Strong electron correlation and nonlinear optics

Abstract

Based on experimental and theoretical research during the last decade, giant optical nonlinearities found in Mott-Hubbard insulators like Sr 2CuO 3,Ca 2CuO 3, Nickel halides ([Ni(chxn) 2X]X 2 where X = Br, Cl and 'chxn' refers to cyclohexanediamine) are presented. These materials are reported to be potential materials for all optical switching devices. The occurrence of nearly degenerate lowest one- and two-photon states, strong Coulomb correlation and strong dipole coupling between the one- and two-photon states are believed to be the reason for such colossal optical nonlinearities in these systems. In some of these materials (at least), the two photon state is below the one-photon state. This leads to the possibility that such material can be excited to the lowest optical state by shinning laser of suitable wavelength, the populations thus generated decays to the two-photon state at ultrafast short time. Thus nonlinear measurements can be made from an excited state (we call as excited state nonlinear optical properties). One dimensional strongly correlated materials are predicted to have several orders-of-magnitude larger excited state optical non-linearities in comparison to that from the ground state, in the wavelength region suitable for terahertz communications. A large number of measurable nonlinear optical properties like Two Photon absorption, Photo induced absorption, Third Harmonic generation, Stimulated Raman Scattering are obtained theoretically and compared with available experimental observations. Then a large number excited state nonlinear optical properties are predicted which are experimentally measurable. We emphasize that the mechanism of nonlinear optics in one dimensional Mott-Hubbard insulators is different from that of the π-conjugated polymers --- in the former spin excitation play an important role. We argue from detailed understanding of nonlinear optics of π-conjugated systems that some features in the Third Harmonic Generation susceptibility measurements are perhaps finger print of gapless spin excitations of Mott-Hubbard insulators. © 2012 American Institute of Physics.