In the FDTD cad, 2D sheet of rectangle and polygon structure is given. But how to make 2D sheet of circle ring or circular arc for graphene material.
For slightly complicated structures, you can use scripting language and defines the vertices for your objects, and then set vertices. The link below has some examples (including ring and circle) for your review:
Defining material in mode solution
A post was split to a new topic: When to use single layer graphene model for multiple layers?
How to make a 2D sheet like shown in the figure for graphene sheet?
While you might be able to construct this ring (maybe by a 2D rectangles with surface normal of x that are rotated along z-axis), our software is only capable of handling flat graphene layers (i.e. graphene is aligned with x,y, or z axis).
Keep in mind that, because the graphene material type generates a surface conductivity that corresponds to a flat surface, it can only be used with geometric structures that represent flat surfaces.
You can read more about the limitations in this link:
https://kb.lumerical.com/en/index.html?other_application_graphene_simulation_tips.html (please check the links for the publications). This might be still a good assumption if ring is very large compared to wavelength, but will not be reliable if they have comparable sizes.
You might have some success using volumetric cylindrical graphene, but please note that since you need to use a very fine mesh for entire geometry to resolve the thickness, this approach might not be very practical.
Please keep me updated with your thoughts and I will be glad to discuss if further based on the specific application that you are trying to model.