Third-order nonlinear optical properties of sol-gel derived alpha-Fe2O3, gamma-Fe2O3, and Fe3O4 thin films

The third-order nonlinear optical properties of sol-gel derived alpha-Fe2O3, gamma-Fe2O3, and Fe3O4 thin films have been investigated by the third-harmonic generation method. Especially, the effects of the valence and coordination number of Fe ions on the third-order nonlinear optical susceptibility, chi((3)), have been examined. The chi((3)) values of alpha-Fe2O3, gamma-Fe2O3, and Fe3O4 thin films were 5.8X10(-11), 2.1X10(-11), and 4.0X10(-10) esu, respectively, which are the highest values among inorganic oxides reported so far. It was considered that chi((3)) of alpha-Fe2O3 and gamma-Fe2O3 was enhanced by the pair excitation process involving the simultaneous excitation of magnetically coupled two neighboring Fe3+ ions as well as by virtual transition from O 2p orbitals to Fe 3d orbitals while chi((3)) of Fe3O4 was enhanced by both one- and three-photon resonances. The higher second hyperpolarizability, gamma(Fex/yO), was obtained when Fe ions are 3+ rather than 2+ and octahedrally rather than tetrahedrally coordinated by oxygens. (C) 1996 American Institute of Physics.