Enhancement of the optical Kerr effect exhibited by an integrated configuration of silicon quantum dots and silver nanoparticles

Abstract

We present nonlinear refractive results for three different systems produced by ion implantation: high purity silica substrates with silicon quantum dots (Si-QDs), silver nanoparticles (Ag-NPs), and one sample containing both. We used a femtosecond optical Kerr gate (OKG) with 80 fs pulses at 830 nm to investigate the magnitude and response time of their nonlinear response. The Ag-NPs samples were prepared implanting 2 MeV Ag2+ ions at different fluencies. A sample with 1×1017 ions/cm2 showed no discernible Kerr signal, while for one with 2.4×1017 ions/cm2 we measured |χ(3)|1111 = 5.1×10-11 esu. The Si-QDs sample required irradiation with 1.5 MeV Si2+ ions, at a 2.5×1017 ions/cm2 fluence in order that the OKG results for this sample yielded a similar |χ(3)|1111 value. The sample containing the Si-QDs was then irradiated by 1 MeV Ag2+ ions at a 4.44 × 1016 ions/cm2 fluence and thermally treated, for which afterward we measured |χ(3)|1111 = 1.7×10−10 esu. In all cases the response time was quasi-instantaneous. These results imply that the inclusion of Ag-NPs at low fluence, enhances the nonlinearity of the composite by a factor of around three, and that this is purely electronic in nature. Pump-probe results show that there is not any nonlinear absorption present. We estimate that the confinement effect of the Si-QDs in the sample plays an important role for the excitation of the Surface Plasmon Resonance (SPR) related to the Ag-NPs. A theoretical model that describes the modification of the third order nonlinearity is also presented.