ABSTRACT
Two-photon absorption (2PA) is a loss mechanism occurring in all materials at sufficiently high irradiance when the combined energy of two quanta of light matches a transition energy between two states of the same parity. The fact that states are coupled that have the same parity makes 2PA transitions complementary to one-photon absorption where states of opposite parity are coupled by electric-dipole allowed transitions. 2PA spectroscopy is used to determine even parity transition energies. It has been suggested that three-dimensional optical memories can be written with two intersecting beams in 2PA materials, where 2PA creates defects or other long-lived optical property changes such as 2PA gratings, and 2PA-induced photorefractivity. For 2PA spectral measurements other methods must be used such as excitation and probe methods or fluorescence measurements. The experimental data on two-photon absorption coefficients are arbitrarily divided into three tables; small band gap materials, large gap materials, and glasses.
