This video is taken from the FDE 100 course on Lumerical University.

## Transcript

Before running a simulation, it’s important to check the material fit in the Material

Explorer to make sure that the simulated material properties for sampled data materials will

accurately represent the physical material.

This is especially important for a frequency analysis where the multi-coefficient material

model is being used.

Open the Material Explorer from the “Check” menu.

The Material menu can be used to select a material.

Click the “Fit and plot” button to generate the plots of the data points for the real

and imaginary parts of the material data, and the material fit curve which is labeled

“Eigensolver model” in the legend.

Under the simulation bandwidth settings, you can choose to fit the material using the multi-coefficient

model and set the wavelength range.

You can also set the wavelength range to show in the plot under the view settings.

The Material settings portion of the Material Explorer window includes the fit tolerance

setting and the max coefficients setting.

The fit tolerance setting specifies the target RMS (root mean squared) error between the

generated material fit and the material data.

You can see the RMS error displayed in the Fit analysis below.

It’s typically safe to set the fit tolerance to 0.

The max coefficients setting sets the maximum number of coefficients of the equation used

for the material fit.

The more coefficients, the more inflection points there can be in the fit curve.

The material fit with the lowest error will be chosen, so the fit may use fewer coefficients

than the number of max coefficients if using more coefficients doesn’t result in a better

fit.

For example here max coefficients is set to 3, but the number of coefficients used is

2.

If the number of max coefficients is too high, it’s possible to get a material fit which

has a lower RMS error, but actually doesn’t generate a better fit, as you can see here

when I change max coefficients to 5, as it leads to extraneous peaks in the fit.

This is one reason why it’s good to visually inspect the fit.

Clicking on the "Show advanced" button exposes additional advanced fitting parameters.

The imaginary weight is the relative weighting placed on the fit of the imaginary part of

the permittivity or conductivity compared to the real part.

By default this is set to 1 to give equal consideration to the real and imaginary part.

A value of 2 will give twice the weight to fit of the imaginary part of the data compared

to the real part and a value of 0.5 will give twice the weight to the real part compared

to the imaginary part.

The weighted RMS error displayed in the Fit analysis is the RMS error with imaginary weight

applied.

It’s important to remember that the weighting factor is applied to the imaginary part of

the permittivity, not the refractive index.

Therefore, it’s best to plot the permittivity when adjusting the imaginary weighting factor.

The improve stability and make fit passive options are selected by default.

These settings are important if running time domain simulations using the varFDTD solver

and typically don't need to be changed.

Specify fit range allows you to set the wavelength or frequency range of the fit.

By default the fit range will be set to the simulation bandwidth.