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 sputterin g technique under precise control of the gas pressure during sputterin g. 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 Sn O2 thin film. A larger susceptibility is observed in amorphous SnO2-x of lower oxygen content. These results are explained on the basis of t he electronic structure of tin oxides and are summarized as follows, ( 1) The introduction of a sufficient amount of oxygen deficiency into S nO2 results in the formation of Sn2+. (2) The optical band gap of amor phous SnO2-x becomes smaller with the formation of Sn2+ than that of S nO2. (3) The non-linear susceptibility becomes large with narrowing of the band gap, as predicted by the quantum mechanical formula of the t hird-order optical susceptibility.