Photorefractive dark conductivity in SBN:60:Cr and SBN:60:Ce,Ca

Abstract

Previous experiments on photorefractive two-wave mixing and phase conjugation in SBN:60:Ce, Ca and SBN:60:Cr have shown that the two-beam coupling constant Γ increased as the crystals were cooled below room temperature despite the decreasing electrooptic coefficient when the crystal temperature was lowered away from the ferroelectric phase transition at ~70°C. The existing Kukhtarev model only predicts that Γ should decrease with temperature due to the lower electrooptic coefficient as the crystals were cooled away from the phase transition point. This model, however, does not include the dependence of the space-charge electric field on the dark conductivity of the materials. The finite dark conductivity of the material will result in a recombination of the photoionized electrons, which reduces the space-charge field and, therefore, the photorefractive gain. By adding a thermal dark conductivity term σdark proportional to exp(–A/kT), where A is a constant, to the Kukhtarev rate equation, we derived an expression for the steady-state photorefractive gain and response time as Γ = Γ0(1 + σdark/σph)–1 and τ = τ0(1 + σdark/σph)–1, where Γ0 and τ0 represent the low-temperature limit, and σph, the photoconductivity, is a function of total intensity. Using an SBN:60:Cr sample grown at Rockwell International Science Center, we experimentally verified our theoretical predictions.