1 Nonlinear upconversion enhancement from a single semiconductor nanoparticle coupled to a plasmonic antenna Heykel Aouani 1†* , Mohsen Rahmani 1† , Miguel Navarro-Cía 2 and Stefan A. Maier 1 1 The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom, 2 Optical and Semiconductor Devices Group, Department of Electrical and Electronic Engineering, Imperial College London, London SW7 2BT, United Kingdom, † These authors contributed equally to this work * e-mail: h.aouani@imperial.ac.uk The ability to convert low energy quanta into a quantum of higher energy is of great interest for a variety of applications such as bioimaging 1,2 and photovoltaic light harvesting 3,4 . Although phase matching processes enable achieving high nonlinear conversion efficiency rates through macroscopic nonlinear crystals, the characteristic dimensions of subwavelength materials prevent exploiting such coherent enhancement processeses 5 . Generating high nonlinear effects at the nanometric scale is essential for the future development of on-chip frequency conversion, as well as for ultrafast switching and modulation of optical signals 6 . Here, we demonstrate that the third harmonic generation from a single indium tin oxide (ITO) nanoparticle can be dramatically increased after being decorated with a plasmonic gold dimer, thus enabling to overcome the strong intrinsic nonlinear signal from the metal itself. Such a hybrid nanodevice provides visible third harmonic generation enhancements up to 10 6 fold compared to an isolated ITO nanoparticle, which leads to effective third order susceptibilities up to 3543 nm 2 /V 2 and conversion efficiencies of 0.0007%. Reciprocally, the enhanced third harmonic signal can be exploited to probe the local excitation intensity at the gap of plasmonic dimers, thus introducing new routes for the near-field characterization of nanoscale elements by far-field detection techniques via nonlinear interactions of light.