Plasmon enhanced Second Harmonic Generation in mixed semiconductor/metal nanoparticles

VON BILDERLING C.; BRAGAS A.V.

Buenos Aires, Argentina

Conferencia; NFO10: “10th International Conference on Near-field Optics, Nanophotonics and related techniques”; 2008

We present measurements of the optical second-harmonic generation (SHG) in colloidal semiconductor quantum dots (QDs), metal nanoparticles (NPs) and mixed QDs-NPs systems. The experiments are performed in transmission, using a tunable modelocked Ti:Sa laser (50 fs pulse width, 400 mW average power, 80MHz repetition rate, 780-800 nm). Samples were prepared by evaporating a drop of the particles in solution, onto a glass substrate. For the mixed QDs-NPs samples we first dried a drop of the metal NPs, and then we deposited a drop of CdS QDs on top of them. The average diameter of the CdS QDs is 3nm, as it is shown by the absorption spectrum and the AFM characterization. We used silver and gold NPs, with 12nm and 80nm of average diameter, respectively. Due to the concentrations used, SEM and AFM studies reveal that metal NPs are isolated and homogeneously distributed on the substrate, while the CdS QDs cover densely the entire sample. We did not detect second harmonic (SH) signal for any of the centrosymetric NP samples, and obtained a small SH peak, raising among the two-photon fluorescence spectrum (fig.1A), for the QDs (non-centrosymetric). Conversely, we observed a strong SH response for the mixed QDs-NPs systems, enhanced by more than one order of magnitude respect to the QDs signal (fig. 1B). We propose that the QDs-AgNPs signal is enhanced because the Ag-NP plasmon is in resonance with the second harmonic (fig1C). On the other hand, for the QDs-Au NPs we found that the enhancement is due to the resonant excitation of the Au plasmon (fig. 1C) with an external 532 nm light, which is collinear with the Ti:Sa (coming from the Ti:Sa pump). In figure 1D we show the dependence of the SH signal on the 532nm intensity for both QDs-Au and QDs-Ag NPs samples. For QDs-Au NPs we see that SH signal depends strongly on this external light, and falls to the noise level when we turn it off, meanwhile for QDs-Ag NPs the SH keeps on constant. Other authors reported SHG in metallic [1] and semiconductor [2-4] particles, but this is, in our knowledge, the first report of SHG on semiconductor-metal interacting systems.