Measurements of absorption and proton activation in crystals with applications to holographic storage

(Formula presented) is a material under consideration for electro-optic and photorefractive applications. Single crystals of (Formula presented) have been grown using the vertical Bridgman method in the composition range (Formula presented) The formation and thermal fixing of holographic gratings in these crystals were recently demonstrated. In this study the (Formula presented) absorption spectra were measured by fourier-transform infrared spectrometry. As-grown crystals showed a strong (Formula presented) absorption near (Formula presented) revealing the existence of protons (Formula presented) which are essential for the thermal fixing of holograms. The dependence of the integrated (Formula presented) optical-absorption intensity on polarization showed that all of the O-H bonds were polarized either perpendicular or parallel to the (Formula presented) axis, with a greater proportion perpendicular. The proton concentration could be increased by heat treatment in a humid atmosphere and decreased in an ambient or oxygen atmosphere. The diffusivity of protons perpendicular to the (Formula presented) axis was measured in (Formula presented) by proton in-diffusion experiments and was found to be (Formula presented) at 750 °C, which is comparable to the proton diffusivity in (Formula presented) and (Formula presented) at the same temperature. Measurements of the thermal decay of holograms showed that the proton activation energy, (Formula presented) in (Formula presented) was (Formula presented) which is close to the proton activation energies reported for (Formula presented) and (Formula presented) 1.10-1.40 eV and 0.81-1.04 eV, respectively.