Discrete Dipole Approximation Calculations Research Articles

Stokes–Mueller polarization-based analysis of model SARS-CoV-2 virions

Understanding the virology of the coronavirus at the structural level has gained utmost importance to overcome the constant and long-term health complications induced by them. In this work, the light scattering properties of SARS-CoV-2 of size 140 nm were simulated by using discrete dipole approximation (DDA) for two incident wavelengths 200 nm and 350 nm, respectively. Three different 3-dimensional (3D) models of SARS-CoV-2 corresponding to 15, 20, and 40 numbers of spike proteins on the viral capsid surface were constructed as target geometries for the DDA calculations. These models were assessed by employing Stokes–Mueller polarimetry to obtain individual polarization properties such as degree of polarization (DOP), degree of linear polarization (DOLP), and degree of circular polarization (DOCP). Irrespective of its spike numbers, all the coronavirus models were found to display higher DOP and DOCP values and negligibly small DOLP values for circularly polarized incident light, indicating the presence of chiral structures. On the other hand, the lack of understanding about the dependence of the Mueller matrix on its microstructural properties was overcome by transforming 16 Mueller elements into sub-matrices with specific structural and physical properties using Lu–Chipman-based Mueller matrix polar decomposition method. The obtained properties such as retardance, diattenuation, and depolarization were used for investigating the composition and microstructural information. The approach presented in this work has the potential to understand the virology of the coronavirus at the structural level and, therefore, will be beneficial in developing effective detection strategies by exploiting their characteristic electromagnetic scattering signatures.

Optical Absorption Spectra of Star-Shaped Au–Ag Nanoparticles by Discrete Dipole Approximation Calculation Considering Highly Symmetrical Models

Optical Absorption Spectra of Star-Shaped Au–Ag Nanoparticles by Discrete Dipole Approximation Calculation Considering Highly Symmetrical Models

Cautious note on using the discrete dipole approximation for inhomogeneous, spherical scatterers with high optical contrast

The discrete dipole approximation (DDA) is capable of treating scatterers with arbitrary shape and composition. However, large values of the refractive index require additional considerations. DDA calculations are performed for small spheres with a stronger absorbing inclusion and compared to T-matrix results. A natural phenomenon with strong optical contrasts is the melting of ice hydrometeors at microwave frequencies, and hence corresponding refractive indices are chosen. The obtained extinction and absorption efficiencies are found to depend mainly on the dipole size, whereas the phase function closely follows the T-matrix results by choosing a smaller stopping criterion and changing the polarizability formulation.

Toward laser-induced tuning of plasmonic response in high aspect ratio gold nanostructures

Abstract High aspect-ratio gold nanostructures sustain Fabry–Perot-like surface plasmon responses from infrared to visible light energies. We show that some resonances can be tuned by means of laser irradiation, where low energy modes stay unperturbed. After laser irradiation, gold nanowires’ tips are transformed into nanoparticles of various sizes joint to gold nanowires, producing high aspect-ratio half-dumbbells and dumbbells structures. The plasmonic behaviour of both the nanowires and the newly created nanostructures has been characterised by in-depth monochromated electron energy loss spectroscopy (EELS) developed in a transmission electron microscope (TEM) and state-of-the-art discrete dipole approximation (DDA) calculations. All these analyses serve as experimental proof of the selective tuning (or robustness) of the plasmonic modes of the nanostructures in a specific spectral range, which is of critical interest regarding applications for sensing devices, nano-sources or nanophotonic waveguide, as well as optical remote control.

Revisiting the Hail Radar Reflectivity–Kinetic Energy Flux Relation by Combining T-Matrix and Discrete Dipole Approximation Calculations to Size Distribution Observations

