L’irradiation des matériaux par des impulsions laser ultrabrèves déclenche un agencement anisotrope de la matière à l’échelle nanométrique: des structures de surface périodiques induites par laser (LIPSS). L'énergie laser déposée et distribuée de manière inhomogène dans le matériau induit des contraintes thermiques locales et des changements de phase transitoires entraînant ainsi des modifications microstructurales. Cette thèse porte sur le rôle de l'altération de la surface irradiée ainsi que les modifications microstructurales en profondeur dans la contribution à la formation des LIPSS, en établissant une corrélation entre l'auto-organisation de la matière et la génération de défauts en tenant en compte de l'orientation cristalline. Comme les LIPSS sont générés au seuil de transition de phase, l’étude de la corrélation avec les défauts induits est alors pertinente. Une étude expérimentale couplée à des simulations de dynamique moléculaire effectuées à l’Université de Virginie suggère que l'altération de surface générée par une irradiation d'échantillons monocristallins de Chrome dans le régime de spallation est susceptible de jouer un rôle majeur dans le déclenchement de génération de LIPSS de haute fréquence spatiale. La microscopie à force atomique ainsi que les résultats de simulations attestent que les caractéristiques de rugosité de surface à l'échelle nanométrique dépendent de l'orientation cristalline. La forte rugosité de surface générée par la première impulsion laser active la diffusion de la lumière laser et l’exaltation du champ local lors des irradiations ultérieures, ce qui génère des structures LIPSS de haute fréquence plus prononcés du côté (100) que celle du (110). Une étude expérimentale approfondie, utilisant la microscopie électronique rétrodiffusés et transmission, a révélé que le Cr (110) est plus susceptible d'être endommagé que les autres orientations cristallines de surface. On constate que les défauts induits par le laser peuvent altérer la topographie de surface et la région sous-jacente, ce qui peut avoir un impact sur les caractéristiques des centres de rugosité favorisant la formation de structures de fréquence spatiale élevée. Afin d’accéder à la transition de phase subie dans la région de formation des LIPSS, une approche d'analyse microstructurale à haute résolution couplée à des calculs hydrodynamiques est utilisée, comprenant la croissance épitaxiale et la nanocavitation. La formation de structures de fréquence spatiale élevée est le résultat de nanocavités périodiques piégés sous la surface, ainsi que des nanocavités apparues à la surface des matériaux cubiques faces centrées.De plus, étant donné que le feedback dans la formation des LIPSS est souvent évoquée, le comportement dynamique des surfaces a été sondé par microscopie électronique à photoémission et étayé par des calculs électromagnétiques. Un caractère périodique des photoélectrons émis par les creux des LIPSS a été mis en évidence, ce qui a permis de vérifier la modulation du dépôt d'énergie.Le travail effectué contribue non seulement à progresser vers l'objectif général d’élucider le phénomène complexe multi-échelles de la formation des LIPSS, mais ouvre une nouvelle voie expérimentale pour générer des structures non conventionnelles avec des périodicités extrêmes (~60nm), offrant ainsi de nouvelles opportunités pour le traitement laser ultrarapide des métaux. / Irradiation of materials by ultrashort laser pulses triggers anisotropically structured arrangement of matter on the nanoscale, the so-called laser-induced periodic surface structures (LIPSS), or ‘ripples’. Ultrashort laser energy deposited and distributed inhomogeneously in the material launches local thermal stresses and transient phase changes yielding microstructural modifications. This thesis focuses on the role of irradiated surface alteration as well as in-depth microstructural modifications in promoting LIPSS formation, by establishing a correlation between self-organization of matter and defect generation taking into account crystalline orientation. Since LIPSS are generated at the threshold of phase transition, then the correlation with defects formation is relevant. An experimental study coupled with molecular dynamic MD simulations performed in the University of Virginia suggest that surface alteration generated by a single pulse irradiation of monocrystalline Cr samples in the spallation regime is likely to play a main role in triggering high-spatial frequency LIPSS generation upon irradiation by multiple laser pulses. Atomic force microscopy as well as computational results suggested that the nanoscale surface features are crystalline orientation dependent. The higher surface roughness generated by the first laser pulse activates scattering of the laser light and the local field enhancement upon irradiation by the second laser pulse, leading to the formation of much more pronounced high-spatial frequency structures on the (100) surface as compared to (110) one. An extended in-depth experimental study, using electron backscattered and transmission microscopy, combined with large-scale two-temperature model TTM-MD simulations revealed that Cr (110) is more likely to get damaged. It is found that laser-induced defects can alter the surface topography and the region beneath it which can impact in turn the roughness center features promoting high-spatial frequency structures formation. In order to infer the phase transition undergone in the LIPSS region, a high resolution microstructural analysis approach coupled with hydrodynamic calculations is employed, including epitaxial regrowth and nanocavitation. High-spatial frequency structures formation is found to be the result of periodic nanovoids trapped beneath the surface as well as nanocavities emerged at the surface on fcc materials. Furthermore, since optical feedback in LIPSS is often evoked, the behavior of dynamical surfaces was probed by photoemission electron microscopy and supported by electromagnetic calculations. A periodic character of photoelectrons emitted from nanoholes was unveiled, which in turn verified a modulated energy deposition. The performed work not only contributes to the progress towards the general goal of untangling the complex multiscale phenomenon of the LIPSS formation, but unlocks a new experimental setup to generate unconventional structures with extreme periodicities (~60 nm), which offers new opportunities in ultrafast laser processing of metals.

