Photorefractive properties of ${\mathrm{LiNbO}}_3$ crystals doped by copper diffusion

Lithium niobate volume crystals (up to 1 mm thick) are doped by indiffusion of thin layers of evaporated copper. The obtained samples are investigated by conventional electrical and holographic methods. Saturation values $\Delta n_{\mathrm{s}}$ of the refractive-index changes, bulk-photovoltaic current densities $j_{\mathrm{phv}},$ and photoconductivities ${\sigma }_{\mathrm{ph}}$ are measured utilizing samples with copper concentrations in the range $\left(2.2\mathrm{}-145\right)\times {10}^{24}{\mathrm{m}}^{-3\mathrm{}}.$ Comparison with experimental data for melt-doped lithium niobate shows that the photorefractive properties do not depend on the doping technique. Another important outcome is that the refractive-index changes saturate at $7\times {10}^{-4}$ (ordinary light polarization, green light) for copper concentrations larger than $60\times {10}^{24}{\mathrm{m}}^{-3\mathrm{}}.$ This is caused by a strong increase of the photoconductivity for high doping levels.