Abstract The retrieval of hail kinetic energy with weather radars or its simulation in numerical models is challenging because of the shape complexity and variable density of hailstones. We combine 3D scans of individual hailstones with measurements of the particle size distributions (PSD) and T-matrix calculations to understand how hail reflectivity Z changes when approximating hailstones as spheroids, as compared to the realistic shapes obtained by 3D scanning technology. Additionally, recent terminal velocity relations are used to compare Z to the hail kinetic energy flux . We parameterize the hail backscattering cross sections at L, S, C, and X bands as a function of size between 0.5 and 5.0 cm, matching the range of the observed PSDs. The scattering calculations use the T-matrix method for size parameters below 1.0 and the discrete dipole approximation (DDA) method otherwise. The DDA calculations are done for 48 digital models of realistic hailstones of sizes between 1 and 5 cm. The DDA cross sections are calculated for multiple orientations and averaged assuming a fully random orientation distribution to provide a single value per hailstone. The T-matrix reflectivity assuming solid ice spheres presents negligible differences to DDA results for size parameters below 1.0. Therefore, T matrix was used to fill in the gaps left by the DDA calculations. The results are mapped to the same size bins of the observed PSDs, allowing the calculation of the radar reflectivity. This is then correlated to , allowing a potential improvement of past retrieval methods of from Z in multiple wavelengths.

Colloidal Nanospheres of Amorphous Selenium: Facile Synthesis, Size Control, and Optical Properties

AbstractThis article reports a facile method for the colloidal synthesis of nanospheres made of amorphous selenium (ɑ‐Se). The diameters of the nanospheres can be tightly controlled in the range of 45–157 nm by varying the reaction time and temperature, as well as the amount of the reducing agent. The average size of the nanospheres is measured using transmission electron microscopy and dynamic light scattering while their optical properties are characterized using UV‐vis spectroscopy. We also compute their UV‐vis spectra using the discrete dipole approximation (DDA) method and the calculated spectra are consistent with the experimental data based on the UV‐vis spectra and the particle sizes derived from electron microscopy. The data derived from DDA calculations provides a fitting curve for finely controlling the sizes of the ɑ‐Se nanospheres by monitoring the UV‐vis spectra of the reaction solution at different stages of a synthesis.

Formation of CdS/ZnS nanoparticles in polymer matrix by SILAR method: Experiments and exploring its optical properties with DDA calculations

Composite nanostructures of Cadmium Sulfide (CdS) and Zinc sulfide (ZnS) were formed within the Cellulose Acetate matrix using successive ionic layer adsorption and reaction (SILAR) method, and their morphological and elemental characterizations were done employing field emission electron microscopy (FESEM) and energy dispersive X-ray (EDX) analyses. Experimental and theoretical investigations of the optical absorbance spectra of the prepared samples were performed by Uv-Visible spectroscopy and discrete dipole approximation (DDA) calculations respectively. The results show that when the absorbing part of the polymer matrix is composed of only CdS, increasing the number of SILAR cycles results in enlarging the absorbing part and a redshift in the wavelength of the maximum of the absorbance spectra. On the contrary, when the two materials CdS and ZnS have a contribution in the formation of the absorbing part, without paying attention to material distribution topology, and the absorbing part size, the abovementioned wavelength will remain unchanged.

SnowScatt 1.0: consistent model of microphysical and scattering properties of rimed and unrimed snowflakes based on the self-similar Rayleigh–Gans approximation

Abstract. More detailed observational capabilities in the microwave (MW) range and advancements in the details of microphysical schemes for ice and snow demand increasing complexity to be included in scattering databases. The majority of existing databases rely on the discrete dipole approximation (DDA) whose high computational costs limit either the variety of particle types or the range of parameters included, such as frequency, temperature, and particle size. The snowScatt tool is innovative in that it provides consistent microphysical and scattering properties of an ensemble of 50 000 snowflake aggregates generated with different physical particle models. Many diverse snowflake types, including rimed particles and aggregates of different monomer composition, are accounted for. The scattering formulation adopted by snowScatt is based on the self-similar Rayleigh–Gans approximation (SSRGA), which is capable of modeling the scattering properties of large ensembles of particles. Previous comparisons of SSRGA and DDA are extended in this study by including unrimed and rimed aggregates up to centimeter sizes and frequencies up to the sub-millimeter spectrum. The results generally reveal the wide applicability of the SSRGA method for active and passive MW applications. Unlike DDA databases, the set of SSRGA parameters can be used to infer scattering properties at any frequency and refractive index; snowScatt also provides tools to derive the SSRGA parameters for new sets of particle structures, which can be easily included in the library. The flexibility of the snowScatt tool with respect to applications that require continuously changing definitions of snow properties is demonstrated in a forward simulation example based on the output of the predicted particle properties (P3) scheme. The snowScatt tool provides the same level of flexibility as commonly used T-matrix solutions, while the computed scattering properties reach the level of accuracy of detailed discrete dipole approximation calculations.

