Excited state nonlinear optics of quasi-one-dimensional Mott-Hubbard insulators

Abstract

Ground as well as excited state nonlinear optical properties of one-dimensional Mott-Hubbard insulators are discussed in detail. One-dimensional strongly correlated materials are predicted to have several orders of magnitude larger excited state optical nonlinearities in comparison to that from the ground state. Unlike $\ensuremath{\pi}$-conjugated polymers and other classes of one-dimensional systems, strongly correlated materials such as ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{3}$, halogen-bridged Ni compounds, etc., have excited one- and two-photon allowed states almost energetically degenerate, which leads to order(s)-of-magnitude larger dipole coupling between them in comparison to that between the ground and optical states. This causes several orders of magnitude enhancement in the optical nonlinearities obtained from the first two-photon state in the wavelength region suitable for terahertz communications. Our results and conclusions are based on exact numerical calculations of the extended Hubbard model suitable for strongly correlated systems. We argue based on theoretical as well as available experimental data that one-dimensional cuprates would be a good source of terahertz radiation, to be experimentally verified. We also discuss in detail ground state nonlinear properties such as third-harmonic generation, two-photon absorption, and electroabsorption from the theoretical model and compare with experiments. Theoretical calculations of excited state nonlinearities of some $\ensuremath{\pi}$-conjugated polymers are also presented for comparison.