Suppression of metal grating to surface plasma radiation

Abstract

Surface plasmon polaritons (SPP) are widely investigated in many fields because of the surface confinement of their electrocmagnetic field. Grating coupling is one of the methods to achieve the momentum match between light in free space and the surface plasmon to excite SPP. Because of the nature of the grating coupling, its parameters will greatly affect the coupling efficiency. Varying the grating modulation depth but keeping other parameters unchanged, we investigate the reflection spectra of onedimensional rectangle metallic grating by rigorous coupled-wave theory under the irradiation of incident light of 780 and 1500 nm in wavelength, respectively. According to Fano theory, the reflectance of metallic grating is the result of interference of two components, i.e., a directly reflected mode from the metal surface and a resonance radiation mode coupled out by the SPP propagating along the grating surface. We derive the Fano-type expression to describe the reflection spectra, and explain the contributions of directly reflected mode, SPP resonance radiation mode and the interference between these two effects. Near-filed electromagnetic distribution on metallic grating surface proves that the Fano-type expression is accurate enough to reflect the nature of the interference between the direct and radiation modes. Most importantly, our results from the expressions suggest that in some special grating condition, the metallic grating almost completely suppresses the SPP radiation propagating from grating to free space, which means that the energy of light can be completely trapped inside the grating. The phenomenon can be employed in designing light trapping device.