Multipolar effects and strong coupling in hybrid plasmonic metamaterials Arash Farhang 1 , Anantha Ramakrishna 2 , Olivier J.F. Martin 1 1 Nanophotonics and Metrology Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), 1015 Lausanne, Switzerland; 2 Metamaterials and Plasmonics Laboratory, Department of Physics, Indian Institute of Technology Kanpur, 208016 Kanpur, India ABSTRACT Recently stacked metamaterial structures coupled to a conductive plane have been investigated and have been shown to exhibit the same properties as stacked structures with double the layers, due to dipole mirror coupling. Here we study a system of stacked subwavelength metallic grating layers coupled to a metal film and show that this system not only supports the localized modes of a doubly layered structure, but also, for non-normal incidence, supports modes that exhibit a clear propagation and in one case a simultaneous localization of the electromagnetic field in the region between the metal film and the first grating layer. Furthermore we show that this hybridized propagating mode, excited for any N number of periodic layers, is further influenced as it couples with the highest energy localized mode of the periodic layered stack. Additionally it is found that the localized modes of the structure can be spectrally positioned in a directly adjacent manner, resulting in wideband absorption that can effectively be tuned by varying the grating film spacing. Keywords: Localized, delocalized, surface plasmon, plasmonic, multilayer, grating, film, propagating, hybrid, hybridization, absorption. 1. INTRODUCTION Plasmonics, the bridge between the best of optics and electronics, is based on resonant electron plasma oscillations in metallic nanostructures 1, 2 . There are two types of plasmon resonances, localized plasmons and propagating plasmons 3 . The former, confined around nanoparticles, nanoantennas, and various other compact nanostructures, allows for very high near-field enhancements at optical frequencies due to high localizations of the scattered field and has been shown useful in applications ranging from trapping and sensing 4-6 to cancer treatment 7, 8 . The latter, based on the free propagation of surface waves at an extended metal dielectric interface, has been key to the development of surface plasmon based biosensors 9-12 and shows great promise in the development of ultra-fast and compact optical circuitry 13-17 . Metamaterials composed of subwavelength structures are of great interest since they can be tuned to exhibit normally unattainable electromagnetic properties. At optical frequencies, artificial structures such as periodic metallic gratings, nanodots, and splitring resonators have been investigated. A combination of localized plasmonic structures, especially 2D arrays, stacked to give a 3D geometry 18, 19 , has allowed for the progressive development of optical metamaterials 20-30 , which come in as key components for the development of many new plasmonic devices. Stacked composites of such structures have also been proposed and investigated for additional tunability of spectral properties. Branching out even farther we have systems of such stacked structures above a conductive plane 31 . Although these have not been well investigated thus far, studies till this point have shown that a conductive plane in very close proximity to a multilayer acts as a mirror and thus doubles the number of layers 31 . In the case of optical metamaterials, the metallic nanostructures support localized plasmon resonances while a continuous conductive plane such as a metal film supports propagating plasmon resonances. Here we take the simplest optical metamaterial structure, an array of subwavelength Au gratings, and explore its response when up to four layers are stacked directly above a Au film. As shown previously in the case of two cut-wire layers 31 , use of the plasmon hybridization model 32, 33 suggests the existence of 2n number of dipolar modes, which we will refer to as dipole-dipole modes, for n number of layers above a conductive plane. If we apply the rules of mirror coupling, however, only n number of dipole-dipole modes can exist. In addition to verifying this claim and explaining some minor digressions, we also examine as a function of both angle of incidence and wavelength, the effects of near-field coupling between multiple dipole-dipole plasmon modes and Photonic and Phononic Properties of Engineered Nanostructures II, edited by Ali Adibi, Shawn-Yu Lin, Axel Scherer, Proc. of SPIE Vol. 8269, 82691B · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.908923 Proc. of SPIE Vol. 8269 82691B-1