Unfortunately, you cannot directly assign index to be negative. Since negative refractive material is dispersive, you should use dispersive material model, and usually you need both permittivity and permeability negative simutaneously. Please refer to magnetic-electric model
https://optics.ansys.com/hc/en-us/articles/360034394734
and this example:
https://optics.ansys.com/hc/en-us/articles/360041616614-Bulk-metamaterials-based-on-magnetic-and-electric-Lorentz-models
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