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The correlation between transport properties across subnanometric metallic gaps and the optical response of the system is a complex effect that is determined by the fine atomic-scale details of the junction structure. As experimental advances are progressively accessing transport and optical characterization of smaller nanojunctions, a clear connection between the structural, electronic, and optical properties in these nanocavities is needed. Using ab initio calculations, we present here a study of the simultaneous evolution of the structure and the optical response of a plasmonic junction as the particles forming the cavity, two Na380 clusters, approach and retract. Atomic reorganizations are responsible for a large hysteresis of the plasmonic response of the system, which shows a jump-to-contact instability during the approach process and the formation of an atom-sized neck across the junction during retraction. Our calculations demonstrate that, due to the quantization of the conductance in metal nanocontacts, atomic-scale reconfigurations play a crucial role in determining the optical response of the whole system. We observe abrupt changes in the intensities and spectral positions of the dominating plasmon resonances and find a one-to-one correspondence between these jumps and those of the quantized transport as the neck cross-section diminishes. These results reveal an important connection between transport and optics at the atomic scale, which is at the frontier of current optoelectronics and can drive new options in optical engineering of signals driven by the motion and manipulation of single atoms., We acknowledge financial support from Projects FIS2013-41184-P and MAT2013-46593-C6-2-P from MINECO. M.B., P.K., F.M., and D.S.P. also acknowledge support from the ANR-ORGAVOLT project and the Euroregion Aquitaine-Euskadi program. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a Ph.D. grant. P.K. acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union. J.A. also acknowledges support from Grant 70NANB15H321, “PLASMOQUANTUM”, from the US Department of Commerce (NIST).

arXiv:1512.02104v2, We present a study of the optical response of compact and hollow icosahedral clusters containing up to 868 silver atoms by means of time-dependent density functional theory. We have studied the dependence on size and morphology of both the sharp plasmonic resonance at 3–4 eV (originated mainly from sp-electrons), and the less studied broader feature appearing in the 6–7 eV range (interband transitions). An analysis of the effect of structural relaxations, as well as the choice of exchange correlation functional (local density versus generalised gradient approximations) both in the ground state and optical response calculations is also presented. We have further analysed the role of the different atom layers (surface versus inner layers) and the different orbital symmetries on the absorption cross-section for energies up to 8 eV. We have also studied the dependence on the number of atom layers in hollow structures. Shells formed by a single layer of atoms show a pronounced red shift of the main plasmon resonances that, however, rapidly converge to those of the compact structures as the number of layers is increased. The methods used to obtain these results are also carefully discussed. Our methodology is based on the use of localised basis (atomic orbitals, and atom-centered and dominant-product functions), which bring several computational advantages related to their relatively small size and the sparsity of the resulting matrices. Furthermore, the use of basis sets of atomic orbitals also allows the possibility of extending some of the standard population analysis tools (e.g. Mulliken population analysis) to the realm of optical excitations. Some examples of these analyses are described in the present work., This work is supported, in part, by the ORGAVOLT (ORGAnic solar cell VOLTage by numerical computation) Grant ANR-12-MONU-0014-02 of the French Agence Nationale de la Recherche (ANR) 2012 Programme Modèles Numériques. F Marchesin, P Koval and D Sánchez-Portal acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) through the sfb1083 project, the Spanish mineco MAT2013-46593-C6-2-P project, the Euroregion Aquitaine-Euskadi program and from the Basque Departamento de Educación, upv/ehu (Grant No. IT-756-13). Peter Koval acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union.

Electromagnetic field localization in nanoantennas is one of the leitmotivs that drives the development of plasmonics. The near-fields in these plasmonic nanoantennas are commonly addressed theoretically within classical frameworks that neglect atomic-scale features. This approach is often appropriate since the irregularities produced at the atomic scale are typically hidden in far-field optical spectroscopies. However, a variety of physical and chemical processes rely on the fine distribution of the local fields at this ultraconfined scale. We use time-dependent density functional theory and perform atomistic quantum mechanical calculations of the optical response of plasmonic nanoparticles, and their dimers, characterized by the presence of crystallographic planes, facets, vertices, and steps. Using sodium clusters as an example, we show that the atomistic details of the nanoparticles morphologies determine the presence of subnanometric near-field hot spots that are further enhanced by the action of the underlying nanometric plasmonic fields. This situation is analogue to a self-similar nanoantenna cascade effect, scaled down to atomic dimensions, and it provides new insights into the limits of field enhancement and confinement, with important implications in the optical resolution of field-enhanced spectroscopies and microscopies., We acknowledge financial support from projects FIS2013-14481-P and MAT2013-46593-C6-2-P from MINECO. M.B., P.K., F.M., and D.S.P. also acknowledge support from the ANR-ORGAVOLT project and the Euroregion Aquitaine-Euskadi program. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a PhD grant, as well as from Euskampus and the DIPC at the initial stages of this work. R.E. and P.K. acknowledge 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”.

We show that chemically synthesized polycyclic aromatic hydrocarbons (PAHs) exhibit molecular plasmon resonances that are remarkably sensitive to the net charge state of the molecule and the atomic structure of the edges. These molecules can be regarded as nanometer-sized forms of graphene, from which they inherit their high electrical tunability. Specifically, the addition or removal of a single electron switches on/off these molecular plasmons. Our first-principles time-dependent density-functional theory (TDDFT) calculations are in good agreement with a simpler tight-binding approach that can be easily extended to much larger systems. These fundamental insights enable the development of novel plasmonic devices based upon chemically available molecules, which, unlike colloidal or lithographic nanostructures, are free from structural imperfections. We further show a strong interaction between plasmons in neighboring molecules, quantified in significant energy shifts and field enhancement, and enabling molecular-based plasmonic designs. Our findings suggest new paradigms for electro-optical modulation and switching, single-electron detection, and sensing using individual molecules. © 2013 American Chemical Society., This work has been supported in part by the Spanish MICINN (MAT2010-14885, FIS2010-19609-C02-00, and Consolider NanoLight.es), the European Commission (FP7-ICT-2009-4-248909-LIMA and FP7-ICT-2009-4-248855-N4E), and the Etortek program. A.M. acknowledges financial support through FPU from the Spanish MEC. P.K. aknowledges support from the CSIC JAE-doc program, cofinanced by the European Science Foundation. P.N. acknowledges support from the Robert A. Welch Foundation (C-1222) and the Office of Naval Research (N00014-10-1-0989).