ABSTRACT
Nonlinear absorption refers to any effect in which a material's optical transparency depends on the intensity of the illumination. This includes changes in which the transparency increases or decreases. Such processes can occur almost instantaneously as in two-photon absorption, or can accumulate in time as in optically triggered chemical reactions (e.g., the mechanism in many photochromic sunglasses). In general, the magnitude of the nonlinear optical response gets smaller as the response time gets faster. Thus, slow nonlinear optical effects can be observed with sunlight while fast effects require pulsed laser beams. This entry discusses many but by no means all such effects. In general, nonlinear refraction accompanies nonlinear absorption since these processes are intimately connected by causality. Thus, this entry is a companion to the one on nonlinear refraction by M. Sheik-Bahae and M. Hasselbeck. In that entry, the causal relationship is described in more detail.
