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The study is aimed at investigating the radiative properties of soot aggregates at determined morphological features using both experimental and numerical methods. Soot aggregates collected from air monitoring stations in different locations were examined. The locations were divided into three groups. The first group (Case 1) included the coastal and industrial zone; the second group (Case 2) consisted of small and large cities; and the third group (Case 3) included areas in the neighbourhood of thermal power plants. The absorbance measurements of the soot aggregates were conducted in the visible and near-infrared spectra, and in the wavelength range of 2 μm-20 μm. The samples were characterised by scanning electron microscopy (SEM), and their radiative properties were assessed using the discrete dipole approximation (DDA) for numerically generated fractal aggregates with two popular refractive indices of m = 1.60 + 0.60i and m = 1.90 + 0.75i. Calculations were conducted for primary particles in point-contact, with 20% overlapping and with a coating (50% and 80%) in the wavelength range of 0.4–1.064 μm. The largest measured absorbance values in both the winter and summer seasons were found in the cities in Case 1, and the x-ray diffraction (XRD) phases of the samples were also presented. The radiative properties of the aggregates, i.e., Df = 1.78 and kf = 2.0 representing Case 3, were close to those of aggregates with Df = 2.1 and kf = 2.35 representing Case 1 in the investigated wavelength range. The calculated radiative properties and the experimental absorbance measurements for point-contact and overlapping situations showed the same trend in the examined wavelengths. The absorbance properties of the samples of coastal and industrial zones were distinctively higher than others in the wavelength range of 2 μm-20 μm which could be attributed to the PAH effects.

We study and compare spectral properties of various volume-integral-equation formulations. The equations are written for the electric flux, current, field, and potentials, and discretized with basis functions spanning the appropriate function spaces. Each formulation leads to eigenvalue distributions of different kind due to the effects of discretization procedure, namely, the choice of basis and testing functions. The discrete spectrum of the potential formulation reproduces the theoretically predicted spectrum almost exactly while the spectra of other formulations deviate from the ideal one. It is shown that the potential formulation has the spectral properties desired from the preconditioning perspective. (C) 2016 Elsevier Ltd. All rights reserved.

With the development of nanotechnology, many new optical phenomena in nanoscale have been demonstrated. Through the coupling of optical waves and collective oscillations of free electrons in metallic nanostructures, surface plasmon polaritons can be excited accompanying a strong near field enhancement that decays in a subwavelength scale, which have potential applications in the surface-enhanced Raman scattering, biosensor, optical communication, solar cells, and nonlinear optical frequency mixing. In the present article, we review the Green's matrix method for solving the surface plasmon resonances and near field in arbitrarily shaped nanostructures and in binary metallic nanostructures. Using this method, we design the plasmonic nanostructures whose resonances are tunable from the visible to near-infrared, study the interplay of plasmon resonances, and propose a new way to control plasmonic resonances in binary metallic nanostructures.

A microphysically based algorithm (named Snow Aggregation and Melting (SAM)) that models snowflakes composed of a collection of hexagonal columns by simulating a random aggregation process is presented. SAM combines together pristine columns with multiple dimensions to derive complex aggregates constrained to size-mass relationship obtained by data collected from in situ measurements. The model also simulates the melting processes occurring for environmental temperatures above 0 degrees C and thus define the mixed-phase particles structure. The single-scattering properties of the modeled snowflakes (dry and mixed phased) are computed by using a discrete dipole approximation (DDA) algorithm which allows to model irregularly shaped targets. In case of mixed-phased particles, realistic radiative properties are obtained by assuming snow aggregates with a 10% of melted fraction. The single-scattering properties are compared with those calculated through Mie theory together with Maxwell-Garnett effective medium approximation using both a homogeneous sphere and a layered-sphere models. The results show that for large-size parameters there are significant differences between the radiative properties calculated using complex microphysical and optical algorithms (i.e., SAM and DDA) and those obtained from simplified assumptions as the layered-sphere models (even when the radial ice density distribution of the aggregated snowflakes is perfectly matched). Finally, some applications to quantitative precipitation estimation using radar data are presented to show how the resulting differences in the basic optical properties would propagate into radar measurable. Large discrepancies in the derivation of the equivalent water content and snowfall rate from radar measurements could be observed when large-size parameters are accounted for.

