  We demonstrate third harmonic generation in plasmonic antennas made of highly
doped germanium and designed to be resonant in the mid infrared. Owing to the
near-field enhancement, the result is an ultrafast, sub-diffraction, coherent
light source tunable between 3 and 5 micrometer wavelength on a silicon
substrate. To observe nonlinearity in this challenging spectral region, a
high-power femtosecond laser system equipped with parametric frequency
conversion in combination with an all-reflective confocal microscope setup is
employed. We show 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 the 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.
