Linear and non-linear optical properties of bi-metallic heterodimers
Optical Nanospectroscopy III, the third annual conference of Cost Action MP1302, Rome, Italy, 22-25 March 2016
Metal nanoparticles have attracted great scientific and technological interest in many application fields, mainly due to their ability to sustain Localized Surface Plasmon Polaritons (LSPP). Linear optical properties and LSPP resonances of metallic nanoparticles or nano-antennas have been well described, since decades, in the literature by extinction and/or scattering spectroscopies and by radiation patterns in direct or in Fourier space. However, much less is known on their non-linear optical properties, and on the role of LSPP resonances in the enhancement of these effects. Two-photon photoluminescence (TPPL) and second harmonic generation (SHG) are both prominent examples of nonlinear effects observed on metallic nanoparticles. Compared to the TPPL process, a third-order process which intrinsically relies on a high local optical field, SHG additionally requires materials with a high second order susceptibility and an inversion symmetry breaking. Such selection rules have to be considered in addition to the ‘plasmonic molecular hybridization’ model, in order to understand the SHG and the TPPL efficiencies in metallic dimers. In this study, we focused on the linear and nonlinear optical characterization of Au-Al heterodimers (Fig1). These metallic dimers have been fabricated by a two-step electron-beam lithography process where the diameters of the Au and Al nanoparticles have been independently varied, as well as the gap distance. The linear optical properties (single dimer scattering spectra) and the non-linear optical properties (SHG and TPPL spectra) have been compared to understand the main physical phenomena involved.