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 Bi(12)TiO(20) crystal. We determine the polarizability difference afe between alpha(fe) full and an empty photoexcitable deep trap at 633 nm in Bi(12)TiO(20) to be (2.5 - 8.3i +/- 1.0 +/- 3.3i) x 10(-40) Fm(2) in SI units or (2.3 - 7.5i +/- 0.9 +/- 3.0i) x 10(-24) cm(3) in Gaussian units.