This video is taken from the FDTD Learning Track on Ansys Innovation Courses.
FDTD Solutions supports modelling of linear dispersive materials as well as more complex
materials including nonlinear, spatially-varying refractive index, and anisotropic materials.
There are two general types of material models: permittivity models and conductivity models.
Permittivity models are more common, but the type of material model that you can define
for a structure depends on the type of structure and whether it is a 2D structure (a 2D rectangle
or polygon), or 3D structure.
Structure primitives have a “material” property with a drop down menu allowing you
to select a material to assign to the object.
For 3D primitives, the material of the object is represented in the simulation by a permittivity.
You can choose to set the material to and specify the refractive
index of the material in the “Index” field.
In the “Index” field, it is possible to type a single number to represent a uniform
isotropic material, or, you can type three values separated by
semicolons to represent diagonal anisotropy where the first value is the refractive index
in the x-direction, the second value is the refractive index in the y-direction, and the
third value is the refractive index in the z-direction.
You can also type an equation as a function of variables x, y, and z to define a refractive
index profile that varies over space.
For example, here we have an equation which sets the refractive index to be a linear function
of x, a sin function along y and a constant along z.
The limitation of the material is that it can only be used to define
a real refractive index that is constant over frequency.
To represent a dispersive material where the index is a function of frequency, you’ll
need to choose a different material from the drop down.
For 2D structure primitives (2D rectangle and 2D polygon), the material of the object
is represented in the simulation by a surface conductivity.
2D structures are used to represent thin films of conductive materials where the film can
be approximated as being infinitely thin.
The materials list for 2D structures includes Perfect Electrical Conductor (PEC), and any
conductive materials from the Material Database.
You can also use 2D rectangle structures to represent a lumped RLC load by setting the
material to .
You can then type in the values of resistance (R), impedance (L), and capacitance (C), as
well as a current flow direction, and the structure will effectively represent a load
with a sum in parallel of R, L, and C.
Most imported objects added from the Import drop down menu in the main toolbar use the
Materials drop down menu to set the material of the imported object similarly to 3D structure
primitives, except for the (n,k) Material object.
The (n,k) Material import object is used to specify a 3D object with spatially-varying
This is done by specifying the refractive index (which can be anisotropic) over a 3D
rectangular grid of positions.
Note that this is meant for single frequency simulations only and it cannot be used to
specify a spatially-varying refractive index that is also dispersive.
The material database which can be opened from the Materials button in the main toolbar
includes the full list of materials that you can choose from in the Materials list of an
object and lets you add and edit materials.
The Material Explorer which can be opened from the Check menu in the simulation toolbar
allows you to plot the material data and fit as a function of wavelength or frequency.
In the following subsections the Material Database and Material Explorer will be covered
in more detail.