Learning Forum

[Erik M]

The mesh refinement option "precise volume average" in combination with the "Import Surface" (importsurface2) geometry does not lead to the expected result. As an example, we look at the reflection of a plane wave on a sinusoidal grating of some material:

The precise volume average mesh refinement incorrectly assigns the permittivity of the material to the whole rectangular volume, leading to the following intensity profile:

Instead, only the material enclosed by the surface should have the perimittivity of the material leading to an intesity profile like:

How can I set up the simulation with the expected behavior (last figure) and the precise volume average mesh refinement option?

Many thanks,
Erik

[Guilin Sun]

Are the two screenshots from the same structure with the same material, source bandwidth and mesh?

Have you compared the grating results if they are the same structure? 

You may also use Conformal Variant 0 instead of precise volume average. When precise volume average is used, it uses finer mesh than the FDTD mesh to average the permittivity, and it should not change anything for uniform material, eg, away from material interface. It may need more memory. the Conformal Variant 0 is always recommended, except that you find it is not accurate or cause other issues.

Anyway, there are many different methods to treat the effective material property at the material interfaces. While they all work well in many cases they can lead to some differences in results. But the difference from your screnshots seem too large if they are from the same simulation file. 

 

 

[Erik M]

Thank you for your kind reply.

Both screenshots are from the same simulation (structure, material [silver], source, bandwidth) but with a different mesh refinement option. With "Precise volume average, the Import Surface structure behaves like it is a rectangular block instead of sinusoidal surface geometry.

Am I missing an extra setting or is it something else?

[Guilin Sun]

Please use index monitor to check if the index is correct.

The first screnshot shows it has zeroth order only, whereas the 2nd shows very differently.

For sinusoidal shape, you can use "custom" to define it: https://optics.ansys.com/hc/en-us/articles/360034382214-Custom-structure-object-Simulation-Object

[Erik M]

I can confirm that the refractive index is correct, as it is the same for both mesh refinement options after meshing, monitored with the refractive index monitor.

Conformal variant 1:

 

Precise volume averaging (precision 11):

It is clear that the expected diffraction behavior of a sinusoidal grating is lost after meshing with "precise volume averaging".

As I would like to use the "precise volume averaging" mesh refinement for arbirarily shaped surfaces, it is unfortunately not an option to use a custom simulation object.

Is this an incompatibility between the importsurface2 function and precise volume averaging or can this problem be resolved by changing some settings?

[Guilin Sun]

https://optics.ansys.com/hc/en-us/articles/360034928993-importsurface2-Script-command

https://optics.ansys.com/hc/en-us/articles/360034382614-Selecting-the-best-mesh-refinement-option-in-the-FDTD-simulation-object

I did not find such limitation from the websites. It seems you used very fine mesh, you may try to use relatively coarser mesh and see if there is any improvement.

I will contact you later.

 

[Erik M]

I also did not find such a limitation in the documentation; that's why I think you would like to know.

The issue occurs at all tested mesh sizes 1--10 nm, e.g., at 10 nm the intensity profile (top) and refractive index (bottom):

I am looking forward to your input.

[Guilin Sun]

So you mean at 10nm mesh, it works fine. But finer mesh than 10nm such as 1nm it causes issue? 

[Erik M]

No, unfortunately, this issue is always there

[Guilin Sun]

But the field profile in the top figure looks normal as expected. I thought you only concern this result:

Do you mean when you use such mesh refinement, the result is always like the above figure? I am a little bit confused. Please check your email and give me more details.

[Erik M]

Thank you for reaching out. I will write you an email once the support website is back online after the scheduled maintenance.

I am afraid that you have misunderstood what the expected intensity profile is. The field should reflect from the surface of the sinusoidal grating (like in your post above), not from some imaginary surface above it. The figure below shows the unphysical result obtained with the precise volume average mesh refinement option.

I hope that this resolves our misunderstanding.

[Erik M]

Dear Dr Sun,

As an academic partner, we were not able to reply to the ticket/e-mail you sent me. Instead, we have been forwarded to CADFEM support. They will take it from here.

Kind regards,
Erik

[Guilin Sun]

ok. But the imaginary line should be correctly indicative that the reflection is a plane wave. Please ask CADFEM forward email to me.