Hello Amrita,

I solve it using PML boundary condition and 5nm*5nm mesh. Could you please let me know what is the effective index of the 3rd mode you are getting? Is it less than nSiO2 (1.44)? Here is the file I simulated:

Could you please check it or share your file? 

Also, seeing the snapshots of the 3rd mode for all the 3 configurations I gave in the previous response, how may I understand that they are artifacts?