Nanohole chains for directional and localized surface plasmon excitation

Abstract

Arrangements of subwavelength sized holes in metal films are often used to launch surface plasmon polaritons (SPPs) onto metal-dielectric interfaces. They are readily fabricated and can also be used to generate a variety of near- and far-field intensity patterns. We use a short chain of equally spaced subwavelength sized holes to launch SPPs onto a gold-air interface in complex patterns of hotspots. With a phase-sensitive near-field microscope, we visualize the electric field of the excited SPPs. We observe self-images of the chain that we attribute to the Talbot effect. Far from the chain we observe the SPP diffraction orders. We find that when the spacing of the holes is of the order of the wavelength, the revivals do not occur on the well-known Talbot distance as derived in the paraxial limit. We present an alternative expression for the Talbot distance that does hold for these small spacings. We study the behavior of both the revivals and the diffraction orders as a function of the number of holes. We find that the Talbot revivals become more pronounced as the number of holes is increased, which is in accordance with numerical calculations. We anticipate that our findings are interesting for multiplexing sensor applications, where control over the local intensity of SPPs is crucial.