I am trying to calculate the attenuation factor of a symmetrical waveguide. To determine the attenuation factor, I need to calculate the free space propagation factor. The wavelength in the travel medium is available to me, so I can calculate the propagation factor k; however, to determine the free space propagation from the result just obtained I am not sure whether I have to divide the propagation factor by n1 or n2. I think it should be n2 because the evanescent field does not propagate in the film, but in the outer layers with a refractive index of n2. The refractive index n1 belongs to the film.
So, here is my question in summary:
free space propagation factor = propagation factor/ n1 or