negative index (metamaterials)

Hi,

I need a program which made it possible to calculate the negative index of refraction according to the parameters S. because I find a major problem with bends of acos.
the formula of n is n = ± accos(X)/kd +2Πm/kd

with X= (1-(s11)2+(s21)2)/2(s21)2

Provided that Im(n)>0

If somebody have some ideas or some interesting sources, it would be great!!!!!

Thanks a lot for your help!!