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 systems, 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 reported. Here we demonstrate an anomalous enhancement of the third-order nonlinear susceptibility in a different category of quantum wires: one-dimensional Mott insulators of 3d 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.