Optimization of photorefractive two-wave mixing by accounting for material anisotropies:KNbO3andBaTiO3

The influence of the anisotropy of the effective dielectric constant, effective electro-optic effect, drift mobility, and photoexcitation cross section on the photorefractive two-wave mixing gain and speed are analyzed in detail. Theoretical expressions that include all these influences and that are valid for the single-level band model are reported. They can give the necessary guidance for optimizing the interaction geometries and the extrinsic crystal properties for systems based on two- and four-wave mixing or self-pumped phase conjugation. Concrete examples are given for ${\mathrm{KNbO}}_3$ and ${\mathrm{BaTiO}\mathrm{}}_3,$ where all possible two-beams interaction geometries are analyzed in the plane of maximum photorefractive nonlinearity $(bc$ and $\mathrm{ac}$ plane, respectively). It is shown that, besides the dielectric constant and the electro-optic effect, also the anisotropy of the photoexcitation with respect to wave polarization plays a major role and strongly influences the optimum geometry, allowing potentially very large enhancement of the exponential gain. Corrections to the standard expressions for photorefractive two-wave mixing amplification in the depleted pump regime are also given. They apply in the case of asymmetric incidence and/or under the presence of anisotropic photoexcitation.