  Localized surface plasmon polaritons in conducting structures give rise to
enhancements of electromagnetic local fields and extinction efficiencies.
Resonant conducting structures are conventionally fabricated with a fixed
geometry that determines their plasmonic response. Here, we challenge this
conventional approach by demonstrating the photo-generation of plasmonic
materials (THz plasmonic antennas) on a flat semiconductor layer by the
structured optical illumination through a spatial light modulator. Free charge
carriers are photo-excited only on selected areas, which enables the definition
of different plasmonic antennas on the same sample by simply changing the
illumination pattern, thus without the need of physically structuring the
sample. These results open a wide range of possibilities for an all-optical
spatial and temporal control of resonances on plasmonic surfaces and the
concomitant control of THz extinction and local field enhancements.
