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I do not know your geometry scale for this problem, but there is a concern about trying to extract a transmission within a short working distance.

Certainly, you could put a boundary probe on the top air boundary to record the transmitted field, but there are several questions to determine. It is far enough away from the GaAs/Au structure to form a uniform field appropriate for retrieval? How do you determine the baseline "incident" power from a point dipole inside a small structure (i.e. in the near field)?

I've set up a short example that is probably not very accurate, but at least demonstrates the challenges of trying to do what you have requested. I set up a boundary probe on both the air and the GaAs/Air interfaces, as well as a point domain probe placed near the dipole. The results of the normalized and averaged power transmittances are shown in the global 1D plots, and show the discrepancies between using the GaAs/air boundary and the point probe.

Keep in mind I am working in "freq" terms, but either you can switch it around for "lamb", or you can just edit my "Parameteric 1" component to sweep what wavelengths you want.


Attachments: GaAs.mph
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Vasanthan Devaraj
November 15, 2013 5:09am EST in response to Bryan Adomanis

Re: Adding Refractive Index

Hi Bryan Adomanis

First of all thanks for your model and try. Its really helpful since I m just beginner in COMSOL. I had attached the geometry of my structure . Previously i used Scattering BC, but now changed to PML as per your ex.

For obtaining transmission(hereafter T or PL) spectra, i think we can adjust the height of transmission monitor to get the best from GaAs top surface.

Coming to your 2nd point - //How do you determine the baseline "incident" power from a point dipole inside a small structure (i.e. in the near field)?// I couldn't understand this ? Why we have to determine the baseline incident power ?

I understood that you used Boundary probes for obtaining T. In Relative permittivity it should be square of refractive index right ?

And in Dipole, you had chosen Dipole moment . How about choosing Magnitude and direction and is it possible for getting T??

The choice of the dipole was just random. There are several things you can do to create a source that suits your needs.

So I mention the incident field/power because that is what is necessary to compute transmittance. When you say "transmission monitor", I am not sure what you are indicating. Certainly the boundary probe records the full field, but it does not compute a transmittance. In order to get transmittance, you must determine a ratio of your recorded field at some region of space over the source field. Normally this is done via a set of ports, where one is both an active "source" boundary AND a "monitor" for the reflected field, while the other is only the monitor for the transmitted field. The source is generated as a plane wave with some known field strength, orientation, power and phase at the boundary, and so the data recorded at the transmission port references these inputs to compute the spectral power transmittance.

I do not know of a way that this is possible for a point source in the near field. Physically, it doesn't make sense to record any field data near the dipole for comparison to the field at the air boundary to get some transmission. First of all, you are measuring the full field, and have no knowledge of the original source field, other than the analytical solution for a field from a point dipole. Secondly, the field is a near field, and thus cannot give you any real comparison to a far-field plane wave needed for a transmittance determination.

Maybe I am still not understanding your problem sufficiently. Could you explain what is it you are trying to acquire, and what recorded data you expect to need to get it?

In our previous discussion in refractive index, I understood the point now for betting using n and k values rather than permittivity. I m really thankful to for giving your time in this and the model u shared.

Coming to this topic, I will explain my focus. Actually I did FDTD simulation with this structure. Why i m preferring dipole source means, I m dealing with Quantum dots (QD). In FDTD, we use dipole source as in meaning of QD. So trying apply the same in COMSOL. I had calculated the transmission spectra vs wavelength. I had attached the copy of result for my structure using FDTD (black line spectra for our current target). It is similar with my experiment results. I think you know my structure now. I think now it will be more clear for you.

So for each wavelength n and k values differ and thats y we are importing n and k values via interpolation. So in COMSOL we have to prefer QD by means of Dipole source right? So with this we have calculate Transmission spectra vs wavelength.

Though I do not have experience with QD, I understand what you are trying to do. However, I am trying to understand how you are planning on doing it in COMSOL.

In FDTD, you generally have good information about the source pulse, as well as the information as to how it was transformed to a continuous wave approximation, so recording transmission data requires less consideration in setup. In Lumerical FDTD, for example, you record transmission simply by setting up a transmission "monitor", which takes the complex field and power data and automatically normalizes it to the source field/power to give you the complex transmission coefficient(t) and the spectral transmittance (T). You do not need to explicitly reference the source information for the t or T calculations.

