Near-field analysis of the transmission properties of subwavelength periodic H-shaped arrays in thin metal film

We theoretically investigated the transmission properties of an H-shaped periodic array built on a metallic thin film by combining the transmission spectra with the near-field amplitude and phase distribution analysis. Because of the geometric anisotropy of the H-shaped structure, the structure exhibits very different transmission behaviors for x-polarization and y-polarization incident light, in particular, the number, position, lineshape and broadness of the resonant peaks. In order to understand the root of these different transmission behaviors for the two polarizations, the characteristics of the transmission near-field, both the amplitude and phase distribution behind the metal structure, are analyzed in detail. At the transmission peaks, the high field amplitude domains always have a homogeneous phase distribution. Both the global and local surface plasmon resonance (SPR) effect can play an active role in shaping the transmission behaviors, and their existence can be uncovered by the optical near-field patterns at the transmission peaks. Whether the global or local SPR effect plays a dominant role strongly depends on the incident light polarization. Even for the same polarization state, the situation can still be very different for different resonant peaks. In some special cases these two resonance mechanisms can act collectively and result in a very sharp SPR peak in the transmission spectrum, which can find potential applications in sensitive optical sensors. Our studies clearly indicate the importance of optical near-field analyses in revealing the major physical mechanisms underlying light interaction with complex metallic nanostructures.