I read this great answer to this question:
The main thing that I am trying to clear in my head is the following:
Given that plane waves are eigenmodes of maxwell's equations in free space in the absence of sources, shouldn't I be able to use this to propagate a planar e/m field exactly (ignoring aliasing effects)?
For example I know the field that exits in the x, y plane, can't I talk about what it will look like at +/- z0? From the answer I would assume you cannot and the closer that comes to using this type of thinking is the "angular spectrum propagation" as the poster describes it, which seems to me to be the same as the "beam propagation method" but not sure about this either.
If the question requires further clarification please let me know.