**1. Introduction**

Nowadays, the possibility to investigate optical nonlinearities offers great insights on material properties and the interaction between light and matter. After the interaction with a strong optical field, the response of the material will be no longer linearly dependent on its strength, instead nonlinear effects start playing major roles. Due to crystal structures and symmetries of media, the third order is the nonlinearity of the lowest order that can be observed in all media [1], and thus has become a valuable tool to investigate structural and dynamic aspects

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

of matter. Among the third‐order nonlinear effects (e.g., third‐harmonic generation, optical Kerr effect), the four‐wave mixing (FWM) is the mostly explored since it generalizes all the third‐order nonlinearities. The FWM relies on the mixing of three input signals, which results in the generation of a fourth output field. When one of the input signals is resonant with the frequencies of the material, the FWM process can be enhanced and is called *stimulated Raman scattering* (SRS). Coupling this process with laser pulses delayed in time, namely using a pump‐ probe setup, it is possible to investigate the temporal behavior of the material and the evolution of its properties. Nonlinear Raman spectroscopy is an example of such combination between third‐order nonlinear optical effect and pump‐probe technique [2].

In this chapter, we discuss one of the FWM processes which contributes to the stimulated Raman scattering, called inverse Raman scattering (IRS). The theory behind the IRS effect will be explained, resorting to Feynman dual‐time line (FDTL) diagrams [3], as well as its appli‐ cation as a spectroscopic tool. Furthermore, the connection held between the IRS and the femtosecond transient absorption (FTA) spectroscopy will be clarified, pointing out the important role the IRS effect plays on the temporal evolution of relaxation dynamics in FTA.
