Nonlinear Optical Susceptibility of Tetragonal BaTiO3

For tetragonal phase of BaTiO3 in framework of vibronic theory of ferroelectricity components and of nonlinear susceptibilities responsible for second harmonic generation (SHG) and optical rectification (OR) are calculated. A model with split in tetragonal phase conduction band of symmetry E+A1 in the Γ point of Brilouin zone and nondegenerate valence band of symmetry A1 is used. Selection rules in Γ point are extended over the whole Brilouin zone. Calculated components of quadratic susceptibilities for both effects are proportional to low symmetry lattice distortion (spontaneous polarisation). They have experimentally observed order of magnitude. The spectral behaviour of and is rather week