Input files

Parameters specified in the input file

In the following, the parameters which can be specified in the input file are listed:

Length unit

Declare here the units in which you want to specify lengths and angles. The length unit has no influence on the calculations and can be chosen arbitrarily. This field is mainly there to remind the user that all lengths have to be specified in consistent units. In addition, it is used for the axis annotation of output plots. The angle units can be ‘degree’, otherwise radians are assumed.

length unit: nm

angle unit: degree

Vacuum wavelength

The vacuum wavelength \(\lambda\) of the electromagnetic field, in the specified length unit:

vacuum wavelength: 550

Layer system

Define the background geometry of the layered medium. A layer system consists of \(N\) layers, counted from bottom to top. Each layer is characterized by its thickness as well as its (real) refractive index \(n\) and extinction coefficient \(k\) (the latter is equivalent to the imaginary part of the complex refractive index \(\tilde{n}=n+jk\)). Provide the thickness information in the form of \([d_0, d_1, ..., d_N]\), where \(d_i\) is the thickness of the \(i\)-th layer. As the outermost layers are infinitely thick, specify them with a thickness of \(0\). Analogously, provide the refractive indices and extinction coefficients in the form of \([n_0, ..., n_N]\) and \([k_0, ..., k_N]\).

For example, the following entry:

layer system:
- thicknesses: [0, 500, 0]
  refractive indices: [1.5, 2.1, 1]
  extinction coefficients: [0, 0.01, 0]

would specify a single film of thickness \(500\), consisting of a material with complex refractive index \(n_1=2.1+0.01j\), located on top of a substrate with refractive index \(n_0=1.5\), and below air/vacuum (refractive index \(n_2=1\)).

Scattering particles

The ensemble of scattering particles inside the layered medium.

For spherical particles, specify shape: sphere, the radius, refractive index, extinction coefficient and the [x, y, z] coordinates of the particle position.

For spheroids, specify shape: spheroid, the half axes along (half axis c) and transverse (half axis a) to the axis of revolution, refractive index, extinction coefficient and the [x, y, z] coordinates of the particle position, as well as the Euler angles defining the rotation of the axis of revolution relative to the z axis (currently rotations other than [0, 0, 0] are not implemented).

For finite cylinders, specify shape: finite cylinder, the cylinder height, cylinder radius, refractive index, extinction coefficient and the [x, y, z] coordinates of the particle position, as well as the Euler angles defining the rotation of the axis of revolution relative to the z axis (currently rotations other than [0, 0, 0] are not implemented).

The coordinate system is such that the interface between the first two layers defines the plane \(z=0\).

In addition, specify l_max and m_max, which refer to the maximal multipole degree and order used for the spherical wave expansion of that particle’s scattered field. These parameters should be chosen with reference to the desired accuracy and to the particle size parameter and refractive index contrast, see for example https://arxiv.org/ftp/arxiv/papers/1202/1202.5904.pdf A larger value leads to higher accuracy, but also to longer computation time. l_max is a positive integer and m_max is a non-negative integer and not greater than l_max.

In the case of non-spherical particles, you can also specify use discrete sources (default is True), nint (default is 200) and nrank: 8 (default is l_max + 2). These parameters specify the calculation of the T-matrix using the NFM-DS module. For further information about the meaning of these parameters, see the NFM-DS documentation.

The parameters for the scattering particles can be listed directly in the input file, in the following format:

scattering particles:
- shape: sphere
  radius: 100
  refractive index: 2.4
  extinction coefficient: 0.05
  position: [0, 100, 150]
  l_max: 3
  m_max: 3
- shape: finite cylinder
  cylinder radius: 120
  cylinder height: 150
  refractive index: 2.7
  extinction coefficient: 0
  position: [350, -100, 250]
  euler angles: [0, 0, 0]
  l_max: 4
  m_max: 4
  use discrete sources: true
  nint: 200
  nrank: 8
- shape: spheroid
  semi axis c: 80
  semi axis a: 140
  refractive index: 2.5
  extinction coefficient: 0.05
  position: [-350, 50, 350]
  euler angles: [0, 0, 0]
  l_max: 3
  m_max: 3
  use discrete sources: true
  nint: 200
  nrank: 8

Alternatively, the scattering particles can be specified in a separate file, which needs to be located in the SMUTHI project folder. This is more convenient for large particle numbers. In that case, specify the filename of the particles parameters file, for example:

scattering particles: particle_specs.dat

The format of the particle specifications file is described below, see The particle specifications file.

