Abstract

Small metal particles embedded in dielectrics have been widely studied as resonant type third-order nonlinear optical materials for photonic devices. However, nonlinear optical properties of these materials are not well understood in the femtosecond time region.¹ We have investigated the third-order nonlinear optical susceptibility, χ⁽³⁾, and response time, τ, of various types of metal/dielectric composite thin films such as Au/SiO₂, Au/Al₂O₃, Au/TiO₂, Ag/SiO₂, Cu/SiO₂ films by means of femtosecond Z-scan (130 fs) and pump-probe experiments (100 fs). The films with high concentration of metal particle (~27 at%) were successfully prepared by the multitarget sputtering system. The diameter of the metal particle was controlled in the region of 3-14 nm by the heat treatment of the films. Figure 1 shows the optical-absorption spectra measured at room temperature for the Au/SiO₂ Ag/SiO₂ Cu/SiO₂ films. The absorption peaks due to the surface plasmon resonance of Au, Ag and Cu particles are clearly observed at 525, 406 and 582 nm, respectively. In the case of Au/dielectric films, the absorption peak is shifted to longer wavelength and the imaginary part of χ⁽³⁾, χIm⁽³⁾, increases with increasing the index of refraction of the dielectric matrices. The χIm⁽³⁾ for the Au/Al₂O₃ film was estimated to be −8.1 x 10⁻⁸ esu which was one order of magnitude smaller than that of the χ⁽³⁾ measured by the degenerated four wave mixing with 20 ns pulses.² Figure 2 shows the temporal evolution of the pump-probe signal for different pumping energy intensity for the Au/Al₂O₃ film. The decay curves show biexponential behaviors with fast, τ fast and slow, τ slow, components. The τ fast was calculated to be about 1 ps. In the case of the Au/Al₂O₃ film, the ratio of τ slow is small in comparison with that of other films with different dielectrics. This is probably due to the high thermal conductivity of Al₂O₃.

© 1998 IEEE