Origin of Giant Optical Nonlinearity in Charge-Transfer–Mott Insulators: A New Paradigm for Nonlinear Optics

Neither pure Mott insulators nor pure charge-transfer insulators have ever been considered as a possible candidate for nonlinear optical (NLO) materials since individually neither the strong correlation $\left(U\right)$ nor the large charge transfer $\left(\Delta \right)$ is favorable to the NLO response. However, in their composites, charge-transfer–Mott insulators, jointly $\Delta$ and $U$ can enhance the hyperpolarizability $\left(\gamma \right)$ by guiding the ground states into the antiferromagnetic phase and the excited states into the charge-transfer phase. These $\Delta$ and $U$ that maximize $\gamma$ form a unique golden $\Delta -U$ line, on which the recently observed giant nonlinear optical effect is just a single point, whose physical origin is that the system is driven into a phase-separated region for the ground and excited states. This novel mechanism may suggest a conceptually new paradigm to explore an even larger optical nonlinearity.