The third-order nonlinear optical properties of sol-gel derived ␣-Fe 2 O 3 , ␥-Fe 2 O 3 , and Fe 3 O 4 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, ͑3͒ , have been examined. The ͑3͒ values of ␣-Fe 2 O 3 , ␥-Fe 2 O 3 , and Fe 3 O 4 thin films were 5.8ϫ10 Ϫ11 , 2.1ϫ10 Ϫ11 , and 4.0ϫ10 Ϫ10 esu, respectively, which are the highest values among inorganic oxides reported so far. It was considered that ͑3͒ of ␣-Fe 2 O 3 and ␥-Fe 2 O 3 was enhanced by the pair excitation process involving the simultaneous excitation of magnetically coupled two neighboring Fe 3ϩ ions as well as by virtual transition from O 2p orbitals to Fe 3d orbitals while ͑3͒ of Fe 3 O 4 was enhanced by both one-and three-photon resonances. The higher second hyperpolarizability, ␥͑Fe x/y O͒, was obtained when Fe ions are 3ϩ rather than 2ϩ and octahedrally rather than tetrahedrally coordinated by oxygens.
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