Optical nonlinearities have been studied for Cu nanoparticle composites in an insulator with a high dielectric constant. Negative Cu ion implantation with the energy of 60 keV is applied to synthesize Cu nanoparticles in SrTiO3. The surface plasmon peak on optical absorption shifts to red and is enhanced with the dielectric constant of matrix substrates. A numerical calculation of the local electric field inside a Cu nanoparticle suggests that optical nonlinearity of the Cu/SrTiO3 composite is enhanced twice as much as that of the Cu/SiO2 composite. Complex nonlinear optical constants were evaluated by the z-scan method with a tunable femtosecond laser system over a range of 2.0-2.3 eV Both the real and the imaginary parts of the constant of the Cu/SrTiO3 composite are negative in the range. The third-order optical susceptibility of the Cu/SrTiO3 composite exhibits values of - 1 to - 2 x 10(-9) esu at 2.03 eV where there is a slope of the surface plasmon resonance. (C) 2004 Elsevier B.V. All rights reserved.