Discrete Dipole Approximation Research Articles

A comprehensive study on the Rod-based plasmonic structures sensing using the modified discrete dipole approximation method

Using the previously introduced modified discrete dipole approximation (DDA) method by applying the full details of the nanoparticle and its surrounding environment, the detection sensitivity of molecules by plasmonic Rod-based nanosensors, including U-shaped and Rod-shaped structures, was investigated. In the calculations, the two factors of the magnitude of the wavelength shift and the ability to distinguish molecules with similar properties were of significant interest. The results indicated that the sensitivity of Rod-based nanostructures is significantly higher than that of spherical nanoparticles. Among the plasmonic Rod-based nanosensors, the silver U-shaped structure performs better than others. The wavelength shift of the absorption spectrum of different nanosensors for a given molecule was very different, making it possible to detect very similar molecules from each other by testing different sensors.

Reducing shape errors in the discrete dipole approximation using effective media.

The discrete dipole approximation (DDA) simulates optical properties of particles with any given shape based on the volume discretization. These calculations cost a large amount of time and memory to achieve high accuracy, especially for particles with large sizes and complex geometric structures, such as mixed black-carbon aerosol particles. We systematically study the smoothing of the DDA discretization using the effective medium approximation (EMA) for boundary dipoles. This approach is tested for optical simulations of spheres and coated black-carbon (BC) aggregates, using the Lorenz-Mie and multiple-sphere T-Matrix as references. For spheres, EMA significantly improves the DDA accuracy of integral scattering quantities (up to 60 times), when the dipole size is only several times smaller than the sphere diameter. In these cases, the application of the EMA is often comparable to halving the dipole size in the original DDA, thus reducing the simulation time by about an order of magnitude for the same accuracy. For a coated BC model based on transmission electron microscope observations, the EMA (specifically, the Maxwell Garnett variant) significantly improves the accuracy when the dipole size is larger than ¼ of the monomer diameter. For instance, the relative error of extinction efficiency is reduced from 4.7% to 0.3% when the dipole size equals that of the spherical monomer. Moreover, the EMA-DDA achieves the accuracy of 1% for extinction, absorption, and scattering efficiencies using three times larger dipoles than that with the original DDA, corresponding to about 30 times faster simulations.

Aerosols are not spherical cows: using discrete dipole approximation to model the properties of fractal particles

ABSTRACT The optical properties of particulate-matter aerosols, within the context of exoplanet and brown dwarf atmospheres, are compared using three different models: Mie theory, modified mean field (MMF) theory, and discrete dipole approximation (DDA). Previous results have demonstrated that fractal haze particles (MMF and DDA) absorb much less long-wavelength radiation than their spherical counterparts (Mie), however it is shown here that the opposite can also be true if a more varying refractive index profile is used. Additionally, it is demonstrated that absorption/scattering cross-sections, and the asymmetry parameter, are underestimated if Mie theory is used. Although DDA can be used to obtain more accurate results, it is known to be much more computationally intensive; to avoid this, the use of low-resolution aerosol models is explored, which could dramatically speed up the process of obtaining accurate computations of optical cross-sections within a certain parameter space. The validity of DDA is probed for wavelengths of interest for observations of aerosols within exoplanet and brown dwarf atmospheres ($0.2-15~\mu$m). Finally, novel code is presented to compare the results of Mie, MMF, and DDA theories (coral: Comparison Of Radiative AnaLyses), as well as to increase and decrease the resolution of DDA shape files accordingly (spherify). Both codes can be applied to a range of other interesting astrophysical environments in addition to exoplanet atmospheres, for example dust grains within protoplanetary discs.

Light-to-Heat Conversion of Optically Trapped Hot Brownian Particles.

