Our paper on Monte Carlo simulations of the second hyperpolarizability has just appeared in the Journal of the Optical Society of America B. The paper was selected to be highlighted on Spotlight on Optics. An excerpt from the summary follows:
Spotlight summary: The paper of Shafei et al. belongs to a series of papers initiated a decade ago by M.G. Kuzyk, oriented toward a theoretical analysis of fundamental limits and the optimization of attainable first- and second-order off-resonant electronic nonlinear hyperpolarizabilities, which describe a molecule’s response—its nonlinear polarization—to optical fields. Nonlinear hyperpolarizabilities are expressed in terms of infinite series. In the absence of a small parameter, the optimization of hyperpolarizabilities and the search for universal features constitute a challenge, which requires, first of all, a deep physical intuition.
In earlier papers, Kuzyk et al. calculated the fundamental limits for nonlinear hyperpolarizabilities using the three-state ansatz, which states that when a hyperpolarizability is near the fundamental limit, the system can be described by a three-level model. To this end they have used sum rules—one of standard approaches in quantum theories—that put constraints on infinite sets of energy levels and transition moments.
Surprisingly, experimental studies and earlier theoretical approaches have revealed a large gap between attainable values of the first and second hyperpolarizabilities and predicted fundamental limits. This casted doubt on the validity of the three-state ansatz...see more