Photochromic-Induced Photorefractive Changes in Barium Titanate
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PHOTOCHROMIC-INDUCED PHOTOREFRACTIVE CHANGES IN BARIUM TITANATE
M. H. GARRETT, J. Y. CHANGt, H. P. JENSSEN AND C. WARDE* Center for Materials Science and Engineering, tDepartments of Materials Science and Engineering, and *Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Ave. Rm 13-3157, Cambridge, MA 02139
ABSTRACT Photochromism in barium titanate was found to affect its deep and shallow trap properties. Argon ion laser illumination at 514.5 nm photo-induced semi-permanent (thermally reversible) changes in the visible absorption spectrum of barium titanate that were stable at room temperature. Increases in absorption at 514.5 nm, also the operating wavelength used to determine photorefractive properties, caused a decrease in the photorefractive response time but also a decrease in the sensitivity. When the photochromism was inactive the crystal shows light-induced absorption. With the photochromism saturated the crystal showed light-induced transparency. A model with two photoactive levels is used to characterize the photorefractive properties of this photochromic crystal.
INTRODUCTION The photorefractive effect is currently being applied to optical information processing applications such as optical memories, associative memories, and neural networks [1]. Barium titanate is often the photorefractive material of choice for these applications because of its high beam coupling gain and high self-pumped phase conjugate reflectivity. High gains, up to 65 cm- 1 , have been demonstrated for crystals in the 45*-cut. However, typically the photorefractive2 response time of commercially available barium titanate is on the order of about 1 sec at 1 W/cm and its theoretical limit is approximately 1 msec at 1 W/cm 2 . We have grown nominally undoped barium 2titanate that is photochromic and displays relatively fast response times (-55 msec at lW/cm determined at the l/e point of the light-induced erasure rate) when the photochromism is activated. The decrease in the response time occurs at the expense of a reduction in sensitivity. Nevertheless, the crystals are, to the best of our knowledge, more sensitive than others reported to date and belong to a class of barium titanate crystals called type B, with high dark conductivity. In this paper we examine the photochromic properties of our crystal and show that the photochromism induces changes in the bulk absorption and also affects the light-induced absorption that arises because of the photoinduced redistribution of charge between two photoactive levels. Our beam coupling measurements, which discern electrooptic and absorptive coupling, confi-rn the predictions of a photorefractive model including deep and shallow traps, first described by Tayebati and Mahgerefteh [2]. The response time and sensitivity as a function of the photochromic state are given, summary and conclusions follow.
Mat. Res. Soc. Symp. Proc. Vol. 228. kc,1992 Materials Research Society
148
CRYSTAL GROWTH Nominally undoped barium titanate, BaTiO 3, w
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