Nonlinear optical properties of Si nanocrystals embedded in SiO2 prepared by a cosputtering method

Nonlinear optical properties of Si nanocrystals (Si-ncs) doped SiO2 prepared by a cosputtering method were studied by z-scan technique in a femtosecond regime at around 1.6 eV. The nonlinear refractive index (n2) and nonlinear absorption coefficient (β) were strongly enhanced compared to those of bulk Si and found to be about ∼2×10−13 cm2/W and ∼0.8 cm/GW, respectively. In the photon energy region from 1.48 to 1.65 eV, the n2 and β spectra followed the absorption spectra and no enhancement was observed in the band-edge photoluminescence region. In the diameter range of 2.7–5.4 nm, the size dependence of n2 coincided well with that calculated by a pseudopotential approach, suggesting that the discrete energy states of Si-ncs are responsible for the observed enhanced optical nonlinearity.