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
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