In some materials, an effective x3 or an effective n2 can be defined as well. For instance, in a secondorder nonlinear material, an effective x3 is established through cascading of x2s, and in the presence of phase mismatch between the fundamental and the second harmonic. In other materials, such as liquid crystals, an effective n2 can be defined due to the nonlinear dependence of the reorientation of the director axis on the optical field. In photorefractive materials, the induced nonlinearity stems from the nonlinear dependence of the induced space charge distribution in the material on the optical field. Some of the techniques used to measure the third-order nonlinearity in materials include degenerate four-wave mixing (DFWM) or phase conjugation, interferometry, ellipse rotation, and beam self-refraction and/or distortion and bending.