Coupling of orthogonally polarized waves in BaTiO3

The first observation of anisotropic diffraction (with a 90° change of the polarization of the diffracted wave) in BaTiO3 from a grating recorded by two orthogonally polarized waves, an ordinary and an extraordinary wave, is reported. Three possible origins of this effect are considered: (i) direct one-step diffraction from a grating recorded anisotropically by spatially oscillating photovoltaic currents, (ii) indirect, sequential diffraction from two gratings, each recorded by one incident wave and an additional wave due to light-induced scattering, (iii) direct one-step diffraction from a grating with the grating vector equal to the difference of the grating vectors of two conjugated 'noisy' gratings. The experimental evidence indicates that the process (iii) contributes most, and process (i) more than 10% to the overall diffraction efficiency, whereas process (ii) seems to be of minor importance.