Phase-sweep method for characterization of cw-laser-induced dielectric gratings

We present a new phase-sweep method for characterizing laser-induced complex dielectric gratings. This characterization requires four parameters: the amplitudes and the spatial phases of both the refractive-index and the absorption components. We apply this phase-sweep method to study a complex polarizability grating in a nominally undoped cubic photorefractive Bi₁₂TiO₂₀ crystal. We determine the polarizability difference a_fe between a full and an empty photoexcitable deep trap at 633 nm in Bi₁₂TiO₂₀ to be (2.5 - 8.3i ± 1.0 ± 3.3i) × 10^-40 F m² in SI units or (2.3 - 7.5i ± 0.9 ± 3.0i) × 10^-24 cm³ in Gaussian units.