Cataloged from PDF version of article. Optical trapping and manipulation of micrometre-sized particles was first reported in 1970. Since then, it has been successfully implemented in two size ranges: the subnanometre scale, where light-matter mechanical coupling enables cooling of atoms, ions and molecules, and the micrometre scale, where the momentum transfer resulting from light scattering allows manipulation of microscopic objects such as cells. But it has been difficult to apply these techniques to the intermediate - nanoscale - range that includes structures such as quantum dots, nanowires, nanotubes, graphene and two-dimensional crystals, all of crucial importance for nanomaterials-based applications. Recently, however, several new approaches have been developed and demonstrated for trapping plasmonic nanoparticles, semiconductor nanowires and carbon nanostructures. Here we review the state-of-the-art in optical trapping at the nanoscale, with an emphasis on some of the most promising advances, such as controlled manipulation and assembly of individual and multiple nanostructures, force measurement with femtonewton resolution, and biosensors.

International audience; Understanding the plasmon coupling between metal nanoparticles under light irradiation remains a challenging issue for optimizing plasmonic devices for chemical and biological sensing. Here, the optical properties of dense spatially ordered two-dimensional arrays of 50 nm gold nanoparticles are investigated in this aim. Microspectrophotometry experiments are carried out on square arrays and parallel chains, elaborated by electron beam lithography, having different periodicities ranging from 80 to 170 nm. The wavelength, width, and amplitude of the localized surface plasmon resonance (SPR) are quantitatively monitored as a function of both the incident light polarization and the interparticle distance. The experimental findings are then compared with calculations based on the discrete dipole approximation. They match remarkably well these numerical results, not only for the spectral location and the width of the SPR but also for the absolute value of the extinction cross section of the nanoparticles that is linked with the local field enhancement. The variation of the SPR band characteristics as a function of the periodicity of the arrays is investigated in terms of near field and far field coupling effects by discussing the topography of the field amplitude and phase in the arrays Moreover, in order to highlight the influence of phase retardation and radiative effects on the plasmon coupling, the optical properties of equivalent arrays scaled by 1/10, which can be qualitatively described by electrostatic dipolar interactions, are also studied by numerical calculations The discrepancies then observed between the two scales are interpreted. The contributions of the radiative and nonradiative dampings to the width and magnitude of the plasmon resonance are also investigated.

The dominant features in the UV/visible spectra of metallic nanoparticles are governed by collective oscillations of valence electrons. These resonances are called localized surface plasmon (LSP) and have positions, widths and intensities that depends strongly on the nanoparticle’s geometry and environment but also on the nature of the probing excitation. When one is interested in coupled nanoparticles, the plasmon modes of these systems can be described with an hybridization scheme (Prodan et al. Science 302(5644):419–422, 2003). With help of this approach, modes can be classified in “bright” or “dark” mode according to whether they have a net dipole moment. In the last case, the mode cannot be excited with traditional optical excitation but one has to use a focused beam of fast electrons having electric field with low spatial extension that allows local stimulation (Nelayah et al. Nat Phys 3:348–353, 2007). Here, we are interested in the simulation of the extinction and electron energy loss (EEL) spectra of isolated and coupled metallic particles. To achieve this, we have developed a new approach based on the Discrete- Dipole Approximation (DDA) (Draine, Astrophys J 333:848–872, 1988; Geuquet and Henrard, Ultramicroscopy 110:1075–1080, 2010) that uses eigenvectors of the particle’s propagator to make a truncated basis for physical quantities (Guillaume et al. Phys Rev B 88(245439), 2013). The response of the cluster of particles is then obtained by adding interaction terms which account for multiple scattering between the particles.

