Developing Scattering Morphology Resolved Total Internal Reflection Microscopy (SMR-TIRM) for Orientation Detection of Colloidal Ellipsoids.

Micrometer scale colloidal particles experiencing ∼kT scale interactions and suspended in a fluid are relevant to a broad spectrum of applications. Often, colloidal particles are anisotropic, either by design or by nature. Yet, there are few techniques by which ∼kT scale interactions of anisotropic particles can be measured. Herein, we present the initial development of scattering morphology resolved total internal reflection microscopy (SMR-TIRM). The hypothesis of this work is that the morphology of light scattered by an anisotropic particle from an evanescent wave is a sensitive function of particle orientation. This hypothesis was tested with experiments and simulations mapping the scattered light from colloidal ellipsoids at systemically varied orientations. Scattering morphologies were first fitted with a two-dimensional (2D) Gaussian surface. The fitted morphology was parameterized by the morphology's orientation angle Mϕ and aspect ratio MAR. Data from both experiments and simulations show Mϕ to be a function of the particle azimuthal angle, while MAR was a sensitive function of the polar angle. This analysis shows that both azimuthal and polar angles of a colloidal ellipsoid could be resolved from scattering morphology as well or better than using bright-field microscopy. The integrated scattering intensity, which will be used for determining the separation distance, was also found to be a sensitive function of particle orientation. A procedure for interpreting these confounding effects was developed that in principle would uniquely determine the separation distance, the azimuthal angle, and the polar angle. Tracking these three quantities is necessary for calculating the potential energy landscape sampled by a colloidal ellipsoid.

Numerical method for analyzing the near-field enhancement of nonspherical dielectric-core metallic-shell particles accounting for the nonlocal dispersion.

Over the last few decades, dielectric core and metallic plasmonic shell (Die@Me) nanoparticles have found a wide variety of applications. The trend to reduce the thickness of the metallic coating requires to account for the influence of the nonlocal dispersion on the spectral response of such nanoparticles. In this paper, we use the discrete sources method and the generalized nonlocal optical response model to describe the nonlocality within the plasmonic metal shell. We found that the variation of the plasmonic shell thickness and the elongation of the nonspherical core-shell particle can enlarge the near-field enhancement and the absorption cross section by an order of magnitude. Besides, we show that the nonlocal dispersion can decrease the field enhancement in the wavelength domain up to 2.5 times with a small blue-shift of about 5 nm.

Numerical simulation of Electron Energy Loss Spectroscopy using a Generalized Multipole Technique.

We numerically simulate low-loss Electron Energy Loss Spectroscopy (EELS) of isolated spheroidal nanoparticles, using an electromagnetic model based on a Generalized Multipole Technique (GMT). The GMT is fast and accurate, and, in principle, flexible regarding nanoparticle shape and the incident electron beam. The implemented method is validated against reference analytical and numerical methods for plane-wave scattering by spherical and spheroidal nanoparticles. Also, simulated electron energy loss (EEL) spectra of spherical and spheroidal nanoparticles are compared to available analytical and numerical solutions. An EEL spectrum is predicted numerically for a prolate spheroidal aluminum nanoparticle. The presented method is the basis for a powerful tool for the computation, analysis and interpretation of EEL spectra of general geometric configurations.

Profile estimation for Pt submicron wire on rough Si substrate from experimental data.

An efficient forward scattering model is constructed for penetrable 2D submicron particles on rough substrates. The scattering and the particle-surface interaction are modeled using discrete sources with complex images. The substrate micro-roughness is described by a heuristic surface transfer function. The forward model is applied in the numerical estimation of the profile of a platinum (Pt) submicron wire on rough silicon (Si) substrate, based on experimental Bidirectional Reflectance Distribution Function (BRDF) data.

The T-Matrix method in electron energy loss and cathodoluminescence spectroscopy calculations for metallic nano-particles.

In this paper, we present the application of the T-Matrix method (TMM) for the calculation of Electron Energy Loss Spectra (EELS), cathodoluminescence spectra (CLS) and far-field patterns produced by metallic nano-particles. Being frequently used in electromagnetic scattering calculations, the TMM provides an efficient tool for EELS calculations as well and can be employed, e.g. for the investigation of nano-antennas.

Comparison of numerical methods in near-field computation for metallic nanoparticles.

Four widely used electromagnetic field solvers are applied to the problem of scattering by a spherical or spheroidal silver nanoparticle in glass. The solvers are tested in a frequency range where the imaginary part of the scatterer refractive index is relatively large. The scattering efficiencies and near-field results obtained by the different methods are compared to each other, as well as to recent experiments on laser-induced shape transformation of silver nanoparticles in glass.

Experimental verification of an exact evanescent light scattering model for TIRM.

Total internal reflection microscopy (TIRM) is a method for the precise measurement of interaction potentials between a spherical colloidal particle and a wall. The method is based on single-particle evanescent wave light scattering. The well-established model used to interpret TIRM data is based on an exponential relation between scattering intensity and particle wall distance. However, applying this model for a certain range of experimental parameters leads to significant distortions of the measured potentials. Using a TIRM setup based on a two-wavelength illumination technique, we were able to directly measure the intensity distance relation revealing deviations from an exponential decay. The intensity-distance relations could be compared to scattering simulations taking into account exact experimental parameters and multiple reflections between a particle and the wall. Converging simulation results were independently obtained by the T-matrix method and the discrete sources method (DSM) and show excellent agreement with experiments. Using the new scattering model for data evaluation, we could reconstruct the correct potential shape for distorted interaction potentials as we demonstrate. The comparison of simulations to experiment intrinsically yields a new method to determine absolute particle-wall distances, a highly desired quantity in TIRM experiments.

Single-particle evanescent light scattering simulations for total internal reflection microscopy.

We simulate and measure light scattering of a micrometer-sized spherical particle suspended in solution close to a glass substrate. The model, based on the discrete sources method, is developed to describe the experimental situation of total internal reflection microscopy experiments; i.e., the particle is illuminated by an evanescent light field originating from the glass-solvent interface. In contrast to the well-established assumption of a simple exponential decay of the scattering intensity with distance, we demonstrate significant deviations for a certain range of penetration depths and polarization states of the incident light.