Size and gap distance dependances of non-linear optical polarization properties of gold dimers
Résumé
Metallic nanoantennas with sub-wavelength dimensions exhibit usually strong plasmon resonances, which influence their optical properties dominantly. We have investigated the linear and non-linear optical effects from various plasmonic systems like nanoparticles, sharp tips and nanotriangles with either confocal optical microscopy or scanning near-field optical microscopy (SNOM). The linear optical properties of dimer antennas consisting of two Au nanospheres with a small gap in between are very commonly investigated. They are known to exhibit strong near field inside a small gap between the two plasmonically coupled single nanoobjects. Based on the gap-field generated by the plasmonic coupling, nonlinear effects like second harmonic generation (SHG) and two photon photoluminescence (2PPL) can be investigated. We choose arrays of Au homo- and hetero-dimers to study the influences of particle size and gap distance on the nonlinear polarisation properties of these dimer structures. Especially the angle resolved 2PPL and the SHG signals were measured and their different dependences on the electromagnetic near-field were discussed. In our recent study, we reported to use SNOM tips studying the plasmonic coupling between a sharp tip apex and the different positions of one single Au nanotriangle. We presented SHG and 2PPL spectroscopy and microscopy in this Au-Au gap mode with sub 30 nm optical resolution. To achieve a more precise and flexible control of the gap distance in the Au-Au dimer antenna especially at small distances of less than 5 nm, we will adopt a similar SNOM configuration where a sharp Au tip is positioned in the close proximity of a Au surface via sensitive shear-force feedback. By switching between the radial and azimuthal polarized doughnut modes, we will compare the dependence of the SHG and 2PPL intensity on the local near-field in the Au tip – Au substrate that is less than 5 nm wide.