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 (U) nor the large charge transfer (Delta) is favorable to the NLO response. However, in their composites, charge-transfer-Mon insulators, jointly Delta and U can enhance the hyperpolarizability (gamma) 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.