Insight on the Coupling of Plasmonic Nanoparticles from Near-Field Spectra Determined via Discrete Dipole Approximations.

Coupling between plasmonic nanoparticles (NPs) in nanoparticle assemblies has been investigated extensively via far-field properties, such as absorption and scattering, but very rarely via near-field properties, and a quantitative investigation of near-field properties should provide great insight into the nature of the coupling. We report a numerical procedure to obtain reliable near-field spectra (Q NF) around spherical gold nanoparticles (Au NPs) using Discrete Dipole Approximation (DDA). The reliability of the method was tested by comparing Q NF from DDA calculations with exact results from the Mie theory. We then applied the method to examine Au NPs assembled in dimer, trimer, and up to pentamer in a linear arrangement. For the well-studied dimer system, we show that the Q NF enhancement, due to coupling in longitudinal mode, is much greater than the enhancement in Q ext. There is a linear correlation between the Q NF and Q ext peak positions, with the Q NF peak redshifted from the Q ext peak by an average of approximately 12 nm. In the case of the multimers, Q NF spectra from individual spheres were not always identical and become dependent on the sphere location. In the longitudinal model, the center sphere has the strongest Q NF spectra. For the transverse mode, we differentiate two different scenario, transverse-Y where both electric field (E) and light propagation vector (k) are perpendicular the chain axis, and transverse-X where k is parallel to the chain axis. In transverse-Y mode, coupling leads to reduced Q NF spectra and the center sphere has the lowest Q NF intensity. In transverse-X mode, there is retardation effect from the front sphere to the back sphere. The Q NF from the front sphere is stronger than from the back sphere. In addition, due to the phase lag in k-direction, the Q NF in transverse-X can differ quite significantly from transverse-Y for large particles. All these results could be understood when one considers how electric field from induced dipoles on neighboring NPs add on or subtract from the incident E-field. These results provide new insight into the coupling properties of Au NPs.

Bias in quantification of light absorption enhancement of black carbon aerosol coated with low-volatility brown carbon

Brown carbon (BrC) is an optically defined class of organic carbon (OC) which strongly absorbs light at shorter visible and ultraviolet-A (UVA) wavelengths. Both light absorbing and non-absorbing OC have been found to coat black carbon (BC) and could modulate the optical properties of BC. Thermodenuders are conventionally used in conjunction with instruments measuring particle light absorption, such as photoacoustic spectrometers, to parametrize enhanced BC light absorption (Eabs) due to OC. This method can introduce unquantified measurement artifacts because thermodenuders do not remove low-volatility compounds at the temperatures they are commonly operated at and due to temperature-dependent loss of particles within the instrument. These artifacts could lead to errors in accurately estimating Eabs for coated BC particles. The value of Eabs can also be estimated by comparing the mass absorption cross-section (MAC) of coated BC aerosol to literature-based MAC values of uncoated BC. In this study, we use two integrated photoacoustic-nephelometer spectrometers, operating at wavelengths of 375 nm and 532 nm, respectively, to quantify the differences and errors associated with measuring Eabs using these two methods. Our results indicate differences ranging from 25% to 65% for Eabs measured using both methods at 375 nm. We observed little to no enhancement at either wavelength using the thermodenuder method. In contrast, the Eabs value increased with the OC/BC ratio at 375 nm using the literature-based MAC method. The difference between the two methods was attributed to the presence of low-volatility brown carbon, and these results were corroborated using discrete dipole approximation calculations. Theoretical calculations predicted that these low-volatility OC could have values of the imaginary part of the refractive index up to 0.32 at a wavelength of 375 nm, which is in line with previous studies that found large absorption by low-volatility organics.Copyright © 2021 American Association for Aerosol Research

Absorption and scattering of light by soot aggregates with uniform and pendular ring coatings

