Learning Forum

[oyesilyu]

Hello,

I have been trying to realize far field summing method demonstrated for the metalens far field calculation with "farfieldexact3d" script command. Originally, the script uses "farfieldvector3d" command to calculate the far field of the metalens. The results I obtained with "farfieldexact3d" clearly indicate incorrect calculation of the far field. I have the necessary files to replicate my results. Please let me know if they (.lsf script and .mat file) are needed alongside with the instructions for file upload as I cannot seem to find a way to do it. You will notice that the focal point is no where close to intended distance and the field distributions seem incorrect.

The reason I am attempting to use "farfieldexact3d" command is to calculate exact fields/profile at desired coordinates with field summing method, which is applicable to massive metasurfaces (1000-10000*wavelength).

I hope this is enough background about the problem.

Thank you,

Omer

[oyesilyu]

I would really appreciate your timely response with this.

Thank you,

Omer

[Guilin Sun]

Sorry to reply you late as our team is busy in other urgent projects.

NO need to upload anything as the forum does not allow file load and does not allow us downloand and check the files by forum policy.

I think it might be some reasons for the discrepancy. Is there any higher order diffraction from each element? is there sufficent number of elements? are the simulation settings including source are correct without any issue? 

The summation method uses pure plane wave results whereas in FDTD simulation a plane wave will be affected by the aperture (if any) or PML, which will cause diffraction. and sometimes the diffraction interacts with the metalens and can create secondary (or higher-order) images.

Please check the simulation files and see if there is anything to improve Not every design can lead to expected results.

 

 

[oyesilyu]

Thank you for the reply. I am trying to replicate the example results for "far field summing" method with "farfieldexact3d" command instead of using "farfieldvector3d". All the calculated near fields and other parameters are identical. All the underlying physics should stay the same. Essentially, my question boils down on how we can perform the far field summing with "farfieldexact3d" command? If you have an example script, that would solve my issue.

 

Best,

Omer

[Guilin Sun]

It seems you want to use farfieldexact to replace farfieldvector3d from the full metalens simulation? yes you can directly use farfieldexact. However it cannot be used for summing method (this requires periodic structure), and farfieldvector3d is for a distance of 1 meter on sphere, but farfieldexact is for specified xyz. Do you use extactly the same paramters? in fdtd_full_lens_plot_field.lsf it uses farfieldexact. Please check your script again and not be confused by the scopes of the two script commands.

[oyesilyu]

Yes, I want to replace farfieldvector3d with farfieldexact from the full metalens simulation. I want to use the farfieldexact with summing method. Essentially, I want to use the summing method for a very large metasurface to calculate it's farfield at an exact location described by the user. How could we use Lumerical script commands to achieve this? Also, can you explain why we can use farfieldvector3d with summing method and why we can not use farfieldexact?

[Guilin Sun]

" I want to replace farfieldvector3d with farfieldexact from the full metalens simulation. " this is ok.

however, "I want to use the farfieldexact with summing method" you cannot do it, as the summing method requires periodic strucuture, and both farfieldvector3d and farfieldexact cannot be used for periodic structure. Periodic structure is graing-structure with isolated diffraction orders and support plane wave in nature.

I hope this is clear.

[oyesilyu]

Thanks for the clarificaiton. However, the structure I am interested in is a unit cell under periodic conditions. Also, farfieldvector3d is used in the example file, and it has been shown to work nicely. Based on this input, is there a way using farfieldexact3d for periodic unit cells and sum their far fields?

[Guilin Sun]

I understand your intention however you cannot do something that violates physics or math. farfieldvector3d is used in the example file for the whole lens, non-periodic structure.

farfieldexact3d cannot be used for periodic structure, since it does not count for the periodic contribution, and it requires the field intensity at the monitor edges be close to zero, if not exactly zero. This is its theoretical base.

Again, you can either simulate the whole lens with farfieldexact3d, or simulate the unit cell with periodic boundary and plane wave with summing method. Periodic simulation results can only use grating analysis, which is farfield. Then it needs stitching back to the focal plane.