Abstract
We report an extraordinary transmission (EOT) of gold-capped silica microsphere monolayers in the mid-infrared range of 5-25 μm. The observed transmittance is significantly greater than that of a flat gold film with the same thickness, although the surface of the microsphere monolayer appeared to be completely covered with gold when observing from above. The calculations based on the finite difference time domain method indicate that light passes through the openings between the gold coating on the substrate and that on the microspheres. The EOT-type studied here occurs over the mid-infrared wavelength range, thus indicating that it is not attributable to the resonance of the surface plasmons. This type of EOT is absent in the visible and near infrared wavelength range, where gold does not function as an ideal metal. In addition, spectral modification originating from localized phonon polariton resonance (LPhPR) in silica microsphere is observed. LPhPR can be interpreted based on the analogy of the localized surface plasmon resonance in metallic nanospheres, in the visible or ultraviolet wavelength range.
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