The magnitude of radiative forcing by soot is strongly dependent on its morphology and mixing state, which continuously evolve during atmospheric aging. One of the major soot aging mechanisms is via interaction with condensable trace-gas chemicals, and our previous study has shown the formation of two distinct coating distributions on fractal soot aggregates, primarily depending on the radius of monomer spheres and supersaturation of the condensing vapor. The two coating distributions, spherical shells around monomers and pendular rings in junctions between monomers, differ profoundly in their ability to restructure soot aggregates. Here, we investigate the impact of these two distributions on the optical properties of soot, using Discrete Dipole Approximation (DDA) calculations, which are also compared against the Rayleigh Debye Gans (RDG) approximation and the core-shell model. For most optical parameters, the difference between the two coating distributions was minor, except for the asymmetry parameter, where it was significant (5–13%). We found that the placement of condensate in junctions significantly depressed the asymmetry parameter for most coating volumes, thus promoting isotropic scattering by soot. Compared to a uniform condensate distribution, the presence of coating between monomers always resulted in slightly larger absorption, scattering and single scattering albedo (SSA), with the difference reaching ~10% for scattering at high coating volumes. We found that the RDG approximation underestimated absorption, but was able to predict scattering to within ~3%. On the other hand, the core-shell model significantly overestimated absorption, scattering, and SSA, especially for thinly coated soot.

Capabilities of ADDA code for nanophotonics

The open-source code ADDA is based on the discrete dipole approximation (DDA) – a numerically exact method derived from the frequency-domain volume-integral Maxwell equation. It can simulate interaction of electromagnetic fields (scattering and absorption) with finite 3D objects of arbitrary shape and composition. Besides standard sequential execution on CPU or GPU, ADDA can run on a multiprocessor distributed-memory system, parallelizing a single DDA calculation. This together with almost linear scaling of computational complexity with number of dipoles (discretization voxels) allows large system sizes and/or fine discretization levels. ADDA is written in C99 and is highly portable. It provides full control over the scattering geometry (particle morphology and orientation, incident beam) and allows one to calculate a wide variety of integral and angle-resolved quantities, including those related to point-dipole excitation. Moreover, ADDA can rigorously and efficiently account for plane homogeneous substrate near the particle, and employ rectangular-cuboid voxels. It also incorporates a range of state-of-the-art DDA improvements, increasing both the accuracy and computational speed of the method. At the conference we will describe the main features of current version of ADDA with special emphasis on nanoparticles and present several simulation examples.

Near-field and far-field optical properties of magnetic plasmonic core-shell nanoparticles with non-spherical shapes: A discrete dipole approximation study

The combined optical and magnetic properties of magnetic-plasmonic core-shell nanoparticles (NPs) makes them ideal candidates for many applications in biomedical fields. Plasmonic properties of the shell gives rise to Surface Enhanced Raman Scattering (SERS) that can be utilized for sensitive detections, while magnetic properties are useful for magnetic separation and magnetic guided delivery. The plasmonic properties of the shell depends on both the size and shape of the core and shell, and this property, in principle, can be calculated using the Discrete Dipole Approximation (DDA) method. However, since the DDA is an approximation method, its accuracy to calculate the plasmonic properties of the shell, especially the near-field enhancement relevant to SERS, has not been examined carefully. We present a systematic test on the accuracy of the DDA to calculate the plasmonic properties in terms of both the extinction spectra and the near-field enhancement of the magnetic-plasmonic core-shell NPs. Accuracy of the DDA method was first investigated in comparison to Mie theory results for spherical core-shell NPs, since Mie theory gives the exact solution to spherical shaped particles. DDA calculations were further extended to core-shell nanoparticles with octahedral cores. We elucidate convergence of the DDA results by considering the effects of dipole distance and shell thickness in regard to the NP spectral properties. This work validates application of the DDA methods for calculating electrodynamic properties of core-shell NPs and highlights plasmonic properties of core-shell with non-spherical cores.

Up-Conversion Luminescence Properties of Lanthanide-Gold Hybrid Nanoparticles as Analyzed with Discrete Dipole Approximation.

