Thomas Wriedt

Refractive-index measurements in the near-IR using an Abbe refractometer

A novel method to measure the refractive index n in the near-infrared by simple extensions to a standard Abbe refractometer is described. A technique is derived to correct for the dispersion of the glass prism and experimental results of refractive-index measurements at are compared with published data. These results prove the suitability of the described method, the accuracy being comparable to that of an Abbe refractometer used in the visible range; that is, the refractive index n can be measured to an accuracy of . Finally, new refractive-index data at 830 nm are given for methanol, water, acetone, ethanol, cyclohexane, glycol, di-2-ethyl hexyl-sabacate (DEHS), carbon tetrachloride, glycerol, toluene, ethyl salicylate, methyl salicylate and cinnamaldehyde at 20 and .

A Review of Elastic Light Scattering Theories

Mie scattering is an important tool for diagnosing microparticles or aerosol particles in technical or natural environments. Mie theory is restricted to spherical, homogeneous, isotropic and non-magnetic particles in a non-absorbing medium. However, as microparticles are hardly ever spherical or homogeneous, there is much interest in more advanced scattering theories. During recent decades, scattering methods for non-spherical and non-homogeneous particles have been developed and even some computer codes are readily available. Extension of Mie theory covers coated spheres, stratified spheres and clustered spheres. For homogeneous non-spherical particles such as spheroids, ellipsoids and finite cylinders, surface discretization methods have been developed. Scattering by inhomogeneous particles may be computed by volume discretization methods.

Comparison of computational scattering methods

Abstract There are various methods to compute electromagnetic scattering by arbitrarily shaped particles. The aim of this article is merely to give a short introduction to three very different types of methods and have a look at the applicabilities and shortcomings of each. At first some comments are made. • to the Discrete Dipole Approximation (DDA), alias Coupled Dipole Method (CDM), as a special form of the Volume Integral Equation Method (VIE); • to the Finite Difference Time Domain (FDTD) and • to the Extended Boundary Condition Method (EBCM). As an example the results and the parameters of different codes for a cube are compared to give just a hint of the computational demands.

Light scattering from a particle on or near a surface

Abstract The problem of light scattering by a particle on or near a surface is treated using the extended boundary condition methods solution for scattering by a particle in a homogeneous medium and the integral representation of spherical vector wave functions over plane waves for the calculation of the reflection of the scattered field by the surface. An approximate solution is also given by assuming that the scattered field, reflecting off the surface and interacting with the particle, is incident upon the surface at near-normal incidence. The range of validity of the approximate method is checked from a numerical point of view

Using the T-Matrix Method for Light Scattering Computations by Non-axisymmetric Particles: Superellipsoids and Realistically Shaped Particles

Light scattering by non-axisymmetric particles is commonly needed in particle characterization and other fields. After much work devoted to volume discretization methods to compute scattering by such particles, there is renewed interest in the T-matrix method. We extended the null-field method with discrete sources for T-matrix computation and implemented the superellipsoid shape using an implicit equation. Additionally, a triangular surface patch model of a realistically shaped particle can be used for scattering computations. In this paper some exemplary results of scattering by nonaxisymmetric particles are presented.

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.

Piezoelectric Droplet Generator for the Calibration of Particle-Sizing Instruments

Using a piezoceramic tube and a continuous glass capillary, droplets in a diameter range between 10 μm and 100 μm can be generated. This corresponds to a volume of up to 0.6 pl. The velocity of the generated droplets depends on droplet size but is constant for each diameter. The liquid can be dosed as single droplet, as an accumulation of droplets or as a chain of droplets in a frequency range between 1 Hz up to 3 kHz. The lifetime of the droplets depends on droplet size and the chemical and physical properties of the dispersed liquid. In addition, for water droplets the humidity of the air near the droplet trajectory influences the lifetime of the droplets.

Simulation of light scattering by biconcave Cassini ovals using the nullfield method with discrete sources

In this paper the nullfield method with discrete sources (NFM-DS) is applied to analysis of light scattering by biconcave Cassini-like particles, which can be described as oblate discspheres with central concavities on their top and bottom. As far as we know this is a first attempt to apply a modification of the T-matrix method to model such a nonconvex object. The numerical results for different particles under different incident angles obtained by the NFM-DS are presented in the last section of the paper. For result verification the discrete source method (DSM) has been chosen. The comparison of results obtained by using both methods shows very good agreement.

Calculation of the T matrix in the null-field method with discrete sources

The problem of computing the transition matrix (T matrix) in the framework of the null-field method with discrete sources is treated. Numerical experiments are performed to investigate the symmetry property of the T matrix when localized and distributed vector spherical functions are used for solution construction.

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.