Initial field

Currently, plane waves and beams with Gaussian transverse cross-section are implemented, as well as single or multiple electric point dipole sources.

For plane waves, specify the initial field in the following format:

initial field:
  type: plane wave
  angle units: degree
  polar angle: 0
  azimuthal angle: 0
  polarization: TE
  amplitude: 1
  reference point: [0, 0, 0]

Angle units can be ‘degree’ (otherwise, radians are used). For polarization, select either TE or TM.

The electric field of the plane wave in the layer from which it comes then reads

\[\mathbf{E_\mathrm{init}}(\mathbf{r}) = A \exp(\mathrm{j} \mathbf{k}\cdot(\mathbf{r}-\mathbf{r_0})) \hat{\mathbf{e}}_j,\]

where \(A\) is the amplitude, \(\mathrm{j}\) is the imaginary unit,

\[\begin{split}\mathbf{k}=\frac{2 \pi n_\mathrm{init}}{\lambda} \left( \begin{array}{c} \sin(\beta)\cos(\alpha)\\ \sin(\beta)\sin(\alpha) \\ \cos(\beta) \end{array} \right)\end{split}\]

is the wave vector in the layer from which the plane wave comes, \(n_\mathrm{init}\) is the refractive index in that layer (must be real), \((\beta,\alpha)\) are the polar and azimuthal angle of the plane wave, \(\mathbf{r_0}\) is the reference point and \(\hat{\mathbf{e}}_j\) is the unit vector pointing into the \(\alpha\)-direction for TE polarization and into the in the \(\beta\)-direction for TM polarization.

If the polar angle is in the range \(0\leq\beta\lt 90^\circ\), the k-vector has a positive \(z\)-component and consequently, the plane wave is incident from the bottom side. If the polar angle is in the range \(90^\circ\lt\beta\leq 180^\circ\), then the plane wave is incident from the top.

For Gaussian beams, specify the input in this format:

initial field:
  type: Gaussian beam
  angle units: degree
  polar angle: 0
  azimuthal angle: 0
  polarization: TE
  amplitude: 1
  focus point: [0, 0, 0]
  beam waist: 1000

The Gaussian beam amplitude corresponds to the electric field value at the focus point. The beam waist parameter describes the transverse width of the beam near the focus point.

More precisely, the beam is designed to fulfill

\[\mathbf{E}(\mathbf{r}) = \exp \left[\frac{(x-x_G)^2+(y-y_G)^2}{w^2}\right] \mathbf{A}_G\]

for \(z=z_G\), where \((x_G,y_G,z_G)\) are the coordinates of the focus point, and \(w\) is the beam waist parameter and \(\mathbf{A}_G\) is the amplitude vector given by the amplitude parameter and the polarization.

For a single electric point dipole source, use an input of the format:

initial field:
  type: dipole source
  position: [100, 10, 350]
  dipole moment: [3e7, 3e7, 0]

The dipole moment vector \(\mathbf{\mu}\) specifies the amplitude and the orientation of the dipole oscillation. It corresponds to a current density of

\[\mathbf{j}(\mathbf{r}) = -j \omega \mathbf{\mu} \delta(\mathbf{r} - \mathbf{r}_D),\]

where \(\mathbf{r}_D\) is the dipole position.

For multiple point dipole sources, specify the parameters in this format:

initial field:
  type: dipole collection
  dipoles:
  - position: [150, -100, 90]
    dipole moment: [1.5e7, 1.5e7, 0]
  - position: [-100, 100, 290]
    dipole moment: [0, 1.5e7, 1.5e7]

Numerical parameters

The radial wavevector component of a plane wave expansion is defined by a sequence n_effective in the complex plane, where n_effective = k_parallel / omega refers to the effective refractive index of the partial wave:

n_effective resolution: 1e-3

max n_effective: 3

n_effective imaginary deflection: 5e-2

‘n_effective resolution’ determines the sampling of the expansion/contour, where n_effective = k_parallel / omega refers to the effective refractive index of the partial wave (default=1e-2). A smaller value leads to more precise results and to a longer computation time. ‘max n_effective’ specifies where the expansion is truncated. It should be chosen somewhere above the maximal refractive index of the layers (default=max(refractive indices)+1). ‘n_effective imaginary deflection’ determines how much the contour is deflected into the lower complex half plane to avoid the vicinity of waveguide or branch point singularities (default=5e-2).