Up-conversion nanoparticles (UCNP) under near-infrared (NIR) light irradiation have been well investigated in the field of bio-imaging. However, the low up-conversion luminescence (UCL) intensity limits applications. Plasmonic modulation has been proposed as an effective tool to adjust the luminescence intensity and lifetime. In this study discrete dipole approximation (DDA) was explored concerning guiding the design of UCNP@mSiO2-Au structures with enhanced UCL intensity. The extinction effects of gold shells could be changed by adjusting the distance between the UCNPs and the Au NPs by synthesized tunable mesoporous silica (mSiO2) spacers. Enhanced UCL was obtained under 808 nm irradiation. The original theoretical predictions could not be demonstrated to full extend by experimental data, indicating that better models for simulation need to take into account in homogeneities in particle morphologies. Yet, one very certain conclusion resulting from the DDA calculations and experiments is that the absorbance can blue-shift with more Au NPs added and the absorbance can-red shift for samples with enhanced silica sizes in the UCNP@mSiO2-Au structures. Furthermore, when the DDA model is more consistent with the practical structure (dispersed Au NPs instead of Au shell), the theoretical absorbance occurs almost the same as the practical absorbance. All in all, the DDA could fit the extinction effect of Au perfectly and be suitable for guiding how to design the UCNP and Au.

Plasmon-Enhanced Photodegradation of Ionic Liquids with Ag Nanocubes/ZnO Microsphere Composites

Ionic liquids (ILs) have been a hot topic in the past decades because of their unique properties and promising applications in various areas. However, the toxicity of ILs and their possible risk to the environment and aquatic organisms have also been confirmed. Therefore, it is imperative to seek an efficient way to remove or degrade ILs in the polluted aqueous system. In this work, Ag nanocubes/ZnO microspheres composites have been designed, prepared, and used for the photocatalytic degradation of eight commonly used imidazolium-based ILs in aqueous solution. It is found that ZnO semiconductor photocatalyst with an Ag content of 1.12 at. % can degrade 90% of all these ILs within 7 h. The structural characterization, photoluminescence analysis, and discrete dipole approximation calculations showed that the loading of Ag nanocubes onto ZnO microspheres significantly improves the performance of ZnO for the IL degradation through hot electron transfer and electrical field enhancement. In addition, the formed...

The effect of poly vinyl alcohol matrix on the light absorbance of Ag2S nanoparticles; experimental and discrete dipole approximation results

Well-ordered films of silver sulfide (Ag2S) nanoparticles (NPs) embedded in the matrix of poly vinyl alcohol (PVA) were fabricated using Successive Ionic Layer Adsorption and Reaction (SILAR) technique. Enlarging the size and increasing the concentration of Ag2S NPs by increasing the number of SILAR cycles confirmed by surface scanning electron microscopy (SEM) images of the produced films. The material composition was studied using energy dispersive X-ray (EDX) analysis. UV–visible experimental spectra of samples were compared with discrete dipole approximation (DDA) calculations results and good agreements were achieved. Increasing the number of peaks and valleys and redshift of peaks by increasing the number of SILAR cycles were obviously observed from the results. The most interesting conclusion of this work raised from electric near-field contour distributions investigations and implies that a matrix of insulator material (PVA) can cause an asymmetry in the intensity distribution of electric field around Ag2S NPs and let more dipole oscillations happen and more peaks appear in extinction/absorption spectra.

Soot light absorption and refractive index during agglomeration and surface growth

Abstract Optical characterization of soot (e.g., Laser Induced Incandescence, LII) and its impact on climate largely depend on light absorption. As soot grows, its morphology changes affecting its optical or radiative properties. Here, the impact of soot maturity on its light absorption is investigated by coupling Discrete Element Modeling (DEM) with Discrete Dipole Approximation (DDA) during soot surface growth and agglomeration. The Mass Absorption Cross-section, MAC, of nascent and mature soot agglomerates is estimated by DDA and validated against atomistic point dipole interactions and mesoscale DDA calculations. Using a refractive index, RI, for mature soot yields constant average and absorption function overestimating the nascent soot light absorption up to 75%. The RI is interpolated between those of nascent and mature soot for wavelengths, λ = 532 and 1064 nm to account for quantum confinement and evolving number of clustered sp2-bonded rings that affect the optical band gap, Eg. This results in excellent agreement of the DEM-derived evolutions of , and ratio R = E ( λ = 532 nm ) > E ( λ = 1064 nm ) > with the corresponding LII measurements in methane and ethylene premixed flames. The nascent soot Eg decreases during aging and agglomeration, increasing and by 65%. The R decreases from 1.34 to 0.95 by aging and coagulation and slowly converges to the asymptotic 0.89 of mature soot measured in diffusion flames. The good agreement between DEM and LII data confirms that soot dynamics by surface growth and agglomeration strongly correlate with soot maturity, composition and RI that are essential for quantifying soot light absorption and scattering.