In COMSOL, you must explicitly reference the source data. This is why you normally set up TWO ports--one to define the plane-wave source and record complex reflection data, another to record complex transmission data. Without this source port--i.e. with only the dipole source--I do not know how you plan to identify the input field/power and phase to perform the t and T calculations. Certainly you could extend the air boundary or do a far-field projection to get a plane wave solution for your "output", but how do you handle the near-field "input" to get your normalization factor? Where do you place your monitor?

Hi Bryan Adomanis

I clearly got your point this time regarding COMSOL in dealing with dipole sources. Since I m a beginner in COMSOL, I just thought placing monitor kind of study will be similar to get data as like Lumerical FDTD. May be i think its better I will do this wavelength vs T calculations in FDTD. I m really thankful for your responses, explanations and your solutions. I learned in this progress how to use COMSOL.

Actually i think finding electric field with Point source will be easy using COMSOL. I think for this we dont need port conditions and all. I m now interested in this which will be useful for my studies.

I want to simulate the Vertical and Horizontal polarization of dipole source. May be better I can post this as new topic of discussion which will be easier for us to discuss. I would be glad and thankful if you could help me.

Anyways I can give a description here too.

Its the same structure with electric dipole source. For this we need to import n and k values for both GaAs and Gold right ? I did it and imported via interpolation. Since I m dealing in Lambda I made arguments in nm. The data source file for GaAs and Au is attached below. I got that from refractive index.info site. I think for each wavelength, the values are not listed.So this would be fine for simulation?? And I had defined the relative permittivity in GaAs and Au. Can you check if thats right ?? Can we use Scat B.C or PML for this ?

For Vertical and Horizontal Polarization, I chose Dipole : Magnitude and direction. So in this x = 0 and y=1 for V-Pol and vice versa for H-Pol right ? Correct me if I m wrong.

Thank you.

First, a few comments on the model. You may want to consider Impedance BCs instead of modeling the entire gold structure. The IBC should still accurately account for the loss into the metal, and your model size is nearly cut in half. Test it out! Also, you will not want to eliminate the PML on the air boundary--air boundaries will not act like a scattering boundary.

Second, it looks like the material properties are properly set: the indexes (and thus the permittivities) are functions of "lambda" and this is the variable you are parametrizing. You can check if you have done it properly by selecting the interpolation and hitting "plot", and look at the associated curve on RefractiveIndex.Info site. You ask " I think for each wavelength, the values are not listed.So this would be fine for simulation??" This is precisely what the interpolation has done for you--converted discrete data points (from published data) into a continuous curve that can be sampled for ANY wavelength in that range!

Now, back to the dipole. Yes, you can control the polarization in the manner in which you state. You could even place multiple dipoles of various magnitudes and directions on top of each other to simulate coherent, unpolarized emission (I think there is a laborious way to do incoherent emission). However, the question remains as to what kind of data you would like to retrieve. You mention "finding electric field with Point source will be easy using COMSOL"--you are correct, it is! Just place the boundary/point probe where you want to sample (average, integrate, etc.) the field generated (and scattered) from the point source. But so what? Is the full-field output all the information you need? You will not get a transmittance that way! You won't even isolate the scattered field that way. So what is it you WANT to get out of the simulation?

Hi Bryan

Thanks for your reply.

If you dont mind, I had posted the reply in my relevant topic since this is getting diverted here.

www.comsol.com/community/forums/general/thread/40765/#p110687

I am trying a similar simulation of transmission and reflection spectra vs wavelenght but in my case the input and and reflectance ports are the same and there is a transmission port on the other end. The base material is silicon and I have periodic structures on the surface (2D simulation). I am using version 4.4 and the RF module.
I have set up n and k interpolations for the wavelenght range of interest similar to the description above.

When I plot the reflectance I am getting a highly oscillatory result. Does anyone know why this might be happening? When I dont have any structures and keep the surface flat, I get the right reflectance values from literature.
More details on my problem are on this thread www.comsol.com/community/forums/general/thread/80041/#p184421
I have attached images of the results and the structure of interest.

Any help on this would be great. Thank you.

I had replied in your topic . Lets continue there