Maximizing two-photon absorption cross section within few essential state model

We compare the experimental absolute two-photon absorption (2PA) cross sections and spectra of various organic molecules with those obtained from the few (two or three) essential states model, assuming certain parameter values that are either previously calculated theoretically or measured. We study conditions under which the two- or three-level models are applicable for quantitative description of 2PA, and estimate the corresponding maximum realistic attainable peak 2PA cross sections and spectral widths. Based on our observations we formulate the steps required to increase the intensity and broaden the spectral coverage of instantaneous 2PA by optimizing intrinsic molecular parameters such as transition dipole moments, permanent dipole moments and excited states energies.