Calculation of the Relationship between Exciting Light Intensity and Two-photon Absorption Cross-section of Organic Dyes

On the basis of a semi-classical point view,by using the quantum-mechanical derivation theory,the fact that two-photon absorption rate is proportional to the intensity square is demonstrated.The incident light is assumed to be unpolarized and the electric field of the incoming radiation is simplified as single-frequency field,which may not be the real case for exciting light in the experiments,but for the purpose of demonstrating the intensity-squared dependence of two-photon absorption is sufficiently accurate.The two-photon-absorption cross-section expression of the organic dyes is given,and the value of two-photon absorption cross-section is calculated to be about 10-50 cm4·s/photon.