Anisotropic hybrid nanostructures stand out as promising therapeutic agents in photothermal conversion-based treatments. Accordingly, understanding local heat generation mediated by light-to-heat conversion of absorbing multicomponent nanoparticles at the single-particle level has forthwith become a subject of broad and current interest. Nonetheless, evaluating reliable temperature profiles around a single trapped nanoparticle is challenging from all of the experimental, computational, and fundamental viewpoints. Committed to filling this gap, the heat generation of an anisotropic hybrid nanostructure is explored by means of two different experimental approaches from which the local temperature is measured in a direct or indirect way, all in the context of hot Brownian motion theory. The results were compared with analytical results supported by the numerical computation of the wavelength-dependent absorption efficiencies in the discrete dipole approximation for scattering calculations, which has been extended to inhomogeneous nanostructures. Overall, we provide a consistent and comprehensive view of the heat generation in optical traps of highly absorbing particles from the viewpoint of the hot Brownian motion theory.

Convergence of the DDA for ensembles of objects of irregular shape

The discrete dipole approximation (DDA) is commonly used to compute light-scattering properties of irregularly shaped particles. The DDA maps the particle into an array of cubic cells with side d < λ. For a randomly oriented irregularly shaped particle, DDA has been shown accurate when kd|m| ≤ 1, where k is the wavenumber and m is the particle refractive index. We demonstrate that the DDA yields robust results even when kd|m| ≈ 1.2 when applied to ensembles of irregularly shaped dielectric particles and kd|m| ≈ 1.3 for conductive particles. This finding can greatly reduce the computational load required for performing such computations.

Optics Determines the Electrochemiluminescence Signal of Bead-Based Immunoassays.

Electrochemiluminescence (ECL) is an optical readout technique that is successfully applied for the detection of biomarkers in body fluids using microbead-based immunoassays. This technology is of utmost importance for in vitro diagnostics and thus a very active research area but is mainly focused on the quest for new dyes and coreactants, whereas the investigation of the ECL optics is extremely scarce. Herein, we report the 3D imaging of the ECL signals recorded at single microbeads decorated with the ECL labels in the sandwich immunoassay format. We show that the optical effects due to the light propagation through the bead determine mainly the spatial distribution of the recorded ECL signals. Indeed, the optical simulations based on the discrete dipole approximation compute rigorously the electromagnetic scattering of the ECL emission by the microbead and allow for reconstructing the spatial map of ECL emission. Thus, it provides a global description of the ECL chemical reactivity and the associated optics. The outcomes of this 3D imaging approach complemented by the optical modeling provide insight into the ECL optics and the unique ECL chemical mechanism operating on bead-based immunoassays. Therefore, it opens new directions for mechanistic investigations, ultrasensitive ECL bioassays, and imaging.

Design and fabrication of Cu conical (Spiral+Helical) sculptured thin films and their application in SERS

Cu conical (spiral + helical (spiral (3 turns) + helical (2, 12 and 17 turns)) nanostructures were grown on stainless steel mesh (SSM) and glass microscope slide by glancing angle deposition (GLAD). The SSM substrates were used in surface enhanced Raman spectroscopy (SERS) for detection of 4,4′-Bipyridine (4,4′-Bipy) molecules. This design fulfilled the main electromagnetic mechanisms of enhancement of Raman signals, which are near field coupling and the lightning rod effect. The size of the sculptured nanostructures produced on the SSM substrates matched the wavelength of the laser beam of the Raman spectrometer and satisfied the requirements for the enhancement of the surface Raman spectroscopy of the 4,4′-Bipy molecules. A comparison is made on the morphology of the produced nanostructures on SSM, microscope slide and previously published works on other surfaces with the view to different surfaces with different surface roughness. Field emission scanning electron microscope (FESEM) analysis of the samples clearly revealed the morphology of the designed and fabricated nanostructure as conical (spiral + helical) features on both SSM and glass substrates. Crystal structures and optical spectra of the produced samples obtained using grazing angle incident x-ray diffraction (GIXRD) and Vis-NIR spectroscopy. The SERS results showed that although according to the GIXRD data the coated Cu films were oxidized, good enhancement is obtained. Hence, this can be considered as a favor to the applied fields of these samples. Vis-NIR results showed that as the number of turns of the helical part of the structure increased, the absorption of the spectra increased consistent with the discrete dipole approximation (DDA) results reported in the literature. Surface-enhanced Raman spectroscopy results of this work when compared with the available published data showed higher/comparable enhancement irrespective of the type of material used (e.g., Ag, Au and Mn), 4,4′-Bipy concentration, Bipy substrate shape, the method of fabrication and the excitation wavelength of the Raman spectrometer.

