The intense monochromatic radiation from a laser can induce profound changes in the optical properties of a material. Nonlinear absorption refers to the change in transmittance of a material as a function of intensity or fluence. At sufficiently high intensities, the probability of a material absorbing more than one photon before relaxing to the ground state can be greatly enhanced. As early as 1931, Göppert-Mayer derived the two-photon transition probability in a system using second order quantum perturbation theory . Since the invention of the laser, not only has this phenomenon of the simultaneous absorption of two photons been observed in a wide variety of materials, multiphoton (>2) absorption has also been widely studied. In addition, population redistribution induced by intense laser fields leads to interesting counterplays of stimulated emission and absorption, complicated energy transitions in complex molecular systems, and the generation of free carriers in solids. These phenomena are manifested optically in a reduced (saturable) or increased (reverse saturable) absorption.