Self-diffraction through cascaded second-order frequency-mixing effects in b-barium borate

The large effective third-order nonlinearity produced by cascaded second-order processes is evidenced by the spatial self-diffraction of two beams interacting in a b-barium borate crystal. In this configuration, both the real and imaginary parts of the induced third-order susceptibility play a role, and the effect reaches its maximum
around the phase-matching condition