Getting different neff and loss for same design


#1

I have downloaded model from link:
https://apps.lumerical.com/pic_passive_dispersion_analysis.html

When I am checking for modes and corresponding loss, I can see neff are having imaginary values and there is some loss present as well as shown in below image (for attached model waveguide_dispersion.lms)

waveguide_dispersion.lms (8.0 MB)

But when i have made similar structure by myself using new file, neff are not showing imaginary values and also there is no significant loss this time(all zero) as shown in below image(attached model waveguide_dispersion_1.lms)

waveguide_dispersion_1.lms (7.6 MB)

Both structures look same to me. Do I need to change some settings to get imaginary neff and can you clarify on loss as well.?