Nonlinear optics in mesoscopic composite materials

We review the dominant processes that shape the optical nonlinearities of mesoscopic composite materials with particular emphasis on the photoinduced modifications of their spectral features that are related either to the electron or photon confinement in metal and semiconductor nanoparticles embedded in a transparent dielectric in low concentration. We also derive fluctuation-dissipation relations for the nonlinear regime in such systems. These modifications are related to the strong charge-photon coupling regime in confined space and can be exploited in different ways in applications and in particular as photonic nanoprobes with outstanding spatio-temporal resolution.