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

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.

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