Espectroscopia ultra-rápida e estudos de dinâmica eletrônica em nano-estruturas plasmônicas

Abstract

The study of photoexcited electron and plasmon dynamics in nanomaterials presents experimental challenges due to the very short times (femtoseconds) involved in the early stages of energy relaxation. In the last decades studies of these processes in various types of materials are being revisited due to the great advance in the development of ultra short pulse lasers and sensitive detectors that allow access to these early stages of energy relaxation. In this thesis I present results obtained in metallic nanoparticles: gold nanorods, and assemblies of gold nanocrystal in organic matrix, which allowed studies of the photoexcited electron dynamics from the earliest times until the complete thermalization with the crystal lattice. Surface electrons in metal nanoparticles can be coherently excited by an external electric field, generating collective oscillations or Localized Surface Plasmons (LSP). Excitation of LSP modify the optical properties of the material such as the absorption and the optical emission that can be controlled by adjusting the shape, size and type of nanoparticle material or the environment that they are immersed. The physical processes involved in both absorption and emission have been intensively studied since the seventies. However, due to the wealth of possible phenomena in these nanoparticles there are still several open questions. In addition the possibility of modifying their optical properties make these nanoparticles ideal for applications in various areas, plasmonics, nonlinear optics and biological applications. We present results for gold nanorods colloidal solutions that present two vibrational modes of surface plasmons: transverse and longitudinal. In order to understand the dynamics of the electronic processes involved in the optical absorption, we use the non-degenerate pump and probe spectroscopy technique with the probe pulse covering a large spectral range (white probe), that allowed the observation of important variations in time resolved absorption in femtoseconds. We studied also the processes of photoluminescence by two photon excitation. In the nanocrystal assemblies we observed that the absorption is also dominated by plasmonic resonance. The results are described by a semi-classical hot electron energy relaxation model considering three temperatures, the temperature of the electrons, the phonons and the lattice. In all the nanomaterials studied we obtained that the electrons out of thermal equilibrium are important for the complete description of the energy relaxation processes.