I ran a simulation (using CST Studio Suite 2020 version) where   I was expecting negative effective refractive index within some wavelength range. After the simulation I got negative effective refractive index   within the wavelength range of interest when I selected port 2 as my source excitation.  However, when I selected port 1 as my source excitation, the results of the effective refractive index   became positive throughout the wavelength range. Can  someone  explain to me what is happening here?  

Thanks,