| In this thesis, light transmission through a single sub-wavelength slit in a silver thin film with periodic gratings at the exit side of the slit has been studied in both the visible and mid-infrared wavelength ranges.In the visible wavelength range, we first investigate the enhancement of plasmonic light beaming efficiency by near field resonance in a subwavelength metallic slit-groove beaming structure. We show that by varying the film thickness and the separation distance of the beaming grating, the intensity of the near field can be greatly enhanced by resonance, which leads to the increase of the beaming efficiency. Moreover, we obtain a nanocavity above the nanoslit by integrating a metal nanostrip with the beaming structure. The resonance of this nanocavity can further enhance the near field intensity and improve the beaming efficiency.We also show that light beaming in mid-infrared is mainly controlled by "spoof" surface plasmons. We take account of both the dispersion relation and the phase-matching condition of "spoof" surface plasmons to obtain good beaming efficiency. Through engineering the geometry of the grating structures, we can achieve the desired near field and far field distribution in mid-infrared. |
