Gigantic optical nonlinearity in one-dimensional Mott–Hubbard insulators

The realization of all-optical switching, modulating and computing devices is an important goal in modern optical technology. Nonlinear optical materials with large third-order nonlinear susceptibilities (χ(3)) are indispensable for such devices, because the magnitude of this quantity dominates the device performance. A key strategy in the development of new materials with large nonlinear susceptibilities is the exploration of quasi-one-dimensional systems1,2, or ‘quantum wires’—the quantum confinement of electron–hole motion in one-dimensional space can enhance χ(3). Two types of chemically synthesized quantum wires have been extensively studied: the band insulators of silicon polymers, and Peierls insulators of π-conjugated polymers and platinum halides. In these systems, χ(3) values of 10-12 to 10-7 e.s.u. (electrostatic system of units) have been reported3,4,5,6,7. Here we demonstrate an anomalous enhancement of the third-order nonlinear susceptibility in a different category of quantum wires: one-dimensional Mott insulators of 3 d transition-metal oxides and halides. By analysing the electroreflectance spectra of these compounds, we measure χ(3) values in the range 10-8 to 10-5 e.s.u. The anomalous enhancement results from a large dipole moment between the lowest two excited states of these systems.