  The generation of surface plasmon polaritons (SPP's) at isolated nanoholes in
100 nm thick Au films is studied using near-field scanning optical microscopy
(NSOM). Finite-difference time-domain calculations, some explicitly including a
model of the NSOM tip, are used to interpret the results. We find the holes act
as point-like sources of SPP's and demonstrate that interference between SPP's
and a directly transmitted wave allows for determination of the wavelength,
phase, and decay length of the SPP. The near-field intensity patterns can be
manipulated by varying the angle and polarization of the incident beam.