The debris disc of HD 131488: bringing together thermal emission and scattered light

ABSTRACT We show the first SPHERE/IRDIS and IFS data of the CO-rich debris disc around HD 131488. We use N-body simulations to model both the scattered light images and the spectral energy distribution of the disc in a self-consistent way. We apply the Henyey–Greenstein approximation, Mie theory, and the Discrete Dipole Approximation to model the emission of individual dust grains. Our study shows that only when gas drag is taken into account can we find a model that is consistent with scattered light as well as thermal emission data of the disc. The models suggest a gas surface density of 2 × 10−5 M⊕ au−2 which is in agreement with estimates from ALMA observations. Thus, our modelling procedure allows us to roughly constrain the expected amount of gas in a debris disc without actual gas measurements. We also show that the shallow size distribution of the dust leads to a significant contribution of large particles to the overall amount of scattered light. The scattering phase function indicates a dust porosity of ∼0.2…0.6 which is in agreement with a pebble pile scenario for planetesimal growth.

A comparative study of efficient iterative solvers for the discrete dipole approximation

The discrete dipole approximation (DDA) is used to compute the electromagnetic diffraction of a three-dimensional object. Computationally, the DDA involves solving large, dense systems of linear equations through iterative methods such as QMR, GPBiCG and BiCGstab. In this paper, we propose to study two new methods (IDR(s) and GPBiCGstab(L)) for objects larger than the wavelength of illumination. We show that while IDR(s) can present a reduced computation time compared to other methods, it may not converge in some cases. Conversely, GPBiCGstab(L) always converges and also has a reduced computation time compared to QMR, GPBiCG and BiCGstab.

Fast and accurate electromagnetic field calculation for substrate-supported metasurfaces using the discrete dipole approximation

Abstract Metasurface design tends to be tedious and time-consuming based on sweeping geometric parameters. Common numerical simulation techniques are slow for large areas, ultra-fine grids, and/or three-dimensional simulations. Simulation time can be reduced by combining the principle of the discrete dipole approximation (DDA) with analytical solutions for light scattered by a dipole near a flat surface. The DDA has rarely been used in metasurface design, and comprehensive benchmarking comparisons are lacking. Here, we compare the accuracy and speed of three DDA methods—substrate discretization, two-dimensional Cartesian Green’s functions, and one-dimensional (1D) cylindrical Green’s functions—against the finite difference time domain (FDTD) method. We find that the 1D cylindrical approach performs best. For example, the s-polarized field scattered from a silica-substrate-supported 600 × 180 × 60 nm gold elliptic nanocylinder discretized into 642 dipoles is computed with 0.78 % pattern error and 6.54 % net power error within 294 s, which is 6 times faster than FDTD. Our 1D cylindrical approach takes advantage of parallel processing and also gives transmitted field solutions, which, to the best of our knowledge, is not found in existing tools. We also examine the differences among four polarizability models: Clausius–Mossotti, radiation reaction, lattice dispersion relation, and digitized Green’s function, finding that the radiation reaction dipole model performs best in terms of pattern error, while the digitized Green’s function has the lowest power error.

