Is it just the one single slab?  How big is the slab?  What is your wavelength?  What material?  Are there any other materials around like a substrate?  I would need this information to help steer you in the right direction.  Meanwhile, if you want to see an overview of a bunch of numerical methods for this sort of thing, check out Lecture 1 here:

http://emlab.utep.edu/ee5390cem.htm

Some imortant information are missing to answer your question. You need

to specify material properties of your film such as the complex permittivity  and permeability, film thickness,  as well as the frequency.