Towards efficient nonlinear plasmonic nanosources
Topical Meeting on Nonlinear Plasmonics and its applications, in Optical Nanospectroscopy III, the third annual conference of Cost Action MP1302, Rome, Italy, 2016 March 21st
Metallic nanoantennas with sub‐wavelength dimensions exhibit usually strong plasmon resonances, which influence their optical properties dominantly. The linear optical properties of dimer antennas consisting of two gold nanoparticles with a small gap in between are very commonly investigated. They are known to exhibit strong electromagnetic hot spots inside a small gap between the two plasmonically coupled single nanoobjects. 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). Towards the achievement of efficient plasmonic nanosources based on nonlinear effects, we choose arrays of gold homo‐ and hetero‐dimers to study the influence of electromagnetic hot spots on the second harmonic generation (SHG) and two photon photoluminescence (2PPL). We show that the generation of both signals is differently influenced by changing the dimer gap distances and particle sizes. We demonstrate that the nonlinear optical signal intensity provides no direct information about the external fundamental near‐field distributions and intensities around plasmonic nanostructures, despite the fact that these two distributions are closely related to each other. We show that the SHG and 2PPL intensities as well as the spectral position of the 2PPL emission can be selectively tuned by changing the particle material and the dimer geometry. To investigate nonlinear optical signals from a narrow gap of less than 5 nm, we combine an ultrafast laser system with SNOM. Based on the sensitive shear‐force or tunnelling current feedback, we are able to control the gap distance accurately and to study the nonlinear optical processes in extremely narrow gaps that are difficult to be fabricated via conventional lithographical methods. Furthermore, using this combination the nonlinear optical signal generation from different positions of a plasmonic nanostructure can be resolved with an optical resolution of less than 30 nm.