Coupling of surface plasmons in nanostructured metal/dielectric multilayers with subwavelength hole arrays

We demonstrate here that the enhancement of optical transmission originates not only from surface plasmons (SPs) but also from the coupling of SPs in the Ag/SiO2 multilayer with a periodic array of subwavelength holes. The multilayer film is constructed by repeating a building block, which contains two layers: one is a silver film with periodic array of subwavelength holes and the other is a film of SiO2. The multilayers were fabricated by magnetron sputtering, and the array of holes was milled with focused-ion-beam facility. The measured optical transmission properties reasonably agree with our numerical calculations. It is shown that the coupling of SPs strongly depends on the detailed structure, and in our system, the coupled SPs can be characterized by using an effective-permittivity model. In the sandwiched structure with nanostructured silver, the coupling of SPs leads to the shift of transmission peaks, while in a nanostructured Ag/SiO2 multilayer, the coupling of SPs yields a new resonant mode with increased quality factor of the transmission peak, which originates from multiple scatterings and the coupling of electromagnetic waves on the interfaces of the multilayers. These properties may be utilized to tune electromagnetic wave in subwavelength optics.