Hello Matthias,
You have defined the indices of GaN as a list, but the materials need to be defined as a arrays, or using a string for the predefined materials in material library. In python [,,] creates a list data type which you need to convert to np array.
Passing-Data-Python-API
This type conversion between python and lsf can get a little complicated. I would suggest keeping it as simple as possible.
mode.setmaterial(material, "Refractive Index", np.array(n_GaN))
mode.setmaterial(material, "Imaginary Refractive Index", np.array([0, 0, 0]))
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