Extraordinary transmission through aluminum metal with superperiodic micro-cell arranged in a long-range periodic structure

Enhanced optical transmission through an Al film with perforated short-range 3times3, 4times4 and 5times5 hole arrays arranged in a long-range periodic structure is investigated. This three finite squared holes array constitutes an individual micro-cell, respectively. The measured transmission peak wavelength reveals that the surface plasmons are Fabry-Perot type resonance to different integer order. The ratio of periodicity between the super-periodic structure p and the NtimesN micro-cell d determines the integer order (where N is number of isolated hole; N=3, 4 and 5). Namely, when p/d>4, the order 4 and 5 surface plasmon modes with average hole spacing p/4 and p/5 are observed in 4times4 micro-cell array, respectively. The transmission intensities of order 4 mode are the strongest, and those of order 5 modes are barely seen. But as p/d approaches 5, the order 4 mode gradually disappear with the emergence of order 5 mode. The 3times3 and 5times5 micro-cell array have the same result. These phenomena suggest that the surface plasmons are Fabry-Perot type resonance within a super unit cell. The dispersion relations of surface plasmons are also observed.