Guilin Sun
Ansys Employee

In your case this material SiO2 can be practivally considered as pure dielectric with a given refractive index directly at the geometry tab, instead of using the mutil-coefficent fitting model.

Secondly, this small negative value is due to fitting error. If you use much smaller "fit tolerance" , or larger weight for the imaginery in the "show advanced" tab, it can be corrected.

Thirdly, if such small "gain" does not cause simulation diverging or wrong result, it is ok.

Speaking to  negative absorption in RTA analysis, which one is larger than 1? if R or T is larger than one, then the simulation should be modified:

If both R and T are normal then it is the marial "gain" that causes the issue. Choice of #1 is the simpest solution.