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Demonstrated is a new technique for observation of the surface plasmon polariton (SPP) generation in a tapered metal-coated tip (a SNOM tip) terminating a multimode optical fiber. The mode-filtering effect of a SNOM tip is used to virtually eliminate most of the fiber modes, retaining only two of them. We utilize the fact that SPPs in the tip propagate slower than pure photons, and the propagation speed actually depends on the coupling strength between photons and plasmons. This results in an optical path difference (OPD) for different photonic fiber modes having different coupling strength to plasmons. The OPD value for a pair of modes can be easily determined by analyzing the mesoscopic spectral modulation of the output light. The fiber length dependence of OPD is studied to take into account the fiber’s inherent modal dispersion. The modal dispersion is shown to change its sign in the SNOM tip.

plasmonics; nanophotonics