| Nonlinear absorption is a fundamental interaction of light with material. It is closely related to nonlinear scattering, nonlinear lighted refraction and nonlinear fluorescence emission. Excited-state absorption (ESA) and two-photon absorption (TPA) are two kinds of familiar nonlinear absorption and either of them could happen in the transparent regime. Generally it's difficult to tell the real nonresonant absorption mechanism when the excited-state absorption cross section is bigger than the ground state absorption cross section. Because that both of them can enhance the absorption with the increase of the light intensity.In this thesis, the main mechanisms and development of nonlinear absorption are introduced firstly. Then two appropriate models for either of organic material and bulk semiconductor in the pump-probe experiment are established. We describe nonlinear absorption in the organic material whose population density is always small with multiple level model and absorption cross section. On the other hand, we describe nonlinear absorption in the bulk semiconductor whose population density is big with absorption coefficient. At last, a numerical simulation is made.The effect on ESA and TPA of different absorption cross section, the life of the excited states, the intensity of pump beam and the pulse width in the pump-probe measurement are researched by the numerical simulation. The typical normalized transmittance as a function of probe-beam time delay is obtained when the TPA dominanted. So are done when the ESA dominanted and two absorption mechanisms existed. The numerical analysis offers a theoretical instruction of how to distinguish the TPA and ESA.In the end, we have studied the C60 in Toluene and the ZnSe crystal in the self-pump-probe experiment to prove the theory. The wave length is 532nm and the pulse width (HMFW) is 22ps. The experimental results are in agreement with the presented theory well.
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