  Impulsive interband excitation with femtosecond near-infrared pulses
establishes a plasma response in intrinsic germanium structures fabricated on a
silicon substrate. This direct approach activates the plasmonic resonance of
the Ge structures and enables their use as optical antennas up to the
mid-infrared spectral range. The optical switching lasts for hundreds of
picoseconds until charge recombination red-shifts the plasma frequency. The
full behavior of the structures is modeled by the electrodynamic response
established by an electron-hole plasma in a regular array of antennas.
