Abstract

What are the defects or impurities responsible for the photorefractive effect in Bii2MO2o (BMO 6:1)? What happens during illumination at various wavelengths? What are the microscopic centers, either diamagnetic or paramagnetic, responsible for ±e intrinsic or light / impurity induced absorption bands? One must obviously try to answer such questions if one wants to understand the observed effects and to optimize the materials for improved performances. Since 1990 *, we have conducted a systematic spectroscopic study of photochromic effects in a large variety of undoped and doped BMO 6:1 crystals of two origins As already briefly demonstrated in previous communications ^χ3.4, magnetic circular dichroism (MCD) offers an elegant and powerful means for a microscopic study of charge transfer processes and the identification of donnor/acceptor levels. The technique has also long been established 5 as an invaluable tool (i) to characterize the charge state and local environment of a doping rm nation metal ion; (ii> to discriminate among paramagnetic and diamagnetic centers. MCD can be defined as the differential absorption Δα (cm4) for left and right circularly polarized light when the sample is subπüted to a longitudinal magnetic field B. In zero field, BMO crystals present also a natural CD which contains interesting information. My goal here is to present an overview of our most significant results since the Aussois Topical Meeting. Quite naturally, MCD.and CD spectra have been complemented by low temperature absorption experiments in the visible and near IR regions. Note that previous CD 6,7 and MCD 8 work on BMO materials, although scarce, has all been conducted in Eastern Europe.

© 1993 Optical Society of America