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How-to : Transmission Spectra vs Wavelength

November 13, 2013, 9:55 pm
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Hi

I had attached the concept of my simulation as picture where you can understand my idea.

For obtaining Wavelength vs Transmission spectra I need to include all Refractive index (n and K values of GaAs). I had imported Refractive index values via interpolation. I had converted n and k values to Relative permitivity say GaAs_real and GaAs_imag.

Previously in Parameters I had defined Lambda = 1000 and frequency = c_const/Lambda[nm]. I had used nm in Lambda since imported (ex. GaAs_real) wavelength values as say 800 ..... without nm.

In GaAs Material properties I had defined Relative permittivity as = GaAs_real(Lambda)-j*GaAs_imag(Lambda). I think this is correct definition.

So In Parametric sweep now I can do Lambda dependent.

My Goal is to get PL spectra ( Transmission ) vs wavelength. I had placed Point in GaAs and used that as Dipole source. My target is to achieve Transmission spectra coming out from GaAs vs wavelength (say 800-1200nm). I m really confused here how-to-do. For obtaining transmission spectra do I need to use port condition or what ?
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Light reflectivity and transmitivity of periodic surface structures

May 29, 2015, 9:15 pm
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Hello,
I am trying to simulate the reflectivity and transmitivity of different wavelengths of light on a periodic surface structure.

1) I am using a 2D simulation using the RF module to create a basic cell with two media air and silicon and am trying to repeat it for the entire structure (periodic condition on side walls). I plan to have a electromagnetic wave incident on it (first TE and TM then average it out).

2) I plan to input the refractive index of silicon for different wavelenghts and use this for the simulation over the entire wavelenght range using a parametric sweep. Refractive index is the only material property I have specified.

3) I have used two ports, one for the input of a plane wave (and reflectivity) and one after the silicon side to find the transmitivity.

Is this right approach? I am unable to get reliable results as the values of R and T are fluctuating rapidly (between 0 and 1) over the wavelenght range. Could someone please throw some light on the above problem. I have attached images of the geometry and my results below.

Thank you.
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Need to simulate very thin layer with permittivity and permeability tensors

June 22, 2015, 12:18 pm
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I am simulating an antenna structure in the RF module. Unfortunately, due to 3 dimensions of variability, no 2D models are suitable. Therefore, I am using a 3D solver.

I have an impedance sheet covering a radial waveguide that produces a leaky wave. The impedance sheet has both permittivity and permeability tensors. The simulation is cleanest (i.e. smallest and produces best results) with the transition boundary condition. However, though I can input an anisotropic permittivity, the permeability only allows constant inputs.

Is there a module that has the capability to include permeability tensors in a transition boundary condition? I have not found one yet, with some fairly deep searching. Sans that, can a custom boundary condition be written that allows this?

Other attempts I have tried are making an actual 3D bulk material in place of the transition boundary. This is problematic because the material must be very thin, which greatly complicates the simulation. I am unable to get a thin enough layer.

An alternate approach is to define a current source that is dependant upon impingent fields. However, this requires both electric and magnetic current sources, and as of right now, I havent found a way to implement both on the same boundary.

Thank you,
Jason
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Light propagation in silver nanowire

June 23, 2015, 12:43 am
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Dear all,
I am trying to simulate the light propagation in silver nanowire by exciting its one end of nanowire. Kindly please suggest me how to do it.
Thanks

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Recovering desired eigenfrequencies

December 20, 2012, 12:38 pm
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Hi,

I'm performing an eigenfrequency analysis of a model in the Comsol 4.3a RF module. My problem is that, though I ask for only 1 eigenfrequency, Comsol ALWAYS gives me 2 eigenfrequencies: one purely real, and one complex. This is frustrating me because when I try to probe the eigenfrequency, it always probes the wrong eigenfrequecy (the complex one), so the only way I can extract the data for a 50 point sweep is to manually go through and extract the proper eigenfreq. The correct eigenfreq is always at the bottom of the dropdown menu, which in previous versions of Comsol always corresponded to the first solution.

I figure there are two solutions to my problem:

1. Figure out how to make Comsol only give me 1 eigenfrequency (should work if the bottom solution does still correspond to the first one it found).

2. Tell Comsol to probe eigenfrequency 1.

I can't seem to figure out how to execute either of these solutions. I'm attaching a couple screen shots of my model builder to help explain.

Any advice would be greatly appreciated.

