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Thesis by Mattin Urbieta Galarraga for the degree of Doctor of Philosophy in Physics. ; This thesis tackles the theoretical description of atomic-scale features in plasmonic nanostructures asprobed by light and swift electrons. Plasmonic nanostuctures are known to localize and enhanceelectromagnetic fields in their proximity, and thus serve as building blocks to perform improved andenhanced molecular spectroscopy on them. We focus on the analysis of the effect of atomic-scale featuresin the overall response of plasmonic nanoparticles and nanocavities. We apply ab initio atomisticquantum time-dependent density functional theory (TDDFT) to unveil the near-field distribution aroundmetallic antennas, and describe "classically" various atomic-scale features such as continuous protrusionson the surfaces of the metal using a Boundary Element Method (BEM), providing an extra localization ofthe field. Moreover, we propose an analytical model to address the signal increase observed in surfaceenhancedRaman scattering (SERS) spectra related to local variations of the electron density associated toatomic-scale defects. Last, we identify the excitation of confined bulk plasmons (CBP) within theTDDFT calculations for the electron energy loss (EEL) probability of atomistic clusters, and provide asemi-analytical expression within a Hydrodynamic Model (HDM) to address such excitations. ; This thesis was funded by the University of the Basque Country UPV/EHU Grant PIF15/237 with a PhD fellowship and project GIU18/202, Donostia International Physics Center (DIPC) (Reference 2015/11) and Materials Physics Center (MPC) for financial support at the initial steps of the PhD, the Basque Government for projects IT756-13 and IT1164-19, and and by Spanish Ministry of Economy and Competitiveness MINECO for projects FIS2013-41184-P, FIS2016-80174-P and PID2019-107432GB-I00. The author thanks DIPC for the computing facilities and CFM for its support. ; Peer reviewed

Plasmonic gaps are known to produce nanoscale localization and enhancement of optical fields, providing small effective mode volumes of about a few hundred nm. Atomistic quantum calculations based on time-dependent density functional theory reveal the effect of subnanometric localization of electromagnetic fields due to the presence of atomic-scale features at the interfaces of plasmonic gaps. Using a classical model, we explain this as a nonresonant lightning rod effect at the atomic scale that produces an extra enhancement over that of the plasmonic background. The near-field distribution of atomic-scale hot spots around atomic features is robust against dynamical screening and spill-out effects and follows the potential landscape determined by the electron density around the atomic sites. A detailed comparison of the field distribution around atomic hot spots from full quantum atomistic calculations and from the local classical approach considering the geometrical profile of the atoms' electronic density validates the use of a classical framework to determine the effective mode volume in these extreme subnanometric optical cavities. This finding is of practical importance for the community of surface-enhanced molecular spectroscopy and quantum nanophotonics, as it provides an adequate description of the local electromagnetic fields around atomic-scale features with use of simplified classical methods. ; Financial support from Project No. FIS2016-80174-P and MAT2016-78293-C6-4-R of MINECO, grant of Consolidated Groups at UPV/EHU (IT-756-13) of the Basque Government, and project 70NANB15H32 from U.S. Department of Commerce, National Institute of Standards and Technology, is acknowledged. M.U. acknowledges support from the University of the Basque Country through a Ph.D. grant, as well as DIPC and CFM at the initial stages of this work. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a Ph.D. grant, as well as from Euskampus and the DIPC at the initial stages of this work. P.K. acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union, "Una manera de hacer Europa". ; Peer Reviewed

The description of surface-enhanced Raman scattering (SERS) as a molecular optomechanical process has provided new insights into the vibrational dynamics and nonlinearities of this inelastic scattering process. In earlier studies, molecular vibrations have typically been assumed to couple with a single plasmonic mode of a metallic nanostructure, ignoring the complexity of the plasmonic response in many configurations of practical interest such as in metallic nanojunctions. By describing the plasmonic fields as a continuum, we demonstrate here the importance of considering the full plasmonic response to properly address the molecule-cavity optomechanical interaction. We apply the continuum-field model to calculate the Raman signal from a single molecule in a plasmonic nanocavity formed by a nanoparticle-on-a-mirror configuration, and compare the results of optomechanical parameters, vibrational populations, and Stokes and anti-Stokes signals of the continuum-field model with those obtained from the single-mode model. Our results reveal that high-order non-radiative plasmonic modes significantly modify the optomechanical behavior under strong laser illumination. Moreover, Raman linewidths, lineshifts, vibrational populations, and parametric instabilities are found to be sensitive to the energy of the molecular vibrational modes. The implications of adopting the continuum-field model to describe the plasmonic cavity response in molecular optomechanics are relevant in many other nanoantenna and nanocavity configurations commonly used to enhance SERS. ; We acknowledge project Nr. 12004344 from the National Science Foundation of China, joint project Nr. 21961132023 from the NSFC-DPG, project PID2019-107432GB-I00 from the Spanish Ministry of Science and Innovation, project KK-2019/00101 from Eusko Jaurlaritza, project H2020-FET Open "THOR" Nr. 829067 from the European Commission, UK EPSRC EP/L027151/1, and grant IT1164-19 for consolidated groups of the Basque University, through the Department of Education, Research and Universities of the Basque Government. ; Peer reviewed