How Chain Plasmons Govern the Optical Response in Strongly Interacting Self-Assembled Metallic Clusters of Nanoparticles

Self-assembled clusters of metallic nanoparticles separated by nanometric gaps generate strong plasmonic modes that support both intense and localized near fields. These find use in many ultrasensitive chemical and biological sensing applications through surface enhanced Raman scattering (SERS). The inability to control at the nanoscale the structure of the clusters on which the optical response crucially depends, has led to the development of general descriptions to model the various morphologies fabricated. Here, we use rigorous electrodynamic calculations to study clusters formed by a hundred nanospheres that are separated by ∼1 nm distance, set by the dimensions of the macrocyclic molecular linker employed experimentally. Three-dimensional (3D) cluster structures of moderate compactness are of special interest since they resemble self-assembled clusters grown under typical diffusion-limited aggregation conditions. We find very good agreement between the simulated and measured far-field extinction spectra, supporting the equivalence of the assumed and experimental morphologies. From these results we argue that the main features of the optical response of two- and three-dimensional clusters can be understood in terms of the excitation of simple units composed of different length resonant chains. Notably, we observe a qualitative difference between short- and long-chain modes in both spectral response and spatial distribution: dimer and short-chain modes are observed in the periphery of the cluster at higher energies, whereas inside the structure longer chain excitation occurs at lower energies. We study in detail different configurations of isolated one-dimensional chains as prototypical building blocks for large clusters, showing that the optical response of the chains is robust to disorder. This study provides an intuitive understanding of the behavior of very complex aggregates and may be generalized to other types of aggregates and systems formed by large numbers of strongly interacting particles

Os estudantes indígenas em cena : a memória coletiva sobre a inclusão na universidade

Esta dissertação trata de uma pesquisa-ação que tem como objetivo geral desvelar a memória coletiva dos estudantes indígenas no processo de inclusão no ensino superior, no que se refere ao acesso e à permanência deles na Casa do Estudante Universitário – CEU, da Universidade Federal do Rio Grande do Sul – UFRGS, no período de 2008 a 2013, para intervir na realidade acadêmica. Os objetivos específicos são os de conhecer o perfil dos estudantes indígenas para a caracterização e o reconhecimento das origens, da situação familiar e dos cursos em formação; sensibilizar a comunidade estudantil para participar do processo de investigação, a fim de envolvê-la na reflexão sobre a inclusão dos estudantes indígenas no ensino superior e na construção de mudanças para a garantia do acesso e da permanência desses estudantes na UFRGS; conhecer as experiências cotidianas vivenciadas por eles na interação com a comunidade universitária, para identificar os limites e as possibilidades do acesso e da permanência deles na UFRGS; construir coletivamente um vídeo da memória coletiva sobre a importância da inclusão, do acesso e da permanência desses estudantes na comunidade universitária para incidir na realidade acadêmica. Como procedimentos metodológicos, destacamos a revisão teórica realizada sobre a temática e as categorias memória coletiva, ações afirmativas, diversidade cultural, ensino superior, inclusão social e minorias étnicas. O desenvolvimento da pesquisa-ação consistiu em 04 (quatro) principais fases: fase exploratória (diagnóstico), fase principal (planejamento), fase ação (seminário e filmagens) e fase de avaliação (análise dos resultados e produção do vídeo). Participaram da pesquisa 6 (seis) estudantes indígenas da UFGRS que, coletivamente, produziram um vídeo o qual desvela as memórias guardadas acerca do processo de inclusão na UFRGS, os limites e as possibilidades do cotidiano acadêmico, as questões referentes ao acesso e à permanência deles na universidade, bem como as mudanças que propõem. A pesquisa se constituiu em uma importante estratégia para a implementação de mudanças no âmbito acadêmico no que diz respeito ao acesso e à permanência dos estudantes indígenas na UFRGS.This dissertation addresses an action research whose general objective is to unveil the collective memory of indigenous students in the process of inclusion in higher education by considering their access to and permanence in the University Residence Hall of the Federal University of Rio Grande do Sul (UFRGS) from 2008 to 2013, in order to intervene in the academic reality. The specific objectives are the following: to know the indigenous students’ profiles to characterize and recognize their origin, family situation and chosen courses; to sensitize the student community to participate in the investigation process in order to make it reflect on the inclusion of indigenous students in higher education and the production of changes to guarantee their access to and permanence in UFRGS; to know their daily experiences along their interaction with the university community to identify the limitations and possibilities of their access to and permanence in UFRGS; to collectively produce a video about the collective memory regarding the importance of inclusion, access and permanence of those students in the university community to impact the academic reality. As methodological procedures, we can point out the theoretical review of both the topic and the categories of collective memory, affirmative actions, cultural diversity, higher education, social inclusion and ethnic minorities. The development of the action research involved four major stages: exploratory stage (diagnosis), main stage (planning), action stage (seminar and filming) and assessment stage (analysis of the results and video production). Six indigenous students attending UFRGS participated in the research and collectively produced a video that has unveiled the memories of their process of inclusion in UFRGS, their limitations and possibilities in the daily academic routine, issues related to their access to and permanence in the university, and the changes they have proposed. The research has been an important strategy for implementing changes in the academic sphere concerning the access of indigenous students to UFRGS and their permanence in this institution

Electric Field-Induced High Order Nonlinearity in Plasmonic Nanoparticles Retrieved with Time-Dependent Density Functional Theory

The nonlinear response of metallic nanoparticles is obtained from quantum time dependent density functional theory calculations. Without any aprioristic assumption our calculations allow us to identify high-order harmonic generation in canonical plasmonic structures such as spherical single particles and dimers. Furthermore, we demonstrate that under currently available experimental conditions, the application of an external polarizing field to the nanoparticles allows to actively control even-order harmonic generation in otherwise symmetry forbidden situations. Our quantum calculations provide quantitative access to the high-order response of metallic nanoantennas, which is of utmost importance in the design, control, and exploitation of optoelectronic devices as well as in the generation of extreme ultraviolet radiation

Evolution of Plasmonic Metamolecule Modes in the Quantum Tunneling Regime

Plasmonic multinanoparticle systems exhibit collective electric and magnetic resonances that are fundamental for the development of state-of-the-art optical nanoantennas, metamaterials, and surface-enhanced spectroscopy substrates. While electric dipolar modes have been investigated in both the classical and quantum realm, little attention has been given to magnetic and other “dark” modes at the smallest dimensions. Here, we study the collective electric, magnetic, and dark modes of colloidally synthesized silver nanosphere trimers with varying interparticle separation using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). This technique enables direct visualization and spatially selective excitation of individual trimers, as well as manipulation of the interparticle distance into the subnanometer regime with the electron beam. Our experiments reveal that bonding electric and magnetic modes are significantly impacted by quantum effects, exhibiting a relative blueshift and reduced EELS amplitude compared to classical predictions. In contrast, the trimer’s electric dark mode is not affected by quantum tunneling for even Ångström-scale interparticle separations. We employ a quantum-corrected model to simulate the effect of electron tunneling in the trimer which shows excellent agreement with experimental results. This understanding of classical and quantum-influenced hybridized modes may impact the development of future quantum plasmonic materials and devices, including Fano-like molecular sensors and quantum metamaterials

Rabi Splitting in Photoluminescence Spectra of Hybrid Systems of Gold Nanorods and J‑Aggregates

We experimentally and theoretically investigate the interactions between localized plasmons in gold nanorods and excitons in J-aggregates under ambient conditions. Thanks to our sample preparation procedure we are able to track a clear anticrossing behavior of the hybridized modes not only in the extinction but also in the photoluminescence (PL) spectra of this hybrid system. Notably, while previous studies often found the PL signal to be dominated by a single mode (emission from so-called lower polariton branch), here we follow the evolution of the two PL peaks as the plasmon energy is detuned from the excitonic resonance. Both the extinction and PL results are in good agreement with the theoretical predictions obtained for a model that assumes two interacting modes with a ratio between the coupling strength and the plasmonic losses close to 0.4, indicative of the strong coupling regime with a significant Rabi splitting estimated to be ∼200 meV. The evolution of the PL line shape as the plasmon is detuned depends on the illumination wavelength, which we attribute to an incoherent excitation given by decay processes in either the metallic rods or the J-aggregates