  We map the complex electric fields associated with the scattering of surface
plasmon polaritons by single sub-wavelength holes of different sizes in thick
gold films. We identify and quantify the different modes associated with this
event, including a radial surface wave with an angularly isotropic amplitude.
This wave is shown to arise from the out-of-plane electric dipole induced in
the hole, and we quantify the corresponding polarizability, which is in
excellent agreement with electromagnetic theory. Time-resolved measurements
reveal a time-delay of 38? +/- 18 fs between the surface plasmon polariton and
the radial wave, which we attribute to the interaction with a broad hole
resonance.