Effective focusing of electromagnetic (EM) energy to nanoscale regions is one of the major challenges in nano-photonics and plasmonics. The strong localization of the optical energy into regions much smaller than allowed by the diffraction limit, also called nanofocusing, offers promising applications in nano-sensor technology, nanofabrication, near-field optics or spectroscopy. One of the most promising solutions to the problem of efficient nanofocusing is related to surface plasmon propagation in metallic structures. Metallic tapered rods, commonly used as probes in near field microscopy and spectroscopy, are of a particular interest. They can provide very strong EM field enhancement at the tip due to surface plasmons (SP’s) propagating towards the tip of the tapered metal rod. A large number of studies have been devoted to the manufacturing process of tapered rods or tapered fibers coated by a metal film. On the other hand, structures such as metallic V-grooves or metal wedges can also provide strong electric field enhancements but manufacturing of these structures is still a challenge. It has been shown, however, that the attainable electric field enhancement at the apex in the V-groove is higher than at the tip of a metal tapered rod when the dissipation level in the metal is strong. Metallic V-grooves also have very promising characteristics as plasmonic waveguides. This thesis will present a thorough theoretical and numerical investigation of nanofocusing during plasmon propagation along a metal tapered rod and into a metallic V-groove. Optimal structural parameters including optimal taper angle, taper length and shape of the taper are determined in order to achieve maximum field enhancement factors at the tip of the nanofocusing structure. An analytical investigation of plasmon nanofocusing by metal tapered rods is carried out by means of the geometric optics approximation (GOA), which is also called adiabatic nanofocusing. However, GOA is applicable only for analysing tapered structures with small taper angles and without considering a terminating tip structure in order to neglect reflections. Rigorous numerical methods are employed for analysing non-adiabatic nanofocusing, by tapered rod and V-grooves with larger taper angles and with a rounded tip. These structures cannot be studied by analytical methods due to the presence of reflected waves from the taper section, the tip and also from (artificial) computational boundaries. A new method is introduced to combine the advantages of GOA and rigorous numerical methods in order to reduce significantly the use of computational resources and yet achieve accurate results for the analysis of large tapered structures, within reasonable calculation time. Detailed comparison between GOA and rigorous numerical methods will be carried out in order to find the critical taper angle of the tapered structures at which GOA is still applicable. It will be demonstrated that optimal taper angles, at which maximum field enhancements occur, coincide with the critical angles, at which GOA is still applicable. It will be shown that the applicability of GOA can be substantially expanded to include structures which could be analysed previously by numerical methods only. The influence of the rounded tip, the taper angle and the role of dissipation onto the plasmon field distribution along the tapered rod and near the tip will be analysed analytically and numerically in detail. It will be demonstrated that electric field enhancement factors of up to ~ 2500 within nanoscale regions are predicted. These are sufficient, for instance, to detect single molecules using surface enhanced Raman spectroscopy (SERS) with the tip of a tapered rod, an approach also known as tip enhanced Raman spectroscopy or TERS. The results obtained in this project will be important for applications for which strong local field enhancement factors are crucial for the performance of devices such as near field microscopes or spectroscopy. The optimal design of nanofocusing structures, at which the delivery of electromagnetic energy to the nanometer region is most efficient, will lead to new applications in near field sensors, near field measuring technology, or generation of nanometer sized energy sources. This includes: applications in tip enhanced Raman spectroscopy (TERS); manipulation of nanoparticles and molecules; efficient coupling of optical energy into and out of plasmonic circuits; second harmonic generation in non-linear optics; or delivery of energy to quantum dots, for instance, for quantum computations.