Influences of titania concentration on the structure and the three-order nonlinear optical properties of core-shell Ag/TiO2 nanocomposites

Core-shell Ag/TiO2 nanocomposites with different TiO2 concentration were synthesized by the photoreduction method, in the presence of ethanol. The diameter of the nanoparticles, with various TiO2 and Ag content, was determined to be 20 similar to 60 nm using a transmission electron microscope (TEM), and the crystal lattice fringes of silver particles were observed. The prepared nanocomposites were characterized by ultraviolet-visible (UV-Vis) spectroscopy and X-ray diffraction(XRD). The intensity and width of the characteristic peak near 420 nm of the Ag nanoparticles increased as the TiO2 or silver concentration was increased. The nonlinear optical properties of thin films of nanocomposites with different TiO2 Concentrations were analyzed, using Z-scan technique at the wavelength of 790 nm with a pulse width of 130 fs. Two different nonlinear regimes in nonlinear absorption were found by changing the content of TiO2: an optical limiting effect at a low concentration, and a sign change at a higher concentration.