Absolute Two-Photon Absorption Cross Sections and Spectra

Absolute Two-Photon Absorption Cross Sections and Spectra 

Absolute measurements of the 2PA cross section of chromophores are experimentally very demanding and difficult to implement for routine studies of new materials. This is mainly due to the fact that the 2PA excitation rate in a sample does not depend simply on the square of the average intensity, (f(r, t)), but on the average of the square of the intensity, (f(r, t) ) (r and t are the special and temporal coordinate) [63,64,78]. These two quantities are not the same if the laser piflse has substructure in space and time [85,86]. Thus, absolute 2PA determinations that rely on the measurements of signals proportional to the excitation rate, such as the 2P-induced fluorescence 

More precisely, the quantity of interest is the photon flux, or the number of photons arriving at the sample per unit area and time. The photon energy is the proportionality factor between the two quantities. 

All the practical applications of 2PA mentioned in Sect. 1 would benefit from the availability of compounds with large 2PA cross section, because this will 

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