Optical measurement of the photorefractive parameters of Bi12SiO20

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Abstract
We employ an optical technique requiring no electrodes to measure three parameters characterizing photorefractive traps in nominally undoped n-type bismuth silicate at 515- and 488-nm wavelengths. These parameters are (1) an effective density of active electron trap sites (∼1016 cm−3), (2) the contribution αpcm−1 of the photorefractive excitations to the total absorption coefficient (∼3.8 cm−1 or 54% at 488 nm and 1.4 cm−1 or 86% at 515 nm), and (3) the average distance over which an optically excited electron moves in the absence of electric fields before being retrapped (∼4 μm). With these parameters (and the crystal properties) standard models give predictions for all photorefractive effects, with or without applied electric fields and crystal motion, provided that there is negligible change in the density of occupied traps during an electron recombination time (∼microseconds). Our technique employs measurement of the light-induced exponential decays of spatial ‘‘gratings’’ of trapped charges as a function of the grating period. The expected three-parameter function is observed from which we determine the above three parameters.