Characteristics of surface photorefractive waves in a nonlinear SBN-75 crystal coated with a metal film

Based on the calculation of the electrostatic field potential of space charges, we have analysed the characteristic features of light-induced scattering of extraordinary polarised light in photorefractive (PR) crystals (for example, an SBN-75 crystal). Using the method of images, the electrostatic field is analysed for surface (aperiodic) waves along the crystal – dielectric (air) interface. It is shown that the field distributions satisfying the boundary conditions can emerge only upon accumulation of a screening electric charge in a narrow transition layer of thickness ~1 mm, the sign of the charge being opposite that of the space charge in the illuminated region of the crystal. A model is proposed to explain the observed features of the surface PR waves in a metal-film coated PR crystal. In considering the contact potential difference at the PR crystal – film interface it is shown that in the crystal layer (adjacent to the film) enriched with charge carriers, i.e., electrons, the refractive index can be significantly reduced. In the case of small excitation angles (0 – 1.5°), this layer can act as an optical barrier, the reflection from which can result in near-surface waves; a characteristic difference from the previously observed oscillatory surface waves is the presence of a broadened intensity distribution shifted inside the crystal.