The Principle, Application and Imaging of TPEF

Simultaneous two-photon absorption (TPA) is a nonlinear optical process. This phenomenon was first predicted by Gppert-Mayer in 1931 who calculated the transition probability for a two-quantum absorption process. Two-photon absorption can create excited states with photons of half the nominal excitation energy, which can provide improved penetration in absorbing or scattering media. TPA is a third-order nonlinear optical process. Two-photon absorption is the \(I^2\) dependence of the process, which allows for excitation of chromophores with a high degree of spatial selectivity in three dimensions through the use of a tightly focused laser beam. Theoretical analysis of TPA have been implemented to interpret experimental measurements of TPA, which is very useful to understand structure-property relations.