[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

1. Getting Started

Angora simulations are run by constructing a text file, called the configuration file that specifies all aspects of the simulation. This file is then given as a command-line option to the Angora executable angora; which reads the configuration file and produces the desired output (see Execution).

Let’s start with a simple example. In the following, we will show how Angora can be used to solve the problem of electromagnetic scattering from a sphere illuminated by a plane wave. The geometry of the scattering problem is shown in the figure below.

sph_sc

Figure 1.1: Scattering from a sphere illuminated by a plane wave incident from the -z direction.

We start by creating a configuration file; say ‘sph_sc.cfg’. This file will be populated by configuration options listed in the following. Some basic parameters of our simulation are determined by the following lines (see Grid Properties for details):

 
dx = 20e-9;
courant = 0.98;
grid_dimension_x = 1e-6;
grid_dimension_y = 1e-6;
grid_dimension_z = 1e-6;
pml_thickness_in_cells = 5;
num_of_time_steps = 1500;

The first variable, dx, is the uniform spatial step size in the FDTD discretization. The second variable, courant, is the ratio of the time step to the maximum time step allowable by the Courant condition. The next three variables determine the physical size of the simulation grid in meters. The thickness of the absorbing layer (PML) is determined by the pml_thickness_in_cells variable. The last line specifies the number of time steps in the simulation.

The sphere from which the electromagnetic plane wave will be scattered is created in two steps. First, we define a spherical "shape object" using the Spheres variable (see Spheres):

 
Shapes:
{
    Spheres:
    (
        {
            shape_tag = "mysphere";
            center_coord_x = 0;
            center_coord_y = 0;
            center_coord_z = 0;
            radius = 320e-9;
        }
    );
};

Next, the material filling the sphere is defined using the Materials variable (see Materials):

 
Materials:
(
    {
        material_tag = "sph_mat";
        rel_permittivity = 2.25;
        electric_conductivity = 3e4; //in Siemens/m
        rel_permeability = 1.7;
        magnetic_conductivity = 4.2578e9; //in Ohm/m
    }
);

The shape and material definitions are then combined in the Objects variable, and the sphere is placed in the grid (see Objects):

 
SimulationSpace:
{
    Objects:
    (
        {
            material_tag = "sph_mat";
            shape_tag = "mysphere";
        }
    );
};

With the above defitions, we have created a sphere of radius 320 nm and made of the material specified by "sph_mat". Next, we define the waveform of the incident plane wave using the Waveforms variable:

 
Waveforms:
{
    ModulatedGaussianWaveforms:
    (
        {
            waveform_tag = "mywaveform";
            modulation_type = "sine";
            tau = 2.12662e-15;
            f_0 = 5.88878e14;
        }
    );
};

We then create the plane wave incident from the -z direction with the above waveform as its electric field using the PlaneWaves variable (see Plane Waves):

 
TFSF:
{
    PlaneWaves:
    (
        {
            theta = 180;
            phi = 0;
            psi = 90;
            waveform_tag = "mywaveform";
        }
    );
};

Finally, we create a near-field-to-far-field transformer to calculate the scattered field in the far zone using the PhasorDomainNFFFT variable (see Near-Field-to-Far-Field Transformer):

 
PhasorDomainNFFFT:
(
    {
        num_of_lambdas = 1;
        lambda_min = 509.09e-9;
        lambda_max = 509.1e-9;
        direction_spec = "theta-phi";
        num_of_dirs_1 = 360;
        dir1_min = 0;
        dir1_max = 360;
        num_of_dirs_2 = 1;
        dir2_min = 0;
        dir2_max = 0;
    }
);

With the above definitions, the far field is calculated at the free-space wavelength 509.1 nm, and 360 equally-spaced angles on the xz plane. The output of the near-field-to-far-field transformer is in HDF5 format, which can be read and manipulated using freely-available tools. For more information, see Near-Field-to-Far-Field Transformer. The absolute value of the phasor component of the far-zone electric field (normalized by 1/r) at 509.1 nm is shown in a polar plot in the figure below.

sph_pattern

Figure 1.2: The absolute scattered electric field phasor amplitude on the xz plane at 509.1 nm.

The scattered electric field can also be obtained theoretically using Mie theory (see Matzler02), which is shown alongside the Angora solution in the above figure.


[ < ] [ > ]   [ << ] [ Up ] [ >> ]

This document was generated by Ilker Rafet Capoglu on December 12, 2012 using texi2html 1.82.