International audience; A novel one-pot synthesis method to prepare gold nanorods of different aspect ratios using radiolysis is presented. The gold nanorods are synthesized by radiolytic reduction of Au-III complexes in a micellar solution formed by a mixture of cationic surfactants (cetyltrimethylammonium bromide and tetraoctylammonium bromide) and in the presence of acetone and silver ions. The mechanisms of reduction of gold complexes and formation of the nanorods are discussed. The nanorods can be trapped in a poly(vinyl alcohol) gel formed under irradiation. The UV-visible absorption spectra of nanorod solutions are compared with calculations performed by using the discrete dipole approximation method, underlying the effect of the rod aspect ratio distribution.

The resonance behavior of localized surface plasmons in silver and gold nanoparticles was studied in the visible and near-infrared regions of the electromagnetic spectrum. Arrays of nano-sized gold (Au) and silver (Ag) particles with different properties were produced with electron-beam lithography technique over glass substrates. The effect of the particle size, shape variations, period, thickness, metal type, substrate type and sulfidation were studied via transmission and reflectance measurements. The results are compared with the theoretical calculations based on the DDA simulations performed by software developed in this study. We propose a new intensity modulation technique based on localized surface plasmons in nanoparticles with asymmetric shapes. (C) 2010 Optical Society of America

In this paper a theoretical study of polarization properties of a silver nanosphere touching a homogeneous silver substrate and covered by oxide layers of increasing thickness, is reported. Oxide layers are often deposited on metallic nanostructures in metal-enhanced fluorescence (MEF) or surface-enhanced raman scattering experiments to avoid nonradiative energy transfer from emitters to the metal, and to increase the nanoparticles stability against thermal processes and laser exposure. Not much has been said on the effect of the oxide on the field enhancement of such kind of plasmonic systems. This work aims at filling this gap by shedding light on the effects of the oxide coverage on the near and far field behavior: numerical simulations performed in the framework of the discrete dipole approximation show the presence of new resonances in the absorption spectra and, of major importance for MEF applications, a strong enhancement of the near field around the nanosphere.

Recent developments in nanofabrication and optical near-field metrology have faced complementary modeling techniques with new demands. We present a surface integral formulation that accurately describes the extreme near-field of a plasmonic nanoparticle in addition to its far-field properties. Flexible surface meshing gives precise control over even complex geometries allowing investigation of the effects of fabrication accuracy and material homogeneity on a particle's optical response. Using this technique, the influence of a particle's symmetry and shape on surrounding "hot spots" of extremely large field enhancement is explored, giving insight into the mechanisms of surface enhanced Raman scattering and single-molecule detection techniques.

Engineering the spectral properties of fluorophores, such as the enhancement of luminescence intensity, can be achieved through coupling with surface plasmons in metallic nanostructures(1-11). This process, referred to as metal-enhanced fluorescence, offers promise for a range of applications, including LEDs, sensor technology, microarrays and single-molecule studies. It becomes even more appealing when applied to colloidal semiconductor nanocrystals, which exhibit size-dependent optical properties, have high photochemical stability, and are characterized by broad excitation spectra and narrow emission bands(12). Other approaches have relied upon the coupling of fluorophores ( typically organic dyes) to random distributions of metallic nanoparticles or nanoscale roughness in metallic films(1-4,6,8). Here, we develop a new strategy based on the highly reproducible fabrication of ordered arrays of gold nanostructures coupled to CdSe/ZnS nanocrystals dispersed in a polymer blend. We demonstrate the possibility of obtaining precise control and a high spatial selectivity of the fluorescence enhancement process.

Calculation of electromagnetic cross sections from surface integral equation simulations, a popular approach in microwave studies and recently also in optics and plasmonics, requires only a single post-processing step, which can, however, be very sensitive to the precision of the simulation result. We investigate the accuracy and robustness of two methods for cross section calculation, displaying when and why errors may occur, in certain cases even unphysical behavior. A calculation recipe which avoids unphysical results is given, ensuring convergence of all obtained cross sections. This study will help judge the accuracy of performed simulations and can prevent misinterpretation of modeling results.

