Nanofabrication, simulation and optical characterization of plasmonic nanostructures
- Autores: Ana Conde Rubio
- Directores de la Tesis: Xavier Batlle i Gelabert (dir. tes.), Amílcar Labarta (dir. tes.)
- Lectura: En la Universitat de Barcelona ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: Juergen Brugger (presid.), Albert Romano Rodríguez (secret.), Antonio Agustin Mihi Cervello (voc.)
- Programa de doctorado: Programa de Doctorado en Nanociencias por la Universidad de Barcelona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
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-oxide-resist) 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 changing 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.
