We examine the modal characteristics of coupled metallic nanoscale rectangular apertures using a simple and accurate separation of variables method, recently proposed by us for a single aperture [Opt. Lett. 33, 333 (2008)]. The study shows that the coupling between the antisymmetric surface plasmon polariton (SPP) modes of the two rectangular apertures results in a coupled SPP mode with very large propagation length, larger than 1 mm at the wavelength of 0.633 (mu)m, provided the aperture size and the separation are sufficiently large. The effective indices and the propagation lengths of various coupled SPP modes are presented.
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