ABSTRACT
Two photon absorption was first predicted by Maria Goeppert-Mayer in 1931 as a single quantum event. Here two photons with equal or different wavelengths are simultaneously absorbed by a molecule, thereby, exciting it from its ground state to an excited estate. This nonlinear effect is associated with the imaginary part of the third order susceptibility, and therefore, its absorption efficiency scales with the light intensity (Boyd 1992). The high intensity is usually reached by using mode-locked lasers, which provides the required high peak powers while at the same time maintains an energy low enough to preserve the sample. When the relaxation to the ground state is radiative the process is known as two-photon excited fluorescence (TPEF).
