# Materials

The Materials element stores information about any materials used in the simulation. To use one of VSim’s pre-defined materials, highlight the Materials element and then switch from 3D View to Database in the Geometry View (see VSim User Guide: Setup Window for Visual-setup Simulations for a picture of the Geometry View). To add one of the pre-defined materials from the table, highlight the material then press Add To Simulation button in upper right hand corner of the VSim Composer window. The material will now be listed under the Materials element. To access a wider selection of materials you may load the emthermal.vmat file by right-clicking on the Materials element and selecting Import Materials. You may also import your own customized material (see Customizing Materials below).

The editable properties of the materials are:

kind (not editable)
The kind of material (eg dielectric, conductor, particle absorber, permeable, etc), as defined in the .vmat file.
heat capacity
The heat capacity of the material.
thermal conductivity
The thermal conductivity of the material.
resistance
The resistance of the material.
conductivity
The conductivity of the material.
relative permittivity
The relative permittivity of the material. Note that when a material is assigned to a geometry, the relative permittivity is only used in the electromagnetic field solve. When using the electrostatic field solve, a spatially dependent relative permittivity due to the addition of different geometries in the simulation is done by defining a SpaceTimeFunctions and inserting this SpaceTimeFunction in the relative permittivity feature under PoissonSolver in the FieldBoundaryConditions tab. See PoissonSolver for more detail on how to include a dielectric in the simulation using the electrostatic field solve. The difference between how dielectrics are handled using electromagnetic and electrostatic field solves is demonstrated in two different examples: “Dielectric in Electromagnetics (dielectricInEM.sdf)” and “Dielectric in Electrostatics (dielectricInES.sdf)”.

## Drude-Lorentz and Debye-Lorentz Materials

VSim 10.0 includes the ability to model frequency dependent dielectrics, using either the Drude-Lorentz or Debye-Lorentz models.

The Drude model focuses on the zero frequency limit of conductance, while the Debye Relaxation focuses on the zero frequency limit of dielectric. The Drude model assumes unbound charge carriers which undergo collisions, resulting in a frequency dependant conduction current. The Debye Relaxation Model assumes bound charge carriers whose dielectric response relaxes at higher frequencies.

In general Drude models are more advantageous for conductivity dominated dielectrics while Debye models are for permittivity dominated dielectrics.

Each model makes use of Lorentz resonances to handle the middle frequencies of the dielectric. The Lorentz model assumes bound charge carriers whose response is resonant at a material specified frequency and line width. Any number of Lorentz resonances may be specified.

The total conduction current of the dielectric is given by the specified Infinity Limit Current + Drude/Debye Current + Sum of all Lorentz Currents.

These materials are most commonly used in plasmonics and photonics simulation problems.

### Drude-Lorentz Material Parameters

collision function
A function describing the collision frequency of the unbound charge carriers used in the Drude model.
conductivity function
A spatial function describing the conductivity of the material.
lorentz oscillator strength
A vector describing the oscillation at each lorentz resonance, the density of bound charge carriers seeing the lorentz resonance. Given in units of 1/(ohms*meters*seconds)
lorentz frequency
A vector of frequencies of the Lorentz resonances, in Hz.
lorentz line width
A vector of the line widths (bandwidths) of the Lorentz resonances, in 1/seconds.
relative permittivity at infinite frequency
The permittivity of the material at infinite frequencies. Used to keep the simulation stable.
conductivity at infinite frequency
The conductivity of the material at infinite frequencies. Used to keep the simulation stable.
background conductivity
The conductivity of the simulation space outside of this material.

### Debye-Lorentz Material Parameters

relaxation function
A function describing the time scale at which dielectric response relaxes from the specified permittivity function to the specified relative permittivity at infinite frequency.
permittivity function
A spatial function describing the relative permittivity of the material.
lorentz oscillator strength
A vector describing the oscillation at each lorentz resonance, the density of bound charge carriers seeing the lorentz resonance. Given in units of 1/(ohms*meters*seconds)
lorentz frequency
A vector of frequencies of the Lorentz resonances, in Hz.
lorentz line width
A vector of the line widths (bandwidths) of the Lorentz resonances, in 1/seconds.
relative permittivity at infinite frequency
The permittivity of the material at infinite frequencies. Used to keep the simulation stable.
conductivity at infinite frequency
The conductivity of the material at infinite frequencies. Used to keep the simulation stable.
background conductivity
The conductivity of the simulation space outside of this material.

## Customizing Materials

Custom materials properties can be created in a text editor and imported into VSim. To import a custom material:

1. Go to the “data” folder at the top level of the VSim installation directory. On Windows this will be in “Program Files\Tech-X (Win64)\VSim-10.0\data". On Mac and Linux, there is no standard location.

2. Create a new text file that ends in the .vmat extension. For example: emthermalcustom.vmat.

3. Open the default materials file emthermal.vmat.

4. Copy a block from emthermal.vmat to emthermalcustom.vmat to use as a sample.

5. Edit the title and properties of the block as needed. For example:

<Material FreshWater>
<strings>
kind = "dielectric"
</strings>
<params>
heat capacity = 4.184
conductivity = 0.005
relative permittivity = 80.4
thermal conductivity = 0.6065
</params>
</Material>


6. Save the new file.

7. Back in VSim, click on the Materials element in your simulation and then click Add –> Import Materials. A file browser will appear with the data directory already open.

8. Select your custom vmat file. If you do not see your vmat file in the directory, navigate to where you saved it.

9. The view will change to the Database tab and the materials added will be available here.