Numerical study of light scattering by micro and nanostructures on substrates. Application to real experimental situations

Resumen

Probing surfaces at the micro- and nanoscale has played an important role in the advancement of science and technology. Among the various optical techniques, conventional microscopy lacks the required resolution to identify subwavelength objects, while near-field microscopy suffers from weak scattering signals and complex relations between samples and probe. This work focuses on the electromagnetic interaction of micro- and nanostructures with surfaces, with the aim of finding novel surface inspection methods based on light scattering. The underlying idea is to understand and harness the effect of subwavelength objects in the far field. It is shown that by judicious choice of far-field quantities, like ¿BR or PL, one can extract relevant information on the position, size and composition of the scattering object. This topic is in close connection with the recent developments in the field of plasmonics, where the optical properties of nanoscale objects can be exploited in various forms to interface far-field quantities with local near-field phenomena. The thesis is divided in two parts. The first one is dedicated to the modeling and numerical analysis of perturbations on the near-field and far-field scattering patterns. This cab be caused by the presence of nano-defects, either located on the surface of a larger (micrometric) object lying on a substrate or directly deposited on the substrate near to the larger object. In addition to the proper description of the calculation methods employed, an analysis of the dependence of scattering pattern modifications as a function of defect position, size, material and substrate material is made to demonstrate that measurements of these changes are likely to be used for detection purposes. The second part deals with the analysis of the Localized Plasmon Resonances excited in a tip probe metallic nanoparticle located close to a substrate. In particular, we investigate in which extent the study of these localized plasmon resonances observed on the backscattering cross-section or on the linear degree of polarization of light scattered at 90° from the excitation direction, can be used for surface inspection. This study is numerically carried out by means of the Discrete Dipole Approximation method. This part ends up with calculations aimed to reproduce plasmon resonances that have been observed experimentally when Gallium nanoparticles are deposited on a SiC substrate, which in particular exhibit a high sensitivity of these resonances to the morphology of the nanoparticles.