On long-range plasmonic modes in metallic gaps.

Satuby and Orenstein [Opt. Express 15, 4247-4252 (2007)] reported the discovery and numerical and experimental investigation of long-range surface plasmon-polariton eigenmodes guided by wide (6 to 12 mum) rectangular gaps in 400 nm thick gold films using excitation of vacuum wavelength lambda(vac) = 1.55 mum. In this paper, we carry out a detailed numerical analysis of the two different types of plasmonic modes in these structures. We show that no long-range eigenmodes exists for these gap plasmon waveguides, and that the reported "modes" are likely to be beams of bulk waves and surface plasmons, rather than guided modes of the considered structures.