7 September 2011 Why do we need three levels to understand the molecular optical response?
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Traditionally, the nonlinear optical response at the molecular level has been modeled using the two-level approximation, under the assumption that the behavior of the exact sum-over-states (SOS) expressions for the molecular polarizabilities is well represented by the contribution of only two levels. We show how, a rigorous application of the Thomas-Kuhn sum-rules over the SOS expression for the diagonal component of the first-hyperpolarziability proves that the two-level approximation is unphysical. In addition, we indicate how the contributions of potentially infinite number of states to the SOS expressions for the first-hyperpolarizability are well represented by the contributions of a generic three-level system. This explains why the analysis of the three-level model in conjugation with the sum rules has lead to successful paradigms for the optimization of organic chromophores.
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Javier Perez-Moreno, Javier Perez-Moreno, Koen Clays, Koen Clays, Mark G. Kuzyk, Mark G. Kuzyk, } "Why do we need three levels to understand the molecular optical response?", Proc. SPIE 8113, Linear and Nonlinear Optics of Organic Materials XI, 81130L (7 September 2011); doi: 10.1117/12.893477; https://doi.org/10.1117/12.893477

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