Thanks, Chris
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How to get only real frequencies

December 15, 2014, 6:21 am
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Hello.
I am running eigenfrequency study in RF module. I set the frequency and amount of solutions to find. And I am getting among good solutions some complex eigenfrequencies corresponding to non physical solutions.
Is there a way how to set COMSOL searching only real frequencies?
Thank you.
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Initial parameter value effects parametric sweep

June 24, 2015, 1:17 pm
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I am using a parametric sweep to vary the wavelength of incident light for a nanophotonics simulation, but I'm finding that the initial value of my wavelength parameter effects the results.

The parametric sweep node is set to sweep global parameter "lda" for a range of values.

The frequency in the Study 1: Frequency Domain node is set to f0. This is a global parameter defined as f0=c_const/lda

The final results (measuring sample reflection and transmission) differ depending on the value of lda set in the Global Definitions node.

The only other thing that is dependent on this value is the refractive index of my particle, defined using an interpolation from imported values, but I get the same effect if I change these to constants.

Can anyone help me fix this? I'm using ver 4.3a
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NaN or Inf found when solving liner system using SOR,RF devices, non-reciprocal Devices

June 25, 2015, 11:02 am
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Hello All,
I designed a Non-Reciprocal device using Ni78-Fe20. I like to plot the real and imaginary part of s-parameters, so I enable the (split complex terms) under solver but when I am trying to do so I am getting an error (NaN or Inf found when solving liner system using SOR). Model runs perfectly without enable complex terms under solver. I have one more question how can we extract touchstone file or file which gives me frequency v/s complete s-parameters reading. I have attached the paper and Any help will be very appreciated.
Regards,
Kumar
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inconsistency between "neff" and "beta"?

June 26, 2015, 11:27 am
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I am using "Mode Analysis" study to find a guided mode in a structure. I can ask Comsol to compute separately the effective index of the mode (neff) and the propagation constant (beta). The relationship between these two must be straightforward:

beta=neff*k=neff*2*pi/lambda

However, if I compare neff and beta as Computed by Comsol, they do not quite follow this relationship. Generally the difference between neff and (beta*lambda/(2pi)) is of the order of 1%.

I wonder why there is this difference and which of the two quantities is computed more accurately by Comsol (if any???)
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Convergence: when should I stop the model?

March 15, 2012, 3:55 pm
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Hi everyone

I have been working on models that take several days to solve. One of them is running at the moment, and I just have a general question.

When I look at the Convergence Plot 2, which is "Reciprocal of step size" vs "Time step", I can see the reciprocal of step size is between 10^5 and 10^6. It has been like this for several hours, and the progress value is at 2% without increasing.

I need results for this simulation by the beginning of next week. When do you know if you should stop the simulation or let it run and solve this convergence difficulty eventually? If I stop it, and the model could have advanced and solved, then I'll lose several hours of solving. But if I don't stop it, it might be at 2% for more than a day.

What should I do? What are the general guidelines on convergence?

Thank you so much!
Sylvana
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applying plane wave

June 26, 2015, 4:03 pm
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I am modeling Shielding effectiveness of the block (in 2D), so I am applying plane wave in the x direction and considering periodic boundary condition for boundires in y direction. To have plane wave I considered scattering BC for left and right sides, is that the appropriate BC for applying plane wave in Comsol? How Can I also find the transmitted and input poewers?
Thanks
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Integral over a arbitrary path

June 29, 2015, 9:00 pm
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I am new to COMSOL. I am interested to calculate the characteristic impedance of a co-planar waveguide in the RF module. I am referring the example "Finding the Impedance of a Coaxial Cable" for the same. In the example to calculate the current, the Magnetic field is evaluated around the external boundary.
I was actually interested to evaluate the integral along any arbitrary path apart from the boundary of the conductors which are Perfect Electrical Conductor.

The issues are facing the problem:
1. I am not able to define an arbitrary path without affecting the Meshing of the geometry which makes the evaluated value very much path dependent.
2. If I try to do a post processing without defining any path in the geometry, the "derived values" section of Results Tab is not allowing me to define any path except of the boundaries which were already present.

Can somebody suggest a best way find the characteristic impedance of the waveguide in RF module?
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Patch Antenna S11 Problem

June 30, 2015, 5:55 am
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Dear All,

I have been simulating very basic patch antennas. However when I compare simulations with measurements I always get very different plots (please see the attachment). Can it be about COMSOL's S11 Parameter or a plot scale problem? What would be reasons for that?

Thanks in advance.
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Initial condition drift when sequencing COMSOL through Matlab

July 1, 2015, 5:57 pm
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Hello,
I’ll start with my question and then explain the problem which gave rise to it. Is there a way to insure that the initial temperature condition in a transient heat transfer problem is the same as the results given at time = 0? I have noticed in running Comsol that in transient heat transfer problems, the initial condition doesn’t always match the result given at 0 s. The difference is usually negligible. However, I have encountered a situation, where this problem is not negligible.