The utilization of the Green's tensor associated with a complex optical background (surface, cavity or stratified medium) leads to a dramatic reduction of the computation effort associated with scattering calculations in that background. This approach is illustrated with examples where a mere change of the background Green's tenser makes possible the investigation of completely different physical situations. Two different discretization approaches are compared and similarities between the Green's tensor technique and the Method of Lines are emphasized; in the latter, the utilization of analytic solutions in one specific direction also reduces the discretization of the system.

We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a non-regular cross-section in the 20-50 nm range. After briefly recalling the physical properties of metals at optical frequencies, we point out the intrinsic difficulties in the computation of the plasmon resonances for nanoparticles with a non-regular shape. We then consider the resonance spectra corresponding to nanowires whose cross-sections form different simplexes. The number of resonances strongly increases when the section symmetry decreases: A cylindrical wire exhibits one resonance, whereas we observe more than 5 distinct resonances for a triangular particle. The spectral range covered by these different resonances becomes very large, giving to the particle specific distinct colors. At the resonance, dramatic field enhancement is observed at the vicinity of non-regular particles, where the field amplitude earl reach several hundred times that of the illumination field. This near-field enhancement corresponds to surface enhanced Raman scattering (SERS) enhancement locally in excess of 10(12). The distance dependence of this enhancement is investigated and we show that it depends on the plasmon resonance excited in the particle, i.e. on the illumination wavelength. The average Raman enhancement for molecules distributed on the entire particle surface is also computed and discussed in the context of experiments in which large numbers of molecules are used. Finally we discuss the influence of the model permittivity which enters the calculation, as well as the resonances shift and broadening produced by a water background.

A surface integral formulation for light scattering on periodic structures is presented. Electric and magnetic field equations are derived on the scatterers' surfaces in the unit cell with periodic boundary conditions. The solution is calculated with the method of moments and relies on the evaluation of the periodic Green's function performed with Ewald's method. The accuracy of this approach is assessed in detail. With this versatile boundary element formulation, a very large variety of geometries can be simulated, including doubly periodic structures on substrates and in multilayered media. The surface discretization shows a high flexibility, allowing the investigation of irregular shapes including fabrication accuracy. Deep insights into the extreme near-field of the scatterers as well as in the corresponding far-field are revealed. This method will find numerous applications for the design of realistic photonic nanostructures, in which light propagation is tailored to produce novel optical effects. (C) 2010 Optical Society of America

We present a new technique for computing the electromagnetic field that propagates and is scattered in three-dimensional structures formed by bodies embedded in a stratified background. This fully vectorial technique is based on the Green's tensor associated with the stratified background. Its advantage lies in the fact that only the scatterers must be discretized, the stratified background being accounted for in the Green's tensor. Further, the boundary conditions at the different material interfaces as well as at the edges of the computation window are perfectly and automatically fulfilled. Several examples illustrate the utilization of the technique for the modeling of photonic circuits (integrated optical waveguides), the study of the optics of metal (surface plasmons), and the development of new optical lithography techniques. (C) 2001 Optical Society of America.

Optical parameters of simulated ambient individual ammonium sulfate and soot-mixed particles were calculated using the discrete-dipole approximation method with different model geometries. Knowledge of the mixing state and the approximation by a suited idealized geometry reduces the errors of the optical properties by +/-50% to +/-10%. The influence of the soot content and the mixing state on the optical properties of the total aerosol was estimated. For the total aerosol population, the size distribution and the absolute soot content had the largest influence. The exact geometry of the ammonium sulfate and soot-mixed particles can be neglected.

