Optical transmission properties of a single subwavelength aperture in a real metal

A subwavelength aperture is a key element in near-field optical devices and many recent photonic structures. When the lateral dimensions of such aperture is smaller than half the wavelength, light cannot propagate through the hole and the transmission is typically very weak. It is usually believed to scale as the fourth power of the aperture diameter, a result first stated by Bethe [Phys. Rev. 66 (1944) 163] in 1944 by analyzing the transmission through a small hole in an infinitely thin perfectly metal screen. However, a real subwavelength aperture is very different because the thickness and the finite conductivity of the metal has significant consequences which are far from being well understood. Here we report that light impinging on isolated subwavelength holes in real metal film, in this case Ag, excite localized surface plasmon modes on the aperture ridge. Their activation gives rise to optical tunneling with unexpected enhanced transmission peaks and directionality. These properties follow from the dipolar nature of the LSP modes and can be tuned by an appropriate design of the aperture shape. These findings are of relevance for the current trends in subwavelength Optics. (C) 2004 Elsevier B.V. All rights reserved.