I am using a Matlab script to make multiple runs of a Comsol 5.1 model for heating in a microwave field. The script divides the total heating time into steps, taking the temperature distribution at the end of the previous step and using it as an initial condition for the next. I change the geometry in the script between steps, but to make sure everything is operating correctly, I tested this approach WITHOUT the geometry change. So, regardless of the number of steps, the results of running Comsol step-wise via Matlab and running Comsol directly, should be the same. To the contrary, I have found that the more steps taken, the more the results diverge. And counterintuitively (at least for me), I saw that the finer the mesh the greater the divergence as well.

The model is run for a total of 30 seconds and the final average, maximum and minimum temperatures of the heated domain are examined. I run the Matlab script with 1, 2, 3 or 10 steps, corresponding to time step intervals of 30, 15, 10 and 3 seconds. Naturally, the results match when the script is run with 1 step. I’ve attached an Excel workbook showing the comparison with other steps. The last tab (30 s temperatures), shows a table of the results. The other tabs show graphs where the Matlab results are individual points, while the Comsol direct results are shown as smooth curves. All three exhibit increasing deviations with increasing step number, though the global average and global minimum temperatures show much smaller deviations than the global maximum temperatures.
Does anyone know of a way to force Comsol to honor initial conditions?
Thanks,
Greg
P.S. In case it’s important, I generate the initial condition for one step by using MPHEVAL in Matlab to evaluate the temperatures in the heating domain from the previous step. I then write the temperatures and corresponding nodal coordinates to an Excel file that the next Comsol step uses as the initial condition via the interpolation function.
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Bi-anisotropic medium

July 2, 2015, 4:34 pm
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Hello,

I am using COMSOL 4.3a and its RF module to simulate electromagnetic structures. I would like to know if it is possible (and if yes, how) to implement fully bi-anisotropic medium ? So far, I was only able to define anisotropic relative permittivity and permeability but I didn't find anything related to the chiral parameters.

Can anyone help ?

Thank you.
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radiative deccay rate & nonradiative decay of the fluorescence molecule

July 3, 2015, 9:51 am
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Hello everyone
I want to calculate the radiative deccay rate & nonradiative decay rate of the fluorescence molecule near the rough metal surface.
I want to know whether the molecule should be simplified to a dipole and how could I to calcule the decay rate.
Anyone have the related model ?

Thanks !
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How to realize PI controller ?

December 15, 2009, 3:45 am
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How to set relate parameters?
For example , voltage control temperature .
thanks.
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About optical force calculation of particles manipulated by microring resonator

April 23, 2012, 11:49 pm
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Hi,everyone.

I want to simulate a structure which includes two bus waveguides and one microring resonator to form an on-chip optical tweezer. One microparticle is placed in the gap area between one waveguide and the resonator.

The structure is quite similar with:
pubs.acs.org/doi/abs/10.1021/nl100501d
(work from Kenneth Crozier, School of Engineering and Applied Sciences, Harvard University)

I want to use the Maxwell Stress Tensor method to calculate the optical force on it.

I tried the following:
In Results-Derived values. I added a surface integration for the whole surface of this ball-shaped particles. However, I am not sure how the expression of the force is.

If you have some ideas about optical force calculation, please feel free to discuss with me.

Thank you so much.
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Does size of Geometry affect the solution ?

July 6, 2015, 9:46 pm
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I am looking for electric field distribution in a coaxial waveguide. Now if my waveguide is of order of mm range, the electric field pattern is uniform and matches the analytical solution for TEM mode. Now if I reduce the dimension to um(10^-6 m) range, i am seeing the anomaly. I am attaching the figures for the same. I am not sure what am I missing ?
I am operating at 1 GHz frequency. On increasing the frequency, the anomaly decreases.
For both cases I have used "Extremely Fine" element size option.
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RF field - initial value - comsol 4.1

January 6, 2011, 2:19 pm
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Dear forum,

I ask for an advice in the following things:

1)
Is there a predefined Poynting vector in RF module?
& where exactly can I list through the comsol-names for physical quantities?


2)
I have defined own analytic function
besselbeam(x,y,z,kz) = besselj(0, kz*sqrt(x^2+y^2)) * exp(-j*z*kz)

When I try to use it for the initial field Ez = besselbeam(x,y,z,kacko_z)

then the solver says

Error in user-defined function.
- Function: besselbeam
Failed to evaluate expression.
- Expression: besselbeam(x/unit_m_cf,y/unit_m_cf,z/unit_m_cf,kacko_z*unit_m_cf)

I am solving for a fluid velocity field, not EM at the moment.

What is wrong? Thanks for ideas.

Lukas Chvatal


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