On Harvesting, Confinement and Conversion of the Energy of Light with Facile Plasmonic Nanostructures

Abstract

Efficient light harvesting and confinement in deeply sub-wavelength dimensions is very challenging but offers simultaneously exciting possibilities which can impact a variety of important applications from biological sample analysis to electronic circuits and to solar energy harvesting technologies. Indeed, light interaction with ultra-thin or nano-sized objects is inherently poor and it constitutes the bottleneck for the miniaturization of many devices. In recent years metallic nano-structures supporting surface plasmons have been proposed as a viable solution to this problem. In fact, they have been shown to enable effective coupling, focusing and manipulation of light at the nanoscale, finally overcoming the diffraction limit. Light-conversion technologies, such as photovoltaic, solar-thermoelectric and photocatalytic cells, would highly benefit from a nearly ideal light harvesting in ultra-thin structures due to the associated decrease in material costs and increase in efficiencies. Nevertheless, sunlight harvesting poses several challenges to the design of plasmonics-based structures due to its broad-spectrum and variable angle of incidence. In the first part of this thesis (Chapter 3) we addressed these aspects by investigating, designing and fabricating a facile, ultra-thin (260 nm) plasmonic sunlight absorber which is capable of harvesting on average 88 % of the Sun energy in the spectral range 380− 980 nm. Our approach is inherently polarization insensitive and preserves its performances for incident angles up to 48◦. In addition, the fabrication of our structure is compatible with large-scale, roll-to-roll processes and the used multilayer design could be easily implemented in a real devices where additional functionalities (e.g. electrical access) are required. Light collection is only the first step toward light-energy exploitation and in the second part of the thesis (Chapters 4− 5) we thus focused on extreme confinement of the collected light. We performed a general study on a class of plasmonic multilayer (metal-insulator-metal) structures, with a dielectric spacer of only few nanometers (7− 13 nm). As front pattern we used facile hexagonal arrays of tapered gold triangles which eventually connect forming an electrically communicating network. We show that, in the disconnected