  Propagation of spoof surface plasmon polaritons (spoof SPPs) on comb-shaped
ultrathin metal strips made of aluminum foil and printed copper circuit are
studied experimentally and numerically. With a near field scanning technique,
electric field distributions on these metal strips are measured directly. The
dispersion curves of spoof SPPs are thus obtained by means of Fourier transform
of the field distributions in the real space for every frequency. Both
fundamental and second order modes are investigated and the measured
dispersions agree well with numerical ones calculated by the finite element
method. Such direct measurements of the near field characteristics provide
complete information of these spoof SPPs, enabling full exploitation of their
properties associated with the field confinement in a subwavelength scale.
