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.