In addition, specify the resolution (in angle units) of the azimuthal angle coordinate of plane wave expansions, as well as polar and azimuthal angle coordinates of far field evaluations:

angular resolution: 1



neff waypoints: [0, 0.5, 0.8-0.1j, 2-0.1j, 2.5, 4]

Post procesing

Define here, what output you want to generate. Currently, the following tasks can be defined for the post processing phase:

  • Evaluation of the far field. If the initial field is a plane wave, the far field is interpreted in terms of the differential scattering cross section and the extinction cross section. For the case of an initial Gaussian beam, the far field denotes the radiative intensity, and relative reflectivity as well as transmittivity figures are displayed in the terminal. You can export images and raw data in ascii format.
  • Evaluation of the electrical near field. You can export images, animations and raw data regarding field components or the field modulus.

Write for example:

post processing:
- task: evaluate far field
  show plots: true
  save plots: true
  save data: false
- task: evaluate near field
  show plots: true
  save plots: true
  save animations: true
  save data: false
  quantities to plot: [E_y, norm(E), E_scat_y, norm(E_scat), E_init_y, norm(E_init)]
  xmin: -800
  xmax: 800
  zmin: -400
  zmax: 900
  spatial resolution: 50
  interpolation spatial resolution: 5
  maximal field strength: 1.2

The show plots, save plots and save data flags deterimine, if the respective output is plotted, if the plots are saved and if the raw data is exported to ascii files.

In the evaluate near field task, the save animations flags deterimines, if the near field figures are exported as gif animations.

The quantities to plot are a list of strings that can be: E_x, E_y, E_z or norm(E) for the x-, y- and z-component or the norm of the total electric field, E_scat_x, E_scat_y, E_scat_z or norm(E_scat) for the x-, y- and z-component or the norm of the scattered electric field, or E_init_x, E_init_y, E_init_z or norm(E_init) for the x-, y- and z-component or the norm of the initial electric field.

To specify the plane where the near field is computed, provide xmin, xmax, ymin, ymax, zmin and zmax. If any of these is not given, it is assumed to be 0. For exactly one of the coordinates x, y or z the min and max value should be identical, e.g. ymin = ymax as in the above example. In that case, the field would be plotted in the xz-plane.

spatial resolution determines, how fine the grid of points is, where the near field is computed. As xmin etc., this parameter is specified in length units. If interpolation spatial resolution is specified, the near field will be interpolated to that finer value to allow for smoother looking field plots without the long computing time of a fine grained actual field evaluation.

With maximal field strength, you can set the color scale of the field plots to a fixed maximum.

Further settings for the generation of output data

The path to the output folder can be specified as:

output folder: smuthi_output

This folder will be created and in it a subfolder with a timestamp that contains all file output of the simulation.

Finally, if:

save simulation: true

is specified, the simulation object will be saved as a binary data file from which it can be reimported at a later time.

The particle specifications file

The file containing the particle specifications needs to be written in the following format:

# spheres
# x, y, z, radius, refractive index, exctinction coefficient, l_max, m_max
0        100     150     100     2.4     0.05    3       3
...      ...     ...     ...     ...     ...     ...     ...

# cylinders
# x, y, z, cylinder radius, cylinder height, refractive index, exctinction coefficient, l_max, m_max
250      -100    250     120     150     2.7     0       4       4
...      ...     ...     ...     ...     ...     ...     ...     ...

# spheroids
# x, y, z, semi-axis c, semi-axis a, refractive index, exctinction coefficient, l_max, m_max
-250     0       350     80      140     2.5     0.05    3       3
...      ...     ...     ...     ...     ...     ...     ...     ...

An examplary particle specifiacations can be downloaded from here.

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