 Twophoton absorption

 
The number of twophoton excitations per molecule are found for a given laser pulse, at the center of a Gaussian beam. Saturation effects are not considered, and negligible linear absorption is assumed. The molecular twophoton crosssection must be provided (usually in GM).
 
Twophoton absorption occurrs only in nonlinear optical molecules, and even then only at high light intensity. Thus, focused lasers are used. Here, the photon flux at the center of a Gaussian beam with given parameters is calculated (enter the instantanious, not the average power), and from this value we find the average number of times each molecule at that point gets excited by the pulse. If the number approaches 1 and the excitedstate lifetime is of the order of the pulse length, then saturation may take place.  