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W.-C. Chen , N. I. Landy , K. Kempa , and W. J. Padilla * IO N The study of light coupling to small apertures in metallic fi lms has a long and illustrious history. In 1897 Lord Rayleigh treated the problem by using the concept of effective dipoles. Most recently, researchers have made these apertures periodic, where they exhibited so-called extraordinary optical transmission (EOT) – transmission effi ciencies far in excess of unity at wavelengths greater than the lattice parameter of the surface. This behavior is not predicted by standard aperture theory as developed by Bethe in 1944, [ 1 ] nor was it noticed before fabricational and measurement techniques had advanced to the point that EOT could be observed at optical wavelengths. However, in 1998 Ebbesen et al. [ 2 ] showed EOT in the near-infrared and posited that this stemmed from plasmon coupling between surfaces. Since then, EOT [ 2–5 ] has been studied with increasing detail in both theoretical and experimental capacities. This result led to a fl urry of intense theoretical and experimental research to probe the nature of this extraordinary optical transmission. Babinet metamaterials [ 6–8 ] may also be described as periodic apertures in metal fi lms. Metamaterials are structured periodic metallic patterns which enable the construction of materials with specifi ed electromagnetic properties, some of which cannot be obtained via naturally occurring materials. [ 9,10 ] Metamaterials exhibit electromagnetic resonances where the resonant wavelength is signifi cantly larger than the physical dimensions of the individual elements. Thus metamaterials are subwavelength media and well described by the optical constants e and μ . [ 11 ] In transmission metamaterials (scatterers) yield an absorptive like minimum feature at resonance, but are otherwise highly transmissive outside of this band. In contrast Babinet metamaterials (apertures)—made by taking the ‘inverse’ metamaterial structure—yield opposite behavior and exhibit a transmission maxima near resonance. [ 12,13 ]