ELECTRONIC-STRUCTURE AND OPTICAL NONLINEARITY OF TIN OXIDE THIN-FILMS

Off-resonant third-order non-linear optical susceptibility, X((3))(-3 omega; omega,omega,omega), of tin oxide thin film is evaluated by the third-harmonic generation method. The tin oxide thin films of various oxygen contents are prepared successively by an rf magnetron sputtering technique under precise control of the gas pressure during sputtering. The value of X((3)) of amorphous SnO2-x ranges from 10(-12) to 10(-11) esu and is larger by several times than that of polycrystalline SnO2 thin film. A larger susceptibility is observed in amorphous SnO2-x of lower oxygen content. These results are explained on the basis of the electronic structure of tin oxides and are summarized as follows, (1) The introduction of a sufficient amount of oxygen deficiency into SnO2 results in the formation of Sn2+. (2) The optical band gap of amorphous SnO2-x becomes smaller with the formation of Sn2+ than that of SnO2. (3) The non-linear susceptibility becomes large with narrowing of the band gap, as predicted by the quantum mechanical formula of the third-order optical susceptibility.