Learning Forum

[Haridas]

Hi,

I have been trying to estimate the Purcell values for a dipole emitter embedded in a thin dielectric film placed closer to an ITO interface using Lumerical FDTD and stack (Please see the attached schematics). The simulation shows completely different results in stack and Lumerical. The Purcell estimated using Lumerical shows values much lower than 1 particularly for vertical dipole at longer wavelengths and different from the values simulated using stack (see attached plots). Why is the Purcell estimated in lumerical less than 1? Why the stack and Lumerical results are different? Please let me know your thoughts.

Please let me know if you need any additional details about the simulation. 

Thank you.

Best,

H.M

Schematics.png

[Kyle]

Ansys staff are not permitted to download attachments from ALF posts, so I can't see your images. Could you please insert them directly into your post (for example, copy and paste them in)?
I would guess that the issue is with the FDTD simulation, the STACK results should be pretty reliable. Some possible causes of inaccurate FDTD results in this case would be:
Mesh isn't fine enough to resolve the layer thicknesses
FDTD uses a fit to the material data, while by default STACK uses an interpolation of the material data. To match the FDTD results, you can use getfdtdindex in the STACK script instead of getindex to use the same material data in STACK that you're using in FDTD
This example has more details on these issues: FDTD vs stackdipole - halfspace emission in a multilayer stack.
If the dipole is in a dispersive medium or the mesh size is very small (~wavelength/1000), you may also need to use a box of monitors around the dipole to measure the emitted power instead of using the automatic Purcell factor calculation. See these pages for more information:
Purcell factor of a microdisk
dipolepower
Fluorescence enhancement

[Haridas]

Hi @kjohnson
Thank you for your reply. Please see the schematics and plots below. The FDTD simulations were done in 3D, where the layers span 6 microns in X and Y direction and extended through the PML layers placed at 4 microns. The FDTD simulations in 2D also showed Purcell values less than 1 at longer wavelength. I have used high resolution meshing (Mesh accuracy 8 with non uniform meshing) and also tried including a mesh override region around the structure , but there was no change in the simulation results. I have also checked the stack simulation using the getfdtdindex command for the ITO layer and did not observe any considerable change the Purcell values. Looking forward to your suggestions and please let me know if you need any other information.

[Kyle]

How fine was the mesh you placed over the dipole source? There are some grey mesh cells around the dipole source, you need to make sure these cells are not overlapping with the interface of the ITO and the dielectric (there is more information on this issue here: https://forum.ansys.com/discussion/comment/124800). So the mesh should be around 5 nm to make sure there are at least a couple of mesh cells between the interface and the source cells.
I would also recommend trying the box of monitors approach to measure the Purcell factor, which is used in the examples I linked in my previous post, to see if it gets the same results as the built-in Purcell factor calculation.

[Haridas]

Hi @kjohnson Thank you for your reply. The mesh size around the source was 2 nm. The source is 20 nm away from the ITO and not overlapping with the interface regions. I am already using the box of monitors to estimate Purcell and in this case it matches well with the built-in Purcell values.
Best Haridas

[Haridas]

@kjohnson I am wondering if you would be able to check my .fsp file for any error in the simulation set up. Please let me know.
Thank you for your help.
Best Haridas

[Kyle]

Sorry for the delay in replying. What materials are above and below the ITO and dielectric layers? Are you including these materials in both the FDTD and STACK simulations? The STACK simulation needs to include layers for these materials as well, even if the thickness is zero.

[Haridas]

Thank you for your reply. The simulations results in stack and FDTD are now matching. However, the Purcell values for vertically aligned dipoles are less than 1 (~ 0.3 around 700 nm) and I am trying to understand how these Purcell values can be related to the emission dynamics of the dipole emitter. Does the less than 1 Purcell values indicate reduced emission power for the dipole emitter? can we relate it to the excited lifetime of the dipole emitter? I would appreciate if you could provide some insights in to it.
Thank you.
Best Haridas