Propagation of femtosecond light pulses through near-field optical aperture probes

The propagation of femtosecond light pulses through near-field optical fiber tips has been modelled numerically in three spatial dimensions by means of the finite integration technique. Ideally conducting as well as real metallic coatings of the tip have been considered, and the influence of surface plasmon polaritons (SPP) on shape, spectrum, and amplitude of the light pulse in the near and far fields of the tip have been investigated in this way. Special attention has been devoted to the superluminal tunneling of light through the fiber tip. The variation of phase and group velocities along the fiber axis has been characterized for a number of real metals and for different tip angles. A maximum of both velocities in the near field of the tip is characteristic for coatings of finite conductivity. For some tip angles negative values of the phase and/or group velocities are observed, which are caused by the propagation of SPP on the outer surface of the coating and their conversion into photons. It is shown, that the excitation of SPP on the metallic coating leads to strongly altered spatial emission characteristics of the tip.