Picosecond nonlinear optical properties of SrTiO₃ composite films doped with gold and nickel nanoparticles

Nanoparticles composite thin films formed by nanometer-sized gold and nickel particles embedded in SrTiO₃ matrices were fabricated on MgO single-crystal substrates by co-depositing the metal and ceramic targets using the pulsed laser deposition technique. The linear optical absorption properties were measured from 350 to 800 nm, and the absorption peak due to the surface plasmon resonance of Au metal particles was observed around 557 nm. The ultra-fast third-order nonlinear optical properties of the films were determined by a single-beam z-scan method at a wavelength of 532 nm with laser duration of 55 ps. The nonlinear refractive index n₂ and the nonlinear absorption coefficient β were determined, respectively, and the figure of merit χ⁽³⁾/α (with χ⁽³⁾ being the third-order nonlinear susceptibility and α the linear optical absorption coefficient) was discussed. Whether gold or nickel, metal particles have little effect on the figure of merit, but significantly affect the ratio of the real part to the imaginary part of χ⁽³⁾ [Reχ⁽³⁾/Imχ⁽³⁾]. The obtained Reχ⁽³⁾/Imχ⁽³⁾ of Au/SrTiO₃ is 1.43, which is more than 3 times as large as that of Ni/SrTiO₃.