Atomistic polarization model for Raman scattering simulations of large metal tips with atomic-scale protrusions at the tip apex

Abstract Tip-enhanced Raman spectroscopy (TERS) has recently been developed to push the spatial resolution down to single-chemical-bond scale. The morphology of the scanning tip, especially the atomistic protrusion at the tip apex, plays an essential role in obtaining both high spatial resolution and large field enhancement at the Ångström level. Although it is very difficult to directly characterize the atomistic structures of the tip, the Raman scattering from the apex’s own vibrations of the metal tip can provide valuable information about the stacking of atoms at the tip apex. However, conventional quantum chemistry packages can only simulate the Raman scattering of small metal clusters with few atoms due to huge computational cost, which is not enough since the shaft of the tip behind the apex also makes significant contributions to the polarizabilities of the whole tip. Here we propose an atomistic polarization model to simulate the Raman spectra of large metal tips at subwavelength scales based on the combination of the atomistic discrete dipole approximation model and the density functional theory. The atomistic tip with different sizes and stacking structures is considered in its entirety during the calculation of polarizabilities, and only the vibrational contributions from the tip apex are taken into account to simulate the Raman spectra of the tip. The Raman spectral features are found to be very sensitive to the local constituent element at the tip apex, atomic stacking modes, and shape of the tip apex, which can thus be used as a fingerprint to identify different atomistic structures of the tip apex. Moreover, our approaches can be extended to the metal tips with sub-wavelength sizes, making it possible to consider both the large scale and the atomistic detail of the tip simultaneously. The method presented here can be used as a basic tool to simulate the Raman scattering process of the metal tips and other nanostructures in an economic way, which is beneficial for understanding the roles of atomistic structures in tip- and surface-enhanced spectroscopies.

Sensitivity of Localized Surface Plasmon Resonance and Acoustic Vibrations to Edge Rounding in Silver Nanocubes.

Precise knowledge of the dependence of nano-object properties on their structural characteristics such as their size, shape, composition, or crystallinity, in turn, enables them to be finely characterized using appropriate techniques. Spectrophotometry and inelastic light scattering spectroscopy are noninvasive techniques that are proving highly robust and efficient for characterizing the optical response and vibrational properties of metal nano-objects. Here, we investigate the optical and vibrational properties of monodomain silver nanocubes synthesized by the chemical route, with edge length ranging from around 20 to 58 nm. The synthesized nanocrystals are not perfectly cubic and exhibit rounded edges and corners. This rounding was quantitatively taken into account by assimilating the shape of the nanocubes to superellipsoids. The effect of rounding on their optical response was clearly evidenced by localized surface plasmon resonance spectroscopy and supported by calculations based on the discrete dipole approximation method. The study of their acoustic vibrations by high-resolution low-frequency Raman scattering revealed a substructure of the T2g band, which was analyzed as a function of rounding. The measured frequencies are consistent with the existence of an anticrossing pattern of the two T2g branches. Such an avoided crossing in the T2g modes is clearly evidenced by calculating the vibrational frequencies of silver nanocubes using the Rayleigh-Ritz variational method that accounts for both their real size, shape, and cubic elasticity. These results show that it is possible to assess the rounding of nanocubes, including by means of ensemble spectroscopic measurements on well-calibrated particles.

Simulating the sensing properties of silver nanoparticles for identifying molecular structures using modified discrete dipole approximation method ‎

Simulating the sensing properties of silver nanoparticles for identifying molecular structures using modified discrete dipole approximation method ‎

Synergistic effects of gold nanoparticles enhanced the sensitivity of enzyme-linked immunosorbent assay for latex allergens

Hevea brasiliensis rubber elongation factor (Hev b1), a non-glycosylated protein, is a notable allergen in natural latex products. This study aims to assess enzyme-linked immunosorbent assay (ELISA) combined with gold nanoparticles (AuNPs) as a Hev b1 detection substrate. AuNP synthesis precursor and reducing agent concentrations were optimized using factorial design and multiple linear regression (MLR). The optimized conditions yielded spherical AuNPs with an average particle size of 73 ± 23 nm. This substrate enhanced detection sensitivity by around 50% compared to the limit of detection of the bare substrate, in the linear range of 0.25–1.75 μg/mL. Additionally, the heightened absorbance intensity observed during Hev b1 ELISA signifies both augmented surface immobilization of the analyte and optical induction arising from the aggregation of suitably small AuNPs (<80 nm) into dimer clusters, supported by discrete dipole approximation (DDA). Overall, the findings highlighted the synergistic effects of AuNPs in enhancing the performance of ELISA for Hev b1 detection.

