External Electric Field Effects on Photorefractive Bismuth Silicon Oxide

Photorefractive bismuth silicon oxide (Bi _{12}SiO_{20 } or BSO) promises great potential as an optical device material. The PRIZ spatial light modulator uses BSO to perform the unique image processing operations of edge enhancement and directional spatial filtering. The performance optimization of a BSO-based device requires a thorough understanding of the photorefractive mechanism. The primary goals of this research effort are to advance our knowledge of the photorefractive process in BSO and study its application in the PRIZ device. The effects of external electric fields on the carrier transport and optical properties of BSO are investigated experimentally. AC photoconductivity measurements indicate that the charge transport process in BSO is dominated by carrier hopping between highly localized groups of band gap sites. The presence of low concentration impurities in the material are shown to decrease the carrier mobility. These results indicate that charge migration in BSO exhibits characteristics of both hopping and multiple trapping conduction. For the first time, an electric field dependence of the photocarrier mobility in BSO is observed and characterized. Through direct measurement, the electrogyratory effect is found to be negligible in BSO. The impact of these electrical and optical properties on the operation of BSO-based devices is discussed. PRIZ devices have been constructed from crystals of BSO. The image processing operations of edge enhancement and directional spatial filtering are demonstrated. The devices are characterized with respect to their spatial resolution, sensitivity, and electric field dependent diffraction efficiency. These devices exhibit characteristics comparable to PRIZ devices of Soviet manufacture.