The paper presents a theoretical study on the excitation of surface plasmon polaritons (SPPs) and their decay by reradiation to light. We consider a free-standing metallic transmission grating being illuminated with a TM-polarized light pulse of 10 fs duration at normal incidence. The SPP decay time is assumed to be much larger than the pulse duration. In particular, we analyze the SPP decay after the exciting pulse disappeared. We find periodic amplitude modulation of the declining light field both in close proximity to the grating, where evanescent waves are dominant and in the far-field region, where the light field consists mainly of propagating waves. Using the example of the magnetic field, we demonstrate that the amplitude modulation of the near field exhibits increasing strength with time due to a drop of the evanescent wave density associated with the SPP decay. The far field above and below the grating shows frequency beating with periods corresponding to the reciprocal width of a frequency gap in the transmission spectrum. Strong coupling between SPP modes on the top and bottom interfaces leads to fixed phase relations between the beat notes of the transmitted and reflected light fields. This coupling is confirmed by a periodic change in the Poynting flow direction perpendicular to the interfaces. Finally, we study also the dynamics of the SPP decay by employing Gabor wavelet transforms for the calculated fields far above and below the grating. In this way, we get access to the spectral contents of the light field at different times. This novel spectral-temporal analysis shows a narrowing of the initial pulse spectrum and reveals spectral features not seen in the Fourier spectrum.

• Received 30 April 2010

DOI:https://doi.org/10.1103/PhysRevB.82.115408