Flexible scattering order formulation of the discrete dipole approximation.

The discrete dipole approximation (DDA) is a well-known method for computation of the scattering of light from nonspherical particles. Here, we present a new scattering order formulation (SOF) of the DDA that allows the user to represent the scattering particle with higher flexibility than in conventional DDAs, while the computer memory required always scales as O(N). In our new SOF, the user can locate each dipole independently, or off-grid, in space, assign each dipole a unique size and a unique dipole shape as appropriate, and assign each dipole a unique magnetoelectric polarizability with no constraints. The cost of this flexibility is that the computation time is increased from O(N l o g N) to O(N 2). To compensate, our model allows the user to vary the range of dipole interaction in a unique manner. We find that, in cases in which the scatterer has at least one dimension that is sufficiently small compared with the wavelength, a relatively small number of iterations is required for convergence of the simulation, and in addition, a small dipole interaction range can be invoked to reduce the computation time to O(N) while still producing results that are sufficiently accurate.

Radiative properties of non-spherical opacifiers doped in silica aerogels for high-temperature thermal insulation

Silica aerogel, a super-insulating material, has garnered significant attention due to its extensive thermal insulation applications. However, the high-temperature performance of silica aerogels is compromised due to their infrared transparency. This study tackles this issue by investigating the influence of opacifier shape on the insulation properties of silica aerogel composites. Employing the Discrete Dipole Approximation (DDA) theory, the radiative properties of opacifiers of varying shapes are investigated. The study also employs a theoretical model to ascertain the effective thermal conductivity of silica aerogels doped with non-spherical opacifiers. To counter oxidation at ultra-high temperatures, an innovative ellipsoidal [email protected] core–shell opacifier is introduced. Non-spherical opacifiers are further optimized to identify the ideal particle diameters and volume fractions for specific temperature ranges. The investigation uncovers that non-spherical carbon black opacifiers with low shape parameter ratios considerably enhance the insulating performance at elevated temperatures. [email protected] core–shell opacifiers, distinguished by high inner/outer radius ratios, offer superior thermal stability and great infrared radiation extinction. In summary, this study offers a pragmatic approach to fabricating aerogels with enhanced insulation properties at high temperatures.

Numerical study on optimization of quantitative treatment conditions for skin cancer photothermal therapy considering multiple blood vessels

BackgroundIn recent years, lasers have gained considerable attention as a potential treatment option in the medical field. Photothermal therapy, in particular, has been investigated as a technique to remove tumor tissue by leveraging photothermal effects. The method involves raising the temperature of the tumor tissue to destroy it and has primarily been studied for skin cancer treatment. ObjectiveThis study aimed to simulate a skin layer with squamous cell carcinoma by using numerical modeling and investigate the effect of different numbers of blood vessels on the temperature distribution in the medium under conditions such as varied laser intensity and gold nanoparticle volume fraction. MethodsOptical properties of the light absorption enhancer were calculated using the discrete dipole approximation method, and the temperature and velocity distribution were computed using continuity, momentum, and energy equations. ResultsQuantitative determination of the apoptotic variable was performed to evaluate the treatment effect for each case, and the treatment condition with the maximum treatment effect was identified. Laser intensity with optimal therapeutic effect was confirmed to be 0.13 W, 0.15 W, 0.18 W, and 0.24 W, depending on the number of vessels, respectively, and the volume fraction of injected GNRs was confirmed to be 10−6 for all vessel numbers. ConclusionThe results of this study can serve as a guide for selecting appropriate treatment conditions when conducting photothermal therapy in the future

Spectral analysis of oxidation on localized surface plasmon resonance of copper nanoparticles thin film

