We report experimental results on the role of aperture shape in middle-infrared enhanced transmission through a metal film perforated with a periodic aperture array. The transmission spectrum is highly dependent on the aperture shape. When the aperture shape changes from circular to cross-dipole or the cross-dipole arm length increases, the transmission spectrum exhibits a large redshift. The enhanced transmission is proposed to be an interplay of surface plasmon polaritions at the metal surface and localized surface plasmons (LSPs) inside the apertures. The shift of the transmission bands is proposed to be due to the redshift of the LSP resonance spectrum of the individual apertures. The role of shape in enhanced transmission in the cascaded structure is also studied.
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