ABSTRACT
This chapter discusses a number of mechanisms that result in nonlinear absorption (nla) or nonlinear refraction (nlr) or a combination of the two. It concentrates on ultrafast third-order bound-electronic nla and nlr and temporally cumulative free-carrier or excited-state nonlinearities. These excited-state nonlinearities can be understood in terms of a sequential cascading of lower order nonlinearities. The chapter describes how frequency conversion nonlinearities can also lead to nla and nlr through another cascading process, in this case, a cascaded second-order nonlinearity. It discusses the Kramers-Krönig relation used to calculate the change in refractive index from the change in absorption due to some external perturbation. The linear Kramers-Krönig relation can be applied both in the presence and in the absence of a perturbation, and the difference taken between the two cases. The ‘two-level atom’ problem is a familiar one and is illustrative of how the nonlinear Kramers-Krönig relation can be applied.
