Theory of the two-photon absorption-fluorescence method of pulsewidth measurement

A three-level model of a two-photon absorber is set up, and the response of the system to a pulsed excitation is calculated, including the effects of homogeneous and inhomogeneous broadening. The results are applied to the two-photon absorption-fluorescence (TPF) mesurement technique and it is shown that the larger of the two broadening mechanisms sets an upper limit to the spectral width of pulses, for which an unambiguous interpretation of the TPF display can be made. Effects that occur when this condition is violated are discussed.