Avi Niv
Ben-Gurion University of the Negev
Avi Niv works at the solar energy center of the Institutes for desert studies of the Ben-Gurion University of The Negev (Israel). He specialized in optics and the physics of solar cells.
Introduction: Recent years saw the agglomeration of nonlinear optics (NLO) and nano-optics yielding new possibilities for nonlinear light matter interactions. Nonetheless the basic understanding of NLO has not changed since it was first introduced. We investigate the SHG from a metal nano-dimer where, unlike former accounts, dimer is made from two different metals. We show that the resulting SHG cannot be accounted for by existing theories if the dimer is made small enough. Encourage by this result we propose a new mechanism, beyond what is so far known, to explain our unique observations.
Methods: Samples are made by drying water immersed gold nanoparticles (GNPs) on a silver covered by SiO2 separation layer. The dimer is obtained from the charges of these GNPs and their images within the silver. Samples are illuminated with a Ti:Sapphire laser, and the reflection is analyzed for SHG as shown in Fig. 1. The symmetry-breaking, which is essential for SHG, is obtained from the optical properties of the different metals, while its amount is controlled by the SiO2 layer thickness - 5 nm for maximum interaction and up to70 nm for disconnected sphere/surface.
Results: Figure 2 shows a scanned SHG images for 680 nm excitation of 5 nm GNPs for different SiO2 layers. Dark regions indicate clustering of GNPs, while bright areas are sparse GNPs monolayers where data is collected from.Variations in SHG versus SiO2 layer thickness are readily shown.
Discussion: Figure 3 shows the SHG of the GNPs nano-dimer in red. Note a maximum for 10 nm SiO2 layer - an example of observation beyond existing theories. The prediction of a coupled harmonic-oscillator model with nonlinear Coulomb interaction is shown in blue. Good agreement indicates that such mechanism might be responsible for this unexpected result.
Summary: It is shown that nonlinear optics may hide yet unknown sources. Good agreement with a model gives rise to new possibilities for SHG as well as for NLO in general.
Photonic & plasmonic nanomaterials , Optical properties of nanostructures , Nonlinear nano-optics
