Continous-Wave two-photon transition in photoretractive crystals

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The purpose of this research is to investigate the dynamic properties of photorefractive (PR) gratings via the continuous-wave two-photon transition process in PR media and to demonstrate nonvolatile two-color holography with high figures of merit. The main results are listed below.1. We have shown that as compared with nondoped lithium niobate (LN) crystals, two-color recording sensitivity S can be increased by two order of magnitude in an Er-doped LN crystal.2. We have shown that figures of merit S and M/# can be increased up to those of near stoichiometric LN crystals when In ions are doped into congruently melted LN crystals.3. Broadband UV-induced absorption is observed in Mg-doped near stoichiometric LN crystals. It is found that the cause for such induced absorption is attributed to the formation of O^- intermediate levels resulting from photo-excited holes trapped at O^<2-> sites. We also demonstrate quasi-nonvolatile two-color holographic recording by use of this novel phenomenon.4. We demonstrate nonvolatile one-photon (near-infrared) holographic recording by use of LN codoped with In and Fe. Exchange of photo-excited electrons between bipolaron and Fe levels are considered to be a main cause for this novel phenomenon. We confirmed such a transport model by measuring near-infrared light-induced absorption changes in the UV and visible spectral regions.