Learning Forum

[pnair]

Hello,

I have a question related to position of the monitor on the thin film. I am running a simulation using a box of monitors, with the z2 monitor on the surface of my thinfilm as shown below

I have a question related to the position of the z2-monitor above the structure. I am getting different far field field plots for different positions of the z2-monitor above the source. 

But, as I was going through this article "https://optics.ansys.com/hc/en-us/articles/360034395194-Changing-the-far-field-refractive-index-analysis-object". As my monitors are on the film, is this farfield analysis group is considering the refractive index of the vaccum or is it extending the substrate to the infinity? So, how can validate my results to see if this method is correct?

 

Thanks,

Prabha

 

[Guilin Sun]

It is not suggested to place a monitor exactly on the interface, since FDTD is discrfete and needs interpolation to get the fields on a specific plane. Please refer the FDTD Yee cell below:

 

Placing the monitor one cell away from the interface does not affect the farfield projection much.

If you are using dipole source, due to its wide-angle radiation, you should place the monitor as close as possible in order to pick up larger angle radiation.

Please note, in order to get accurate far field projection, all the box monitors should be in the same uniform material.

[pnair]

Hello,

Thank you for the reply. Could you please elaborate a lottle bit on this " It is not suggested to place a monitor exactly on the interface, since FDTD is discrfete and needs interpolation to get the fields on a specific plane". ? I run a simulation by placing the box of monitors close to the dipole source as shown below. So, My farfield pattern in this case is a little bit different from the first case I have mentioned here where my z2-monitor of the box of monitors is on the surface. So, how can I verify my farfield results or to know if my results rae correct in this case?

It is not suggested to place a monitor exactly on the interface, since FDTD is discrfete and needs interpolation to get the fields on a specific plane. Please refer the FDTD Yee cell below:

So, now i have placed the box of monitors in my thin film, closer to the dipole source. my second concern regarding this simulation is acccording to this article "https://optics.ansys.com/hc/en-us/articles/360034395194-Changing-the-far-field-refractive-index-analysis-object". So, my monitors are on the film, is this farfield analysis group is considering the refractive index of the vaccum or is it extending the substrate to the infinity?

[Guilin Sun]

As mentioned previously, when a monitor is exactly on the interface, it may pick up data from two different materials, due to discretization. Therefore it is not suggested doing so.

When refractive index is invloved, it will use the refractive index at the center of the monitor. However such farfield will NOT be able being validated through experiments since the monitors are inside the material, and it will assume that the far field is inside such material, not in the backgound vacuum.

Please reconsider the physics: does the bottom monitor contribute to the farfield in upper half space? how can you measure such farfield in experiment?  what is the purpose for such projection? when there is substrate, one can measure the farfield in the upper semisphere (in air). Then you can use 5 monitors in air to pick up the radiation and do the farfield projection, without simulating large area with one single monitor. 

When the substrate has been extended to the outside of the bottome PML, it assumes the substrate is semi-infinity. 

[pnair]

Thank you for the detailed explanation . This is really helpful to have a better understanding of the placement of monitors. Thank you. I am using box of monitors, i mean 5 monitors to collect the data and the monitor at the substrate is deactivated. I am collecting farfield upper xy_half space data usinf 5 monitors. But, what does it mean by "Then you can use 5 monitors in air to pick up the radiation". How can i place 5 monitors in air? because now i have placed everthing inside the thinfilm so that it can be inside the FDTD region. So, my understanding is that monitors should be inside the FDTD region. If it is wrong, please correct me.

[Guilin Sun]

Box monitors should be inside FDTD this is for sure. What I mean is to place them above the thin fiim, in order to get the farfield in air (background material). 

You can place them inside the thin film. However it is projected inside the material, which cannot be measured in experiment. I am not sure the actual purpose doing this.

 

[pnair]

Hello,

Thank you for the reply. I tried to place the monitor at some distance above the thin film as you have suggested here and is shown below. 

This is the perspective view of the monitor:

Now, I think the 5 monitors are above the thin film.

I have generated farfield plot like this, which is entirely different from my previous plot(where the monitors are on the interface as i have mentioned earlier here

How can I verify if my results are correct?

[Guilin Sun]

You also need to modify the monitors not to be inside of the thin film. Farfield projection requires that the monitors are in uniform material. In your case it is in the background material.

The farfield from box complately in background material is different from that from the box monitors inside the thin film, since it is refracted out.

[pnair]

Thank you for the reply. I have placed the monitors in air on the top of thin film as shown below. I think this might be the correct way to place the monitors

I have placed the monitor 400nm above the thin film. I have generated farfield plots. Just to know if this is the ideal position of the box 0f 5 monitors(6th one is deactivated to the substrate) for far field simulations based on your suggestions.

[Guilin Sun]

In such case, the side monitors can touch the upper layer in order to pick up radiation close to 90 deg, as one point interpolation does not introduce significant interpolation error. No need to use 6 monitors.

However, the box monitors can only receive the radiation with the max angle of atan(half-FDTD-x-span/distance_from_dipole_to_upper_surface), as larger than this angle is absorbed by the side PML. If you calcualte that this angle is larger than the total reflection angle, then it should works since it is thin-film type without nanoparticles .

Please do some calculation and confirm the simulaiton spans are wide enough. It may need more memory but we cannot get good result if not all the physics is included in simulation.

 

[pnair]

Thank you for the reply, "the side monitors can touch the upper layer in order to pick up radiation close to 90 deg," I have modified my box of monitors dimensions according to this suggestion and I have marked in the picture shown below. I am assuming this is correct way of placing monitors for farfield calculations.

"Please do some calculation and confirm the simulaiton spans are wide enough. It may need more memory but we cannot get good result if not all the physics is included in simulation". Could you please elaborate this a little bit and it would be really helpful as I am struggling with this farfield simulation to get more sensible results?

[Guilin Sun]

The max radiation angle that the monitor can pick up is like what is shown above (inside the material. Outside it is the refracted angle). If this angle is larger than the angle creating total internal refelction, it should work. Otherwise you need to increase the simulation span.