Hello,
If the material you're using as the core (SU-8) is lossless then ideally the propagation losses in a straight waveguide should be 0. So, I think your results are correct.
I would also like to add that if you want to determine the propagation loss of a mode then you don't have to simulate the structure in time domain or use a Mode Expansion monitor. You can edit the mode source, go to mode selection and choose user select to find and select the desirable mode. Once you have selected the mode from the list of calculated modes, you can obtain modal properties including the propagation losses.
You can also visualize the properties of the selected mode.
Hope this helps.
Regards,
Amrita
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