Consider this example:
Two Si waveguides on top of each other with a spacing of SiO2 and clad of SiO2. I can use FDTD to simulate coupling between the waveguides. However, Mode Solver doesn’t help. I tried following the approach outlined here: https://kb.lumerical.com/en/pic_passive_waveguide_couplers_evanescent.html
But I got stuck at the very first step as the modes found belong to either waveguide but there was no such mode where the field is in both waveguides.
Is there a limitation I am unaware of in Mode Solver, or am I doing something wrong?