ABSTRACT
So far in this book, we have remained within the realm of linear spectroscopy. This means that the induced polarization, the dipole moment per unit volume, is a linear function of the electric field. This is a good approximation for electromagnetic fields which are very small compared to the internal fields of molecules, and it gives us a wealth of conventional spectroscopy techniques bound by the selection rules introduced in Chapter 4. Linear response of matter to electromagnetic radiation also gives us familiar phenomena such as reflection and refraction. In the presence of more intense fields such as those provided by pulsed lasers, nonlinear effects result in many new spectroscopic techniques to probe matter. At the same time, nonlinear media, such as doubling crystals used to generate the second harmonic frequency of incident light, provide a powerful means to manipulate the properties of light beams. To enter into this new territory, we have to surrender the notion that the light-matter interaction is weak enough to leave each relatively unperturbed by the other. For example, as you will show in Problem 1 at the end of this chapter, the electric field amplitude associated with a typical pulsed laser source is not negligible compared to the internal fields of atoms and molecules.
