Abstract

We present a new promising photorefractive crystal: lead germanate Pb₅Ge₃O₁₁. This crystal is ferroelectric and has trigonal symmetry. The crystal itself has been studied before and most of its physical properties are well known. However, to our knowledge, the occurrence of photorefraction in this material has not been reported before. Large (0.9 cm × 0.8 cm × 0.43 cm) samples of Pb₅Ge₃O₁₁ exhibit absorption of 1.5 cm⁻¹ and optical activity of 8° /mm for green light (514.5 nm). Both parameters are smaller than in commonly used BSO crystal whose optical activity is almost 5 times as large. Since the largest electrooptic coefficient r₃₃ is 15 pm/V compared with 5 pm/V for BSO, this crystal seems to be really interesting as a candidate for holographic recording medium. We studied the photorefractive properties of lead germanate in a conventional two-beam coupling experiment. We found that photorefraction occurs for all argon-ion lines and is strongest for λ = 514.5 nm. Weaker photorefractive sensitivity was found for red light. Therefore we used a He-Ne laser beam to read out the grating written by two green beams. In two-beam coupling measurements we obtained a photorefractive intensity gain coefficient of Γ = 1.46 cm⁻¹. The time constant for grating formation was ~1 s for a total writing beam intensity of 200 mW/cm². In our experiments we saw the effect of hologram fixing under long illumination. For short exposure the grating was completely erasable. We also studied the influence of external electric dc fields on the grating properties. It was found that an electric field of 5 kV/cm significantly improved the diffraction efficiency (5 times). Switching of this field leads to interesting transient behavior of the diffracted beam intensity (strong oscillations).

© 1989 Optical Society of America