Modeling the First-Order Molecular Hyperpolarizability Dispersionfrom Experimentally Obtained One- and Two-Photon Absorption

The search for optical materials, particularly organiccompounds, is still an attractive and essentialfield for developingseveral photonic devices and applications. For example, someapplications are based on light scattering with twice the energy ofthe incoming photon for selected compounds, that is, thenonlinear optical effect related to the second-order susceptibilityterm from the electronic polarization expression. The microscopicinterpretation of this phenomenon is called thefirst-ordermolecular hyperpolarizability or incoherent second harmonicgeneration of light. Understanding such phenomena as a functionof the incoming wavelength is crucial to improving the opticalresponse of future materials. Still, the experimental apparatus,hyper-Rayleigh scattering, apparently simple, is indeed achallenging task. Therefore, we proposed a proper alternative to obtain the dispersion of thefirst-order hyperpolarizability usingthe well-known one- and two-photon absorption techniques. By the spectral analysis of both the spectra, we gathered spectroscopicparameters and applied them for predicting thefirst-order hyperpolarizability dispersion. This prediction is based on ann-levelenergy system, taking into account the position and magnitude of transition dipole moments and the difference between thepermanent dipole moment of then-excited states. Moreover, using the presented method, we can avoid underestimating thefirst-order hyperpolarizability by not suppressing higher-energy transitions. Quantum chemical calculations and the hyper-Rayleighscattering technique were used to validate the proposed method