Nanoparticle-Enhanced Silver-Nanowire Plasmonic Electrodes for High-Performance Organic Optoelectronic Devices.

Improved performance in plasmonic organic solar cells (OSCs) and organic light-emitting diodes (OLEDs) via strong plasmon-coupling effects generated by aligned silver nanowire (AgNW) transparent electrodes decorated with core-shell silver-silica nanoparticles (Ag@SiO2 NPs) is demonstrated. NP-enhanced plasmonic AgNW (Ag@SiO2 NP-AgNW) electrodes enable substantially enhanced radiative emission and light absorption efficiency due to strong hybridized plasmon coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) modes, which leads to improved device performance in organic optoelectronic devices (OODs). The discrete dipole approximation (DDA) calculation of the electric field verifies a strongly enhanced plasmon-coupling effect caused by decorating core-shell Ag@SiO2 NPs onto the AgNWs. Notably, an electroluminescence efficiency of 25.33 cd A-1 (at 3.2 V) and a power efficiency of 25.14 lm W-1 (3.0 V) in OLEDs, as well as a power conversion efficiency (PCE) value of 9.19% in OSCs are achieved using hybrid Ag@SiO2 NP-AgNW films. These are the highest values reported to date for optoelectronic devices based on AgNW electrodes. This work provides a new design platform to fabricate high-performance OODs, which can be further explored in various plasmonic and optoelectronic devices.

Probing plasmon resonances of individual aluminum nanoparticles

The plasmon resonances of individual aluminum nanoparticles are investigated by electron energy-loss spectroscopy (EELS) in scanning transmission electron microscope (STEM). Surface plasmon mode and bulk plasmon mode of Al nanoparticles are clearly characterized in the EEL spectra. Discrete dipole approximation (DDA) calculations show that as the particle diameter increases from 20 nm to 100 nm, the plasmon resonance shifts to lower energy and higher mode of surface plasmon arises when the diameter reaches 60 nm and larger.

Optical characterization of multi-scale morphologically complex heterogeneous media – Application to snow with soot impurities

A multi-scale methodology for the radiative transfer analysis of heterogeneous media composed of morphologically-complex components on two distinct scales is presented. The methodology incorporates the exact morphology at the various scales and utilizes volume-averaging approaches with the corresponding effective properties to couple the scales. At the continuum level, the volume-averaged coupled radiative transfer equations are solved utilizing (i) effective radiative transport properties obtained by direct Monte Carlo simulations at the pore level, and (ii) averaged bulk material properties obtained at particle level by Lorenz–Mie theory or discrete dipole approximation calculations. This model is applied to a soot-contaminated snow layer, and is experimentally validated with reflectance measurements of such layers. A quantitative and decoupled understanding of the morphological effect on the radiative transport is achieved, and a significant influence of the dual-scale morphology on the macroscopic optical behavior is observed. Our results show that with a small amount of soot particles, of the order of 1ppb in volume fraction, the reduction in reflectance of a snow layer with large ice grains can reach up to 77% (at a wavelength of 0.3 µm). Soot impurities modeled as compact agglomerates yield 2–3% lower reduction of the reflectance in a thick show layer compared to snow with soot impurities modeled as chain-like agglomerates. Soot impurities modeled as equivalent spherical particles underestimate the reflectance reduction by 2–8%. This study implies that the morphology of the heterogeneities in a media significantly affects the macroscopic optical behavior and, specifically for the soot-contaminated snow, indicates the non-negligible role of soot on the absorption behavior of snow layers. It can be equally used in technical applications for the assessment and optimization of optical performance in multi-scale media.