Comparative study of photorefractive Bi12SiO20 crystals

We present the results of a comparative experimental study of optical and photorefractive properties at room temperature of four nominally undoped, single-crystal, commercially obtained, samples of cubic Bi12SiO20. We studied (1) the steady-state energy exchange between two overlapping 515-nm beams, (i.e., ‘‘beam coupling’’), (2) the decays of photorefractive (trapped-charge) gratings induced by uniform illumination, (i.e., ‘‘grating erasure’’), (3) spontaneous decay of photorefractive gratings in the dark (i.e., ‘‘dark decay’’), (4) photochromic damage, and (5) standard optical properties such as absorption coefficients and rotatory power. Fitting our results to a simple model of photoexcitation of both electrons and holes from a single species of deep traps, we obtained values for the effective density of deep traps, the diffusion lengths of both electrons and holes (i.e., mobility-lifetime products), quantum efficiencies (near unity), and the ratio of the electron and hole contributions to absorption at 515 nm. Although unexplained or anomalous grating decay characteristics were observed, the model gives an excellent overall account of photorefractive behavior, and suggests that this type of crystal would be suitable for making devices with predictable behavior.