INTRODUCTION Most transparent materials pass light without being altered, but photorefractive materials are the exception. These materials are remarkably sensitive to light, and they can integrate the effects of even weak light beams over time, much like photographic film. However, unlike photographic film, photorefractive materials are erasable, so they can be used over and over again. The photorefractive effect is caused by impurities or vacancies in the crystal, which act as charge donors and acceptors. These extra charges (either electrons or holes or both) are rearranged when light strikes the crystal. If the charges are continuously illuminated they eventually arrange themselves into an equilibrium pattern. When the light is turned off, the rearranged charges stay put (if the crystal is a good insulator in the dark), and thereby "store" the light pattern. Even though the charges may be present in very small amounts, typically 1 part per million, their electric fields can significantly distort the crystal lattice. If the crystal lacks inversion symmetry, then the electric field surrounding each charge can cause a large, first-order change in the crystal's refractive index. Some of the most useful photorefractive crystals, such as BaTiO3 and LiNbO 3 , are ferroelectrics whose inversion symmetry has been removed by poling the crystal into a single domain. If the crystal is at a temperature near a phase transition, the lattice can be especially "floppy", and have a large first-order electrooptic effect. How fast the crystal responds to light and builds up its charge pattern depends on the number of photons hitting the crystal per second, the quantum efficiency for exciting a charge, and the transport length of an excited charge. To create the static electric fields observed in these crystals requires the rearrangement of about 10+16 charges per cubic centimeter. Even if each charge required only one photon to move from its initial to its final location in the crystal, it would still take a few milliseconds to supply sufficient photons, assuming an incident light beam with an intensity of 1W/cm2 , which is comparable to that of a weakly focused helium-neon laser. In fact it takes about a hundred times longer than this for the charges to reach equilibrium in crystals of barium titanate, due to the short migration distance (-100A) of a light-excited charge in this crystal. One would expect that increasing the light intensity Mat. Res. Soc. Symp. Proc. Vol. 228. c'1992 Materials Research Society
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