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An experimental study is made of the enhanced optical transmission of nanostructured gold films in the midinfrared region. Results indicate that the excitation of surface plasmon polaritons due to periodicity plays a fundamental role in producing extraordinary optical transmission. The influence of the surrounding claddings, hole shape, and periodicity on the resonance wavelength and the quality factor is investigated. The aim is to use the subwavelength structures as ultracompact optical filters whose spectral features can be easily tuned and scaled. For filter design purposes, the results show that the main parameters affecting the resonance wavelength are the lattice constant and dielectric cladding. The hole shape and size are found to cause transmission enhancement and there is only a small resonance redshift when the hole area is increased. However, a lower quality factor is achieved when the hole area is increased. [ABSTRACT FROM AUTHOR]