Anisotropic photorefraction in congruent magnesium-doped lithium niobate

A remarkable dependence of photorefractive response on c-axis orientation for lithium niobate samples at several magnesium contents has been observed. Above the well-known threshold concentration of magnesium, around 4.6 mol% MgO in congruent melt, the photorefraction almost disappears when the polar axis is parallel to the incidence plane of light beams in the two-wave mixing configuration. However, if the c-axis is perpendicular to the incidence plane the photorefractive effect persists even above threshold and it is strongly diminished below threshold. An explanation for this anisotropic behavior along with an estimation of bulk photovoltaic electric field through a proposed model has also been carried out.