optical current in the VPD in your official website


Hi all
I have learnt the VPD example on your website recently.https://kb.lumerical.com/en/index.html?applications.html I found the metal you use in the FDTD simulation is PEC and finally the responsivity you get from it is 0.93A/W. You said the value is in good agreement with the value in Ref 1.But the metal used in the Ref 1 is aluminum.

1.What is the difference between the PEC and aluminum.Can the PEC be used in the FDTD simulation instead of the Aluminum.
2.I have tried to use the aluminum in the FDTD simulation.Finally the responsivity I get is larger than that got from the PEC. In my opinion,I think that’s not correct.How do you think of this question?
3.When I use the aluminum instead of PEC in the FDTD simulation.I found it spent much longer time to complete the simulation. How can I improve the simulation speed.
Hope to get your feedback. Thanks a lot

the optical current difference between PEC contact and the aluminum contact

Hi. In this particular structure, the key result that we are interested in is the optical absorption and the contribution of the metal to this result is negligible. However, if we use a real metal (Al) in the simulation instead of PEC then the simulation uses a much finer mesh over the metal (since the optical wavelength is much smaller inside the metal due to its higher index). The whole simulation then becomes extremely slow and requires a lot of memory. This is why we have used PEC in this example.

Now, the shortcoming of using PEC in place of a real metal is that PEC will reflect 100% of incident light whereas a real metal like aluminum might let a small fraction of the light to pass through. Also, if the purpose of the metal is to create surface plasmons then PEC will be unsuitable since PEC will not create surface plasmons. In the VPD example, none of these issues are of concern so we should be able to use PEC instead of Al here.

As mentioned earlier, this has to do with the large index of the real metal. However since PEC is a special material, the FDTD solver can handle it in a special way and it does not need to use a fine mesh for PEC.

I did a quick test and I noticed that using Al instead of PEC does result in a small increase in the photocurrent (15% increase) if I run the simulation with the current settings (I did use a coarse mesh in Al though to make the simulation fast). However I believe this happens due to the following reason:

In the simulation file, you will notice that the generation rate analysis group extends slightly inside the metal. In case of Aluminum some light goes into the metal and gets absorbed and therefore this additional absorbed light when gets included in the calculation of photogenerated current results in a larger current. However, in reality any absorption in the metal does not contribute to photogeneration of electron-hole pairs so even though the result in FDTD gives us a larger photocurrent the overall responsivity should not increase much.

When I made the analysis group smaller in Z-direction so it does not go inside the metal, the different in current between PEC and aluminum came down to a mere 2%. This variation is quite insignificant and I believe it shows that we can get accurate result with PEC while making the simulation faster.


dear aalam
Thanks a lot for your feedback. I almost have understood what you said while there is only a small question left.
1.In the answer fo the third quetion,you mention that the generation rate analysis group extends slightly inside the metal.But when I check the maxium of the z by using "z=getdata(“generation rate::field”,“z”,1);"
I found that the maxium of the z is 6.2um,which means that the generation rate analysis group doesn’t extend slightly inside the metal.What is wrong with my understanding.

  1. can you pass me the FDTD file after you made the analysisi group smaller in Z-direction , or tell me how you make the analysis group smaller in Z-direction.
    Thanks a lot.


Hi. I’ve taken a screenshot of the YZ plane of the FDTD file from the online example page. In the figure you can see that the analysis group (yellow box) extends inside the metal by one mesh grid.

You can modify the extent of the analysis group from the setup tab. In this case, make the z span 0.5 micron in place of 0.54 micron.


dear aalam
I have tried as you said.But in the FDTD simulation of my designed PD,I found that the optical current of the aluminum used is larger than that of the PEC.furthermore,the generation rate analysis group didn’t extend inside the metal.So I think that the reason you said is not right.how do you think this question? This two FDTD simulations below are mine.you can have a check.
Hope th get your feedback.Thanks a lotPEC.fsp (657.2 KB)
Al.fsp (663.3 KB)


dear nlui
as you mention above,you suspect that the interpolation error of the monitor data at the interface between Al and Si cause the small amount of current. Does it mean that there still be some bug in your software.If I decrease the z span of the analysis group by 1 mesh cell,it means that the analysis group don’t contain all of the silicon region.The total optical current collected just come from partial of the silicon.Is this true and reasonable??
I hope to get your feedback.Thanks a lot



From the Al.fsp simulation file, I zoomed in to the view of the structure near the interface between Ge and metal and I found that although the monitor does not extend into the metal, the monitor is cutting through a line of mesh cells that includes both metal and Ge.

The field monitor in the analysis group is using the option “None” for the spatial interpolation option, so the problem is not due to interpolation of monitor data which it could be if the interpolation was set to “specified position”. However, when using the “None” option, if the edge of the monitor does not lie exactly on a mesh line, the monitor snaps to the nearest mesh cell, so when you run the simulation the effective size of the monitor extends upwards to include the full mesh cell that extends into the metal, so some of the monitor results that are returned are in the metal material which causes you to get some contribution from the metal.

You should be able to remedy this by modifying the mesh to ensure that there is a mesh line that falls at the interface between metal and Ge. Hopefully this helps!


dear alui
I have understood what you said .I agree with you that this may be the reason that cause the optical current of aluminum is larger than that of the PEC.But can you teach me how to zoom in to the view of the structure near the interface between Ge and metal?I don’t konw how to make the mesh line in the FDTD simulation and can’t get the picture you post above?Thanks a lot.:relaxed:


No problem, to see the mesh and zoom in you can use the options from the toolbars on the left hand side of the design environment. The “view simulation mesh” button will show the orange mesh lines in the CAD view ports:

Then, if you click on the “zoom” button, you can select a smaller region inside the view port to zoom in on:

This video also gives a demonstration on some of the toolbar options:


dear nlui
I have checked in your guidance and got the same result as you said.Now if I want to modify the programm and make the monitor not cut through a line of mesh cells that includes both metal and Ge,can you give me some advice?How to modify the programm.Thanks a lot.
Hope to get your feedback.


Hi. Check out my earlier response on Sep 23 to see how you can change the extent of the analysis group. You basically have to define the x,y,z centers and their spans to do this.


dear aalam
Thanks very much for your answers.I will have a try.:relaxed: