Theory of Raman enhancement by two-dimensional materials: Applications for graphene-enhanced Raman spectroscopy

We propose a third-order time-dependent perturbation theory approach to describe the chemical surface-enhanced Raman spectroscopy of molecules interacting with two-dimensional (2D) surfaces such as an ideal 2D metal and graphene, which are both 2D metallic monolayers. A detailed analysis is performed for all the possible scattering processes involving both electrons and holes and considering the different time orderings for the electron-photon and electron-phonon interactions. We show that for ideal 2D metals a surface enhancement of the Raman scattering is possible if the Fermi energy of the surface is near the energy of either the HOMO or the LUMO states of the molecule and that a maximum enhancement is obtained when the Fermi energy matches the energy of either the HOMO or the LUMO energies plus or minus the phonon energy. The graphene-enhanced Raman spectroscopy effect is then explained as a particular case of a 2D surface, on which the density of electronic states is not constant, but increases linearly with the energy measured from the charge neutrality point. In the case of graphene, the Raman enhancement can occur for any value of the Fermi energy between the HOMO and LUMO states of the molecule. The proposed model allows for a formal approach for calculating the Raman intensity of molecules interacting with different 2D materials.National Science Foundation (U.S.) (Grant DMR-1004147)MIT-Brazil Collaboration progra

Longitudinal Polaritons in Crystals

The collective excitations of solids are classified as longitudinal and transverse depending on their relative polarization and propagation direction. This seemingly formal classification results in surprisingly distinct types of excitations if calculated within the Coulomb gauge. Transverse modes couple to free-space photons and hybridize into polaritons for strong light-matter coupling. Longitudinal modes, in contrast, are seen as pure matter excitations that produce a dynamic polarization inside the material without photon coupling. Here we show that both longitudinal and transverse modes become polaritons in the explicitly covariant Lorenz gauge. Longitudinal excitations couple to longitudinal and scalar photons, which have been considered elusive so far. We show that the dipolar excitations become three-fold degenerate in the long-wavelength limit when including all photonic degrees of freedom, as expected from symmetry. Our findings demonstrate how choosing a gauge determines our thinking about materials excitations and how gauge fixing reveals new pathways for tailoring polaritons in crystals, metamaterials, and surfaces. Longitudinal polaritons will interact with longitudinal near fields located at surfaces, which provides additional excitation channels to engineer scanning near-field microscopy and surface-enhanced spectroscopy

Description of near-tip fracture processes in strain hardening cementitious composites using image-based analysis and the compact tension test

Strain Hardening Cementitious Composites (SHCC) can be distinguished from other types of fiber reinforced cement-based matrix composites by the typical pseudo-strain hardening behavior they develop in tension. The design of Strain Hardening Cementitious Composites (SHCC) towards the development of multiple cracking in tension is based on the micro-mechanisms involved in the cracking process, including the fiber-matrix interaction properties as well as the fiber and the matrix mechanical properties. The cracking mechanisms at the micro-scale assume a key role in the composite behavior of SHCC. Their complexity introduces, however, significant uncertainty in the entire process. The investigation of the cracking processes of SHCC materials at an intermediate level between the micro-scale and the structural length scale is therefore important to further characterize the influence of the fracture processes on the composite tensile behavior. In previous studies the mechanical behavior of SHCC materials, as well as of other strain softening fiber reinforced cementitious composites, was characterized under eccentric tensile loading using the Compact Tension Test (CTT). The present research further extends this investigation, with particular emphasis on cementitious composites reinforced with multiple types of fibers. The experimental tensile load-displacement results are discussed and compared to the numerically derived responses. To numerically predict the tensile load-displacement responses obtained with the CTTs, the cohesive crack model and the tensile stress-crack opening relationships obtained with the Single Crack Tension Test (SCTT) are utilized. Furthermore, the crack initiation and propagation at the early stages of the loading sequence are analyzed. The size of the specimens and the resolution of the digital images acquired allow the detection of relatively small displacements and crack openings. The results are discussed, with special emphasis on the topology of the cracks obtained near the crack tip and on the description of the fracture process zone.Fundação para a Ciência e a Tecnologia (FCT

Dark interlayer plasmons in colloidal gold nanoparticle bi- and few-layers

We demonstrate the excitation of dark plasmon modes with linearly polarized light at normal incidence in self-assembled layers of gold nanoparticles. Because of field retardation, the incident light field induces plasmonic dipoles that are parallel within each layer but antiparallel between the layers, resulting in a vanishing net dipole moment. Using microabsorbance spectroscopy we measured a pronounced absorbance peak and reflectance dip at 1.5 eV for bi- and trilayers of gold nanoparticles with a diameter of 46 nm and 2 nm interparticle gap size. The excitations were identified as dark interlayer plasmons by finite-difference time-domain simulations. The dark plasmon modes are predicted to evolve into standing waves when further increasing the layer number, which leads to 90% transmittance of the incident light through the nanoparticle film. Our approach is easy to implement and paves the way for large-area coatings with tunable plasmon resonance

