  Terahertz fields can be dramatically enhanced as they propagate through
nanometer-sized slits. The enhancement is mediated by significant accumulation
of the induced surface charges on the surrounding metal. This enhancement is
shown here to be dynamically modulated and the nanoslits are gradually shunted
using a copropagating optical beam. The terahertz fields are found to skirt the
nanoscale photo-excited region underneath the slits, scattering to the far
field and rigorously mapping the near field.
