Measurement of dark conductivity at extremely low light levels using photorefractive two wave mixing

Dark conductivity measurements for BaTiO₃ and Ce:SBN:60 using photorefractive two-wave mixing at intensities of less than 10 mW/cm² are reported. It is shown that the most accurate measurements of dark conductivity are made when the y-intercept is computed using a weighted least squares fit including measurements taken near the noise floor for two-wave mixing interactions in a specific crystal. Such a bound is important to determine experimentally when seeking to understand noise mechanisms in a given photorefractive medium. The authors report σ_d=9.2×10^-13 (Ω-cm)^-1 for Ce:SBN:60 and σ_d=5.4×10^-12 (Ω-cm)^-1 for BaTiO₃ when intensities less than 10 mW/cm² are included in the calculation. The results obtained also indicate that diffusion length varies with intensity regime. Therefore, experimental configurations may be affected as well