We present a technique for the computation of the Green's tensor in three-dimensional stratified media composed of an arbitrary number of layers with different permittivities and permeabilities (including metals with a complex permittivity). The practical implementation of this technique is discussed in detail. In particular, we show how to efficiently handle the singularities occurring in Sommerfeld integrals, by deforming the integration path in the complex plane. Examples assess the accuracy of this approach and illustrate the physical properties of the Green's tensor, which represents the field radiated by three orthogonal dipoles embedded in the multilayered medium.

We develop a fully vectorial formalism for the investigation of electromagnetic scattering in polarizable backgrounds, i.e., where the scatterers are not in vacuum but situated in a medium with a dielectric permittivity different from unity. Our approach is based on the Green's tensor technique and the corresponding Green's tensors for two-dimensional (2D) and three-dimensional (3D) systems are developed. The analysis of 2D systems is not restricted to the case where transverse electric (TE) and transverse magnetic (TM) modes are decoupled, but treated in a general manner. Practical examples illustrate the application of the method: scattering by a microcavity for two dimensions and color formation in opal for three dimensions.

We describe a library to compute various types of Green's tensor for three-dimensional electromagnetic scattering calculations. This library includes the retarded and non-retarded (quasi-static) Green's tensors for infinite homogeneous space and the non-retarded Green's tensor associated with a surface. Both standard and filtered Green's tensor can be computed. Filtered Green's tensor can be used to accurately investigate high permittivity scatterers with the coupled-dipole approximation. (C) 2002 Elsevier Science B.V. All rights reserved.

A novel method is elaborated for the electromagnetic scattering from periodical arrays of scatterers embedded in a polarizable background. A dyadic periodic Green's function is introduced to calculate the scattered electric field in a lattice of dielectric or metallic objects. The method exhibits strong advantages: discretization and computation of the field are restricted to the volume of the scatterers in the unit cell, open and periodic boundary conditions for the electric field are included in the Green's tensor, and finally both near and far-fields physics are directly revealed, without any additional computational effort. Promising applications include the design of periodic structures such as frequency-selective surfaces, photonic crystals and metamaterials.

This work gives a detailed derivation of the non-retarded dyadic Green's tensor associated with surfaces in the quasistatic approximation. The derivation is made from a rigorous model where the dyadic is expressed as Sommerfeld integrals. We then assess the domain where this approximation can be used for scattering calculations on surfaces by comparing rigorous and non-retarded solutions. Implications of this work for scattering calculations in near-field optics are finally discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

Among the most popular approaches used for simulating plasmonic systems, the discrete dipole approximation suffers from poorly scaling volume discretization and limited near-field accuracy. We demonstrate that transformation to a surface integral formulation improves scalability and convergence and provides a flexible geometric approximation allowing, e.g., to investigate the influence of fabrication accuracy. The occurring integrals can be solved quasi-analytically, permitting even rapidly changing fields to be determined arbitrarily close to a scatterer. This insight into the extreme near-field behavior is useful for modeling closely packed particle ensembles and to study "hot spots" in plasmonic nanostructures used for plasmon-enhanced Raman scattering. (C) 2009 Optical Society of America

Resonant waveguide gratings (RWGs), also known as guided mode resonant (GMR) gratings or waveguide-mode resonant gratings, are dielectric structures where these resonant diffractive elements benefit from lateral leaky guided modes from UV to microwave frequencies in many different configurations. A broad range of optical effects are obtained using RWGs such as waveguide coupling, filtering, focusing, field enhancement and nonlinear effects, magneto-optical Kerr effect, or electromagnetically induced transparency. Thanks to their high degree of optical tunability (wavelength, phase, polarization, intensity) and the variety of fabrication processes and materials available, RWGs have been implemented in a broad scope of applications in research and industry: refractive index and fluorescence biosensors, solar cells and photodetectors, signal processing, polarizers and wave plates, spectrometers, active tunable filters, mirrors for lasers and optical security features. The aim of this review is to discuss the latest developments in the field including numerical modeling, manufacturing, the physics, and applications of RWGs. Scientists and engineers interested in using RWGs for their application will also find links to the standard tools and references in modeling and fabrication according to their needs.