Imaging of surface plasmon polariton fields excited at a nanometer-scale slit

Nonlinear two-photon photoemission electron microscopy is used to image surface plasmon polariton (SPP) wave packets excited by an obliquely incident laser pulse (similar to 10 fs) at a single slit fabricated in a thin silver film. We image the forward propagating polarization grating formed by the coherent superposition of the external excitation pulse and the SPP wave packet fields. By systematically varying the coupling slit width from sub-to multiple-wavelength scale, we observe a modulated increase of the grating intensity, which is phenomenologically accounted for by distinct contributions to the forward coupling efficiency from the incident to the SPP waves. Full wave, vectorial finite-difference time-domain (FDTD) simulation of the experiments is in good agreement with the experimental observations and explains their origin. In particular, the FDTD simulation illustrates detailed spatial variation of the polarization grating as a function of the geometry of the slit under excitation by ultrafast laser pulses at an oblique incidence.