Plasmon-Polaritons in Nanoparticle Supercrystals: Microscopic Quantum Theory Beyond the Dipole Approximation

Crystals of plasmonic metal nanoparticles have intriguing optical properties. They reach the regimes of ultrastrong and deep strong light-matter coupling, where the photonic states need to be included in the simulation of material properties. We propose a quantum description of the plasmon polaritons in supercrystals that starts from the dipole and quadrupole excitations of the nanoparticle building blocks and their coupling to photons. Our model excellently reproduces results of finite difference time domain simulations. It provides detailed insight into the emergence of the polariton states. Using the example of a face centered cubic crystals we show that the dipole and quadrupole states mix in many high symmetry directions of the Brilouin zone. A proper description of the plasmon and plasmon-polariton band structure is only possible when including the quadrupole-derived states. Our model leads to an expression of the reduced coupling strength in nanoparticle supercrystals that we show to enter the deep strong coupling regime for metal fill fractions above $0.8$. In addition to the plasmon-polariton energies we analyse the relative contributions of the dipole, quadrupole, and photonic states to their eigenfunctions and are able to demonstrate the decoupling of light in the deep strong coupling regime. Our results pave the way for a better understanding of the quantum properties of metallic nanoparticle supercrystals in the ultrastrong and deep-strong coupling regime.Comment: 30 pages, 6 figure

Liquid crystal phase and waterlike anomalies in a core-softened shoulder-dumbbells system

Using molecular dynamics we investigate the thermodynamics, dynamics and structure of 250 diatomic molecules interacting by a core-softened potential. This system exhibits thermodynamics, dynamics and structural anomalies: a maximum in density-temperature plane at constante pressure and maximum and minimum points in the diffusivity and translational order parameter against density at constant temperature. Starting with very dense systems and decreasing density the mobility at low temperatures first increases, reach a maximum, then decreases, reach a minimum and finally increases. In the pressure-temperature phase diagram the line of maximum translational order parameter is located outside the line of diffusivity extrema that is enclosing the temperature of maximum density line. We compare our results with the monomeric system showing that the anisotropy due to the dumbbell leads to a much larger solid phase and to the appearance of a liquid crystal phase. the double ranged thermodynamic and dynamic anomalies.Comment: 14 pages, 5 figure

Avaliação do desempenho de uma técnica para aplicação de pós-tensão em mantas de CFRP no confinamento de pilares de betão armado

Diversos estudos comprovam que o reforço ao confinamento de pilares de betão armado com mantas de CFRP é uma técnica extremamente eficaz no caso de pilares de secção transversal circular, mas muito pouco eficaz no caso de pilares de secção rectangular. Algumas tentativas para ultrapassar esta situação incluíram o pré-esforço do encamisamento de CFRP através da injecção sob pressão de resinas de epóxido. Contudo, os resultados foram marginalmente satisfatórios e esta técnica revelou-se bastante trabalhosa. Neste artigo, apresenta-se um estudo experimental, desenvolvido na sequência dos estudos anteriormente referidos, tendo por objectivo analisar a viabilidade de utilizar um produto comercial, uma resina expansiva na presença de água, para pré-esforçar o encamisamento de CFRP. Apresentamse os resultados das diferentes fases do estudo: ajuste da percentagem de água para obter o nível óptimo de expansão; análise da influência da espessura do espaço entre o reforço de CFRP e o elemento de betão a reforçar; avaliação do comportamento diferido do elemento reforçado durante as primeiras horas; e avaliação do comportamento até à rotura dos elementos reforçados submetidos a compressão uniaxial. Apresentam-se igualmente as conclusões relevantes do estudo

Object oriented programming : data preparation and visualization of FEM models

In this paper two object oriented applications are described. The former is intended to generate data associated with the finite element method (FEM) and the later is a three-dimensional visualization tool named 3DMesh. Both are based on the principles of object oriented programming, namely encapsulation, inheritance and polymorphism. To support the preparation of FEM data, a language named 3DO was developed. Its syntax is similar to a subset of the C++ programming language. 3DO is based on object construction and modification by methods that require a small number of arguments. With this tool, mesh generation, definition of properties and loads and mesh refinement can be performed with limited user effort, even when the model is complex. All the generated information can be visualized with the program 3DMesh. This application is based on the OpenGL library and uses the Microsoft Foundation Classes to simplify its integration in the MS-Windows environment. 3DMesh implements an interactive navigation technique that allows the visualization of the model interior, preserving its integrity. Model attributes and the results of the FEM analysis can also be visualized