Guilin Sun
Ansys Employee
Since your material is lossless, this negative imaginary part is due to numerical calculation error, which is already very small, 13 orders smaller than the real part. You can ignore it. Or you can use Metal BCs. When PML is used, and the material has no loss, the calculated beta and thus effective index will always have imaginary part, which can be positive or negative, from numerical errors.
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