1. The problem statement, all variables and given/known data I am doing a 2D COMSOL simulation (RF module) of a rib waveguide of nanometric structure. My question is if there is any rules of thumb of distinguishing when the mode no longer is confined under the rib-structure. 2. Relevant equations None 3. The attempt at a solution The mode consists of different intensity layers, my idea is that as long as the most intense regions are under the rib structure then the mode can be considered as confined.. however I don't know how valid this argument is.