Potential-dependent Second-Harmonic Generation in Optical Antennas measured Time-resolved

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Second-harmonic generation (SHG) is a nonlinear optical effect, with promising applications in background-free spectroscopy, ultrafast optical switching, and optical information processing. What is missing is the integration of these effects at the nanoscale, to be sensitive down to the singe molecule level and competitive in size with silicon electronics. Metallic nanoparticles, called optical antennas (OAs), show resonances in the infrared and optical wavelength regime. They exhibit plasmons, coupled states of photons and electron density waves, which allow concentrating light much better than conventional optics, down to 10 nm³. OAs enable SHG in nanometer-sized volumes, but until now only with low efficiency. This is due to the complex task of: (i) a resonance to receive light with wavelength A, (ii) routing the energy to the OA surface (efficient SHG due to symmetry breaking), (iii) exploiting a second antenna resonance which can gather the SHG at wavelength B=A/2, and finally (iv) emitting the wavelength B to the far field. In the proposed project PoSHGOAT I will control and optimize SHG in OAs. To this aim, I will introduce four novelties to SHG research: (1) Fabrication of electrically-contacted nanoantennas with ultra-fine tips (r = 3 nm) and ultra-narrow gaps (g = 3 nm) by a subsequent Ga-ion and He-ion milling procedure. (2) Modulation of the surface charges in metallic nanoparticles by applying an external potential. (3) Time-resolved pump-probe spectroscopy of SHG with an applied voltage, eventually even with induced tunnelling of electrons in a highly asymmetric antenna gap. This will establish ultrafast control over the OA surface charge density and, thus, SHG. (4) Numerical modelling of SHG and evolutionary optimization of nanoparticle geometries to maximize SHG in optical antennas. All these efforts together will increase our understanding of nonlinear processes in plasmonic resonators, towards novel design rules for nonlinear plasmonic devices.