You will need to find material data amd import https://optics.ansys.com/hc/en-us/articles/360034915093-Creating-new-sampled-data-materials-in-FDTD
if is dispersive.
If it works at a single wavelength, the simpler way is to use nk material:
https://optics.ansys.com/hc/en-us/articles/360034394654-Tips-for-using-the-n-k-material-model-in-FDTD
and if it is dispersionless and lossless, you can simply assign one single real refractive index at the "index" tab in the geometry.
However, you will need to have your owen data, either from lab measurement or search the literature.
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