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Optical Nonlinearity of Sputtered Co3O4-SiO2-TiO2 Thin Films Hiroki Yamamoto, Takashi Naito1 and Kazuyuki Hirao2 Nanotechnology Glass Project, New Glass Forum, Tsukuba Research Laboratory, Tsukuba Research Consortium, 9-9, 5-Chome, Tokodai, Tsukuba, Ibaraki, 300-2635 Japan 1 Hitachi Research Laboratory, Hitachi, Ltd., 1-1, 7-Chome, Omikacho, Hitachi, Ibaraki, 319-1292 Japan 2 Division of Material Chemistry, Graduate School of Engineering, Kyoto University Yoshida-honmachi, Sakyou-ku, Kyoto, 606-8501 Japan ABSTRACT Optical non-linearity of cobalt oxide with SiO2-TiO2 additives was investigated, and the change mechanism of the refractive index (n) and extinction coefficient (k), based on the relation between band structure and optical non-linearity of the thin films, was discussed. Refractive index and extinction coefficient of Co3O4 thin films in the ground state were 3.17 and 0.42, respectively. Both n and k decreased by irradiation from a pulse laser with 650 nm of wavelength (1.91eV). These values in the excited state were 2.91 and 0.41, respectively. n2 estimated from the change of n and k was –2.8 x10-11 m2/W. The film had a band gap corresponding to 2.06eV, indicating that it was widened by the band filling effect during the laser irradiation at 1.91eV, and this led to the decrease in absorption coefficient and refractive index. INTRODUCTION Spectral analyses [1-3], band structural analyses [4-6] and optical nonlinearity [7], of cobalt oxide thin films have been extensively reported. Recently, we have discovered that cobalt oxide thin films containing glass additives have remarkable optical nonlinearity, that is, their refractive index drastically changes with laser irradiation (λ=650nm, E=1.91eV) [8-11]. Therefore these films are suitable as super resolution films to reduce the spot size of the laser irradiated on optical recording media such as DVDs (digital versatile disks). Furthermore, we have reported the nano structure of some of these thin films and found they had nano-scale Co3O4 precipitated particles with an amorphous grain boundary phase of 1.0nm width [12]. Ando et al. [7] investigated the third order nonlinear optical susceptibility χ(3) of thin films of transition metal oxides including Co3O4 with a phase-conjugation-type degenerate four-wave mixing (DFWM) method. They reported that thin films of transition metal oxides had large χ(3) when 7ns and 35ps pulse lasers with a wavelength of 532nm were irradiated on them. Co3O4 showed significantly more nonlinearity than the other transition metal oxide films because of its high figure of merit, χ(3)/α. In the present study, refractive index and extinction coefficient of ground and excited states were measured by an ellipsometric method to investigate the mechanism of non-linearity of Co3O4-SiO2-TiO2 thin films. EXPERIMENTAL PROCEDURE Thin films of Co3O4 with SiO2 and TiO2 additives were obtained by RF magnetron sputtering at room temperature on borosilicate glass substrates. Composition of the film was V13.20.1

84Co3O4-8SiO2-8TiO2 in molar percentag