We investigate optical properties of a gold structure comprising a two-dimensional array of gold nanoblocks placed on top of a thin gold film. We observe enhanced transmission through asymmetric nanostructured gold films, which can be attributed to the surface plasmon (SP) mode at the air-gold interface leaking to the substrate when the substrate index is larger than the superstrate index. When the substrate and superstrate are the same dielectric, the SPs at both superstrate-gold and gold-substrate interfaces strongly interact with each other and even and odd SPs are then excited. In addition, we investigate effects of oblique incidence and electronic interband transition on SP resonances. Our results provide a guideline for engineering novel devices with enhanced transmission based on nanostructures.
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