CST MWS (Modes in SOI Waveguide)

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Dear all,
I am trying to simulate a SOI waveguide in CST MWS using time domain solver with dimensions of silicon (220 nm thickn and 500 nm wide)on top of silica.I try to simulate it for frequency range(192-200 THZ).

I am not getting the expected result.What should be the boundary conditions and other settings for getting the correct result.
I have attached the jpg file.Please suggest possible solutions.
Thanks

What is your mesh definition?
As your frequency is very high so proper and dense mesh must be used to get accurate results.