Extended-lifetime holograms for image processing in bismuth silicon oxide

Abstract

Extended-lifetime holograms are observed in photorefractive bismuth silicon oxide (BSO) at 785 nm. In the crystal, light beams from a diode laser (~500 μW/cm2) create an index grating with a period of 10 μm. An electric field ~5 kV/cm is optionally applied in the direction of the grating 110 for hologram formation by either diffusion or drift of charge carriers. Different formation procedures lead to different decay processes of the grating. In the first case, the hologram is written with the applied field for 70 s and, upon uniform illumination in the applied field, decays normally, with a time constant ~15 s. In another, the hologram is written without the applied field for 640 s and decays less simply: The initial diffusion grating is extremely weak for this period; however, upon uniform illumination in the applied field, the hologram quickly strengthens to the level of the previous case ~15 s, and then decays more slowly ~2000 s. Two-level electron–hole transport1 can explain these effects: using one level, electrons establish quick-response gratings; using the other, holes establish complementary extended-lifetime gratings. Vainos et al.2 have multiplexed short-lived photorefractive with long-lived photochromic holograms in BSO, leading to many image processing techniques. Multiplexing the normal and extended-lifetime holograms described here may yield similar techniques.