Copper nanoparticles (CuNPs) possess localized surface plasmon resonance (LSPR) effect. Cu thin films composed of individual CuNPs exhibit stronger LSPR than the individual CuNPs due to the LSPR coupling among CuNPs. However, CuNPs are easy to be oxidized, which results in the rapid LSPR damping of the CuNPs thin films. Simulation of the variations of the coupled LSPR of two adjacent CuNPs with the thickness of oxide shells formed during oxidation is of great importance for understanding the mechanisms of the strong LSPR of CuNPs thin films and its rapid attenuation. In this paper, Discrete-dipole approximation method is used to simulate the extinction spectra of two adjacent spherical CuNPs as a function of the shell thickness (t), the ambient refractive index (n), the diameter (D) of the CuNPs, and the inter-nanoparticle spacing (L). The calculation is validated by experimental results. According to our model, for a definite CuNPs thin films, the oxide shell thickness of CuNPs can be calculated only if the extinction spectra and the morphology are provided. Further, it is found when the oxide shell thickness is small (t/R< 0.3), increasing n and decreasing L/D have an obvious synergistic effect on enhancing the coupled LSPR, but this synergistic effect weakens with the deepening of oxidation, and disappeared when t/R > 0.5. This study provides a calculation method for coupled core-shell nanoparticles and throws light on the role of oxidation on the rapid damped LSPR of CuNPs thin films.

Effects of microstructure and moisture content on the radiative properties of porous films for radiative cooling

Global warming is a major concern, and active cooling technology, which consumes substantial energy, contributes to global warming. Radiative cooling (RC) is a form of passive cooling. An optically selective porous film can provide cost-effective and efficient radiative cooling. This study aims to investigate the theoretical effects of the structural parameters on the cooling performance of porous films. An improved diffusion-limited cluster aggregation (DLCA) algorithm is established to reproduce porous structures with moisture content. A combination of discrete dipole approximation and Monte Carlo method was employed to predict the structure-related solar reflectance and “sky window” emissivity. An optimal design of a porous RC film was developed. The results demonstrate that the BaSO4 porous film can achieve high solar reflectance (∼ 97%), high “sky window” emissivity (∼ 99%), and a net cooling power of 102 W/m2 during the day. In particular, the effect of moisture content on the cooling performance of the porous RC film was analyzed. Moisture content enhances the cooling performance at night but decreases it during the daytime. The results provide guidance for designing optically selective porous films for radiative cooling.

Aggregation-induced enhancements in aerosol absorption and scattering across the black-brown continuum

Global radiative forcing by carbonaceous aerosols is one of the largest sources of uncertainty in current climate models. The radiative impacts of brown carbon (BrC), a type of amorphous organic carbonaceous aerosol formed in biomass burning events, such as wildfires, remains poorly understood. Recently, the aggregation of spheres (monomers) of BrC was observed in wildfire smoke from various parts of the world. Aggregation could alter the optical properties and direct radiative forcing of BrC, yet very little is known about this phenomenon. This study improves upon the spectral optical properties of BrC aggregates observed in past field studies.We are motivated by the framework presented by Saleh et al. (2020) for the optical classification of carbonaceous particles across the black-brown continuum. Here, we simplify Saleh et al.'s BrC categorization into two sub-classes: dark brown carbon (d-BrC) and weakly-absorbing brown carbon (w-BrC). We calculate and compare the mass absorption cross-section (MAC), mass scattering cross-section (MSC), single scattering albedo (SSA), and asymmetry parameter (g) of d-BrC and w-BrC and determine their absorption and scattering enhancements due to aggregation. Polydisperse diffusion-limited cluster-cluster aggregation (p-DLCA) simulations generated 90 aggregates of varying monomer diameters, and discrete dipole approximation (DDA) was used to calculate optical properties of these aggregates at relevant refractive indices, monomer diameters, and incident wavelengths.We find that optical properties of BrC aggregates are more sensitive to the monomer number and mean diameter than to polydispersity. d-BrC has almost twice the MAC of w-BrC at 350 nm, but w-BrC has MSC values almost double that of d-BrC. Both MAC and MSC for both types of aerosol decrease with wavelength. Aggregation enhances optical properties, with smaller size parameters and lower imaginary part of the complex refractive index of aggregates resulting in stronger absorption and scattering.