Third-order non-linear optical susceptibilities, chi((3))(- 3 omega; o
mega, omega, omega), for spinel-type oxide thin films prepared by rf m
agnetron sputtering were evaluated using the third-harmonic generation
method in the off-resonant region. The chi((3))(- 3 omega; omega, ome
ga, omega) of MgAl2O4 was about 1.0 x 10(-12) esu and that of CdIn2O4
ranged from 1.8 to 4.5 x 10(-12) esu, which seemed to be relatively la
rge compared with other inorganic materials. The origin of the large c
hi((3))(- 3 omega; omega, omega, omega) was the spinel-type structure
(AB(2)O(4)). Since octahedra with the B-site element in the center are
linked by sharing an edge in space, it was expected that the delocali
zation of the s-orbital of the B-site metal would be large. The reason
why the chi((3))(- 3 omega; omega, omega, omega) of CdIn2O4 was sever
al times larger than that of MgAl2O4 was that the optical band gap of
CdIn2O4 was smaller than that of MgAl2O4.