Resonant absorption of electromagnetic fields by surface plasmons buried in a multilayered plasmonic nanostructure

Abstract

The optical reflectivity of a metal-dielectric-metal microcavity in which the upper layer is periodically perforated by narrow slits is explored. Complete characterization of the observed modes in terms of their resonant electromagnetic fields is achieved by comparison of the experimental data to the predictions of a finite-element model. In particular, we demonstrate that the slits provide efficient diffractive coupling to a surface plasmon mode buried within the microcavity whose propagation is strongly confined to the dielectric layer. DOI: 10.1103/PhysRevB.74.073408 PACS number͑s͒: 78.20.Ci, 42.25.Bs, 42.79.Dj, 73.20.Mf The texturing of metal surfaces on a subwavelength scale is known to strongly modify their electromagnetic ͑EM͒ response. Excitation of surface plasmons ͑SPs͒ at these interfaces plays a crucial role in light-matter interactions ͑see Refs. 1 and 2, and references therein͒. One of the reasons why the field of plasmonics has received so much attention in recent years is the localization of EM fields associated with these modes, together with their ability to guide energy along interfaces. In addition, metallic microcavity structures have also been explored to provide enhanced performance in a range of optoelectronic devices ͑e.g., light-emitting diodes 3 ͒. Furthermore, light-guiding structures that allow subwavelength confinement of the optical mode are critical for the achievement of compact photonic components.4,5 The design and fabrication of metallic nanostructures combining surface wave properties with microcavity resonant behavior opens up substantial new device potential and has already seen application as ultrathin electromagnetic attenuators in the microwave regime. 6 In this paper, we study the visible EM response of a metal-dielectric-metal microcavity where the illuminated metal film is perforated with periodically spaced identical slits ͓Fig. 1͑a͔͒. Both the width of these slits and the thickness of the dielectric core are ϳ /10 ͑where is the incident wavelength͒, the latter being small enough to permit coupling between SPs of the two metal surfaces. The excitation of these coupled-SP modes 7 using radiation polarized orthogonal to the slits results in resonant absorption of the incident power via Joule heating of the metal. In addition, it is found that single-surface SPs are excited at the illuminated face of the slit array. By recording the positions of the resonances in the reflectivity spectra as a function of the angle of incidence, the band structure of the modes supported is elaborated. The predictions of a finite-element method ͑FEM͒ model 8 are then fitted to the experimental data with good agreement being obtained. Each of the modes supported by the structure is identified by studying the numerically predicted EM field distributions.To fabricate the sample, a Ag/ SiO x / Ag trilayer structure was deposited by vacuum evaporation onto a clean glass slide. The first silver layer is optically thick ͑Ͼ100 nm͒, with the thickness of the SiO x dielectric core ͑t c ͒ and up...

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