Optical Nonlinearity of Sputtered Co₃O₄-SiO₂-TiO₂ Thin Films

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 ×10⁻¹¹ 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.