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We demonstrate third harmonic generation in plasmonic antennas consisting of highly doped germanium grown on silicon substrates and designed to be resonant in the mid-infrared frequency range that is inaccessible with conventional nonlinear plasmonic materials.Owing to the near-field enhancement,the result is an ultrafast,subdiffraction,coherent light source with a wavelength tunable between 3 and 5 μm,and ideally overlapping with the fingerprint region of molecular vibrations.To observe the nonlinearity in this challenging spectral window,a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed.We demonstrate spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures.A clear third-order power dependence as well as mid-infrared emission spectra prove the nonlinear nature of the light emission.Simulations support the observed resonance length of the double-rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing.