Tuning Nonlinear Radiation from Plasmonic Nanoantennas via Controlled Symmetry Breaking: From Passive to Active Modulation

Implementing nonlinear optical processes such as harmonic generation or photoelectron emission in nanoscale devices allows to harness these effects for integrated photonic circuits, single-molecule sensing, or microscopy. The required intense optical fields can be realized in metal nanostructures such as plasmonic antennas. Second harmonic generation (SHG), both from a fundamental and application-based point of view one of the most important nonlinear effects, is, however, suppressed in the bulk of plasmonic systems, as plasmonic metals commonly exhibit a centrosymmetric crystal lattice. At the nanoscale, these symmetry considerations do not only concern the crystal lattice of the base material, but also the device geometry and surface as well as the surrounding medium, all of which can be tailored to tune SH radiation in plasmonic devices. In this thesis, two different schemes to modulate the SH radiation from plasmonic nanoantennas are investigated. First, in a passive modulation scheme, the antenna gap geometry is modified to generate an asymmetric field distribution, which results in the efficient emission of SH radiation. To this end, an advanced nanofabrication procedure combining gallium and helium focused ion beam milling of mono-crystalline gold microplatelets is established, reaching single-nanometer accuracy with high reproducibility. Second, an active modulation scheme is presented, which relies on tuning the surface charge density and thus the optical properties of the antenna by applying an external bias voltage. A voltage-controllable enhancement of the SH signal is demonstrated. Measurements of SH radiation patterns and the description of the system in an analytical model taking into account the perturbative effect of the voltage allow to assess the involved modifications of the SH polarization. The results in this thesis shed new light onto asymmetry-induced SH radiation and provide schemes to tailor nonlinear effects from plasmonic nanostructures.