# Section 4: Third Order Nonlinear Optics

As we talked about in the last section, second-order nonlinear phenomena can only occur in certain types of material (non-centrosymmetric to be exact). Third-order phenomena, on the other hand, can occur in media with any type of symmetry! Thus, third-order phenomena comprise the majority of effects for many materials such as glass and certain types of crystals.

1. $\chi^{(3)}$ and Third Harmonic Generation