Third-order nonlinear optical properties of sol–gel derived α-Fe2O3, γ-Fe2O3, and Fe3O4 thin films

Abstract

The third-order nonlinear optical properties of sol–gel derived α-Fe2O3, γ-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, χ(3), have been examined. The χ(3) values of α-Fe2O3, γ-Fe2O3, and Fe3O4 thin films were 5.8×10−11, 2.1×10−11, and 4.0×10−10 esu, respectively, which are the highest values among inorganic oxides reported so far. It was considered that χ(3) of α-Fe2O3 and γ-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 χ(3) of Fe3O4 was enhanced by both one- and three-photon resonances. The higher second hyperpolarizability, γ(Fex/yO), was obtained when Fe ions are 3+ rather than 2+ and octahedrally rather than tetrahedrally coordinated by oxygens.