Study of the optical properties of metallic nano-structured systems with the finite-difference time-domain method

Resumen

As everybody has experienced by looking at a mirror, light is almost completely reflected by metals. But they also exhibit an amazing property that is not so widely known: under some circumstances light can "flow" on a metallic surface as if it were "glued" to it. These "surface" waves are called Surface Plasmon Polaritons (SPPs) and they were discovered by Rufus Ritchie in the middle of the past century. Roughly speaking, SPP modes generate typically from the coupling between conduction electrons in metals and electromagnetic fields. Free electrons loose their energy as heat, which is the reason why SPP waves are completely absorbed (in the visible range after a few tens microns). These modes decay through so short lengths that they were considered a drawback, until a few years ago. Nowadays that situation has completely turned. Nano-technology now opens the door for using SPP-based devices for their potential in subwavelength optics, light generation, data storage, microscopy and bio-technology, giving rise to a new research field called "Plasmonics". The thesis covers four interesting topics on Plasmonics: i-"Extraordinary Optical Transmission" This phenomenon of Extraordinary Optical Transmission (EOT) is characterized by the appearance of unexpected peaks and dips in transmission spectra from metal films drilled with sub-wavelength holes. From the beginning, it was realized that the spectral locations of those resonant features coincide with the corresponding ones of SPPs. Nowadays,it is widely accepted that the excitation of those surface electromagnetic modes is at the origin of EOT. The EOT mechanism has sparked considerable interest for its fundamental implications and also from the applied point of view, as many potential applications based on this phenomenon have been proposed. We study how EOT depends on different parameters: the metal chosen, the shape of the holes and the film thickness. These works are focused on the optical response of two dimensional hole arrays (2DHAs) drilled on metal films, nevertheless another interesting system displaying EOT is also investigated, an array of dielectric microspheres covered with different metals. ii - "Theory of Negative-Refractive-Index response of double-fishnet structures" In 2005 Zhang and co-workers proposed and demonstrated a negative refractive index (NRI) metamaterial working at near infrared frequencies. This metamaterial is composed by a 2DHA penetrating completely in a metal-dielectric-metal film stack, the double-fishnet (DF) structure. Here, we present an alternative physical view to a well established circuitry model of the NRI in DF structures. iii - "Plasmonic devices" The emerging field of Plasmonics is based on exploiting the coupling between light and SPPs. Because of SPP modes are not constrained by the optical diffraction limit, it is hoped they could enable the construction of ultra-compact optical components. Among these components would have SPP sources and waveguides for sending information through, from one place to another on a hypothetical Plasmonic-chip. - In the corresponding chapter we present a novel back-side slit-illumination method which incorporates a periodic array of grooves carved into the front side of a thick metal film. Bragg reflection enhances the propagation of SPPs away from the array, enabling them to be unidirectionally launched from, and even focused to, a localized point. - A second topic is investigated, which relates with our ability to control light on a surface. There, we present a theoretical study on two different proposals for guiding EM fields, namely the modes supported by a carved triangular groove in metal (channel plasmon-polaritons, CPPs) and, in a way, the "complementary" structure, a triangular metal wedge (wedge plasmon-polaritons, WPPs). We present rigorous simulations of guided CPPs and WPPs aimed at elucidating their characteristics at telecom wavelengths, including full vectorial modes, dispersion, and losses. Additionally, we design two optical devices which performance rest on the results previously found. In this way, we propose a WPP to SPP geometry-driven conversion device. Besides, we demonstrate both theoretically and experimentally that "tapered" CPP supporting structures can enhance EM fields near the surface at a subwavelength scale. iv - "Optical field enhancement on arrays of gold nano-particles" Finally, we investigate the optical response (extinction spectroscopy and two-photon luminescence (TPL) emission) of regular arrays of rectangular gold nanoparticles deposited either on glass or on gold substrates.