Nanofabrication, simulation and optical characterization of plasmonic nanostructures

Abstract

This thesis is devoted to the nanofabrication, simulation and optical characterization of different plasmonic nanostructures.

When an electromagnetic wave reaches a metallic nanostructure, it can give rise to collective oscillations of the free electrons in the metal. These oscillations reach a maximum at the so-called surface plasmon resonance, whose intensity and frequency depend in the material, geometry, embedding medium, interparticle interactions, etc.

Based on the tunability of core-shell nanoparticles, hollow cylindrical gold nanostructures (nanocups) have been fabricating using a combination of nanoimprint lithography (NIL) and non-directional metallization. Besides, to overcome the high-aspect ratio limitations of NIL, a trilayer stack (resist-oxide­resist) has been used in such a way that the bottom resist layer, which controls the height of the nanostructure, is not affected by the lithography, which takes place only in the top resist layer. Also, the fabrication method allows for easy changes in the geometry: the height can be changed by changing the thickness of the bottom resist layer, the thickness by modifying the amount of deposited material and the diameter by changing the etching time. By hanging the geometric parameters of the nanostructures, the plasmonic properties can be easily tuned. Besides, for certain dimensions (400 nm in diameter and height and 30 nm of Wall and base thickness), these structures present a peak in the extinction spectra in the visible range that corresponds to a concentration of the electric field within the cavity. This excitation mode has also been reported for other nanostructures with semispherical symmetry. However, the fact of being cylindrical enables a homogeneous enhancement of the electric field along the cavity while in the other case this is not possible due to the lack of symmetry.

Also, based on geometrically frustrated magnetic systems, three particular cases of hexagonal lattices of plasmonic nanoelements have been studied. All of them have been designed so that the pitch is of the order of the resonance wavelength and the gaps between elements small enough to enable near-field coupling. Besides, a metal-insulator-metal configuration has been implemented, designed to have constructive interference, which leads to high absorption peaks.

The samples have been fabricated by electron beam lithography to be able to change easily the design and study the optical response as a function of the geometries. Both simulation and spectroscopy results show that all these systems present high absorption peaks in the visible and/or near infrared. Also, they present a broad absorption peak in the NIR due to the dipolar excitation of the gaps between neighboring elements and sharper peaks in the visible that are assigned to collective modes. Moreover, these systems present an extended time response where the system fluctuates between collective and localized modes. This behavior, characteristic from magnetic frustrated systems, is induced by the frustration of the dipolar excitation of the gaps due to the geometry of the lattice. Besides, the collective modes give rise to enhancements of the electric field in large areas, making these systems of interest for enhanced spectroscopies.

Esta tesis está dedicada a la nanofabricación, simulación y caracterización de las propiedades ópticas de diferentes nanoestructuras de oro.

Por un lado, inspirados por las nanopartículas tipo core-shell, se han fabricado nanoestructuras de oro cilíndricas en forma de taza, combinando litografía por nanoimpresión (NIL) con metalización por pulverización catódica. Para tener la posibilidad de fabricar estructuras de una elevada altura frente a su anchura, se ha utilizado una tricapa de resina-óxido-resina, de manera que la capa inferior de resina controla la altura de las estructuras mientras que la litografía se realiza en la capa superior y por tanto se sobreponen las típicas dificultades que aparecen en NIL para estructuras de elevada relación de aspecto. Estas nanoestructuras, al igual que las nanoparticulas core-shell, presentan tambien gran capacidad de ajuste de sus propiedades como función de su geometria.

Por otro lado, basados en los sistemas magnéticos con frustración geometrica, se han estudiado diferentes redes hexagonales de nanoelementos de oro. Todos los sistemas se han diseñado de modo que el periodo es del orden de la longitud de onda de resonancia y los espacios entre estructuras suficientemente pequeños para tener acoplo de campo cercano. Se ha utilizado una configuración metal-aislante­metal para obtener interferencia constructiva y, en consecuencia, picos de alta absorción. Las muestras se han fabricado utilizando litografía por haces de electrones para poder estudiar los cambios en la respuesta óptica en función de la geometría. Estos sistemas presentan un pico de absorción ancho en el infrarrojo ligado a la excitación dipolar de los huecos entre nanoestructuras y picos más estrechos en el visible que corresponden a modos donde predomina el comportamiento colectivo del sistema. Además, el estudio de la evolución temporal del sistema muestra que este tipo de redes presentan una respuesta extendida en el tiempo inducida por la frustración geométrica del sistema, característica de los sistemas magnéticos frustrados, durante la cual el sistema oscila entre modos localizados y modos colectivos. Por todo ello, consideramos que estas estructuras pueden ser de interés para aplicaciones relacionadas con la absorción de luz.