In the literature and a lot of numerical codes, there are methods for decomposing the field in a waveguide in forward and backward components, such as here: https://support.lumerical.com/hc/en-us/articles/360034902433-Using-and-understanding-Mode-Expansion-Monitors and here: https://meep.readthedocs.io/en/latest/Mode_Decomposition/ which are based on writing the field as forward and backward expansion terms. Above the cut-off frequency of each mode considered in the mode decomposition, this method is straightforward. These pages state that this method is applicable only for modes above their cut-off frequency.
My question is, what is the physical meaning of the scattering parameters below the cut-off frequency? I know that there is no propagation (power purely imaginary) and I suspect that these parameters are probably meaningless below cut-off frequency.
Thank you.
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