Create a new material

[Nathalie Carrier]

Hi everyone,

I am using FDTD 12 (1 month evaluation) and I would like to create a new material in the FDTD profile designer based on experimental data.

These data are stored in a .txt file and contain 3 columns (wavelenght, index real part, index imaginary part). Is it possible to import them directly ?

Thanks for your help !
Nathalie

[Damian Marek]

The materials that are used in FDTD analysis MUST abide by the Kramers-Kronig relation. It is possible to fit the experimental data to a Lorentz drude model and then define a material with those coefficients. You can Matlab code from quick searches that will fit these problems.

http://fdtd.kintechlab.com/en/fitting

An example of a Matlab program can be found at the link.

Regards

[Nathalie Carrier]

Hi Damian, thanks a lot for the reply, I will fit my data with a Lorentz Drude model.
\Nathalie

[Nathalie Carrier]

Hello,

I am using the MatLab code that you provided me.
However fitting my data with ‘Lorentz Drude’ model appears not to be very accurate (see attached the best result I can get using 6 oscillators).
Would it be possible to import the data directly from XML format ?

Thank you again,
\Nathalie

[Damian Marek]

Unfortunately no, the permittivity must be provided in Sellmeier, Lorentz, Drude or Lorentz Drude format for dispersive simulations. You could choose one wavelength you are interested in and run a CW simulation with a single sample of the data or maybe there is a matlab script out there that can perform a better fit.

Regards

[Nathalie Carrier]

Thanks for the reply, I will focus more on the fitting then. It should be possible to find a better fit!
\Nathalie

[Nathalie Carrier]

One last question, I tried to set a new material (with Sellmeier model) with the oscillator parameters I found (even if the fit is not perfect) but it seems that negative amplitudes are not supported. How to overcome that ?
I tried Lorentz-Drude, same issue.

Thank you,
Nathalie

[Damian Marek]

The Lorentz-Drude model should allow negative strengths under the “S” parameter, I just verified that it works. It won’t work for Sellmeier model since it does not model loss. However, I am not sure what the physical meaning of a negative strength is!

[Nathalie Carrier]

Hi thank you for the reply, I do not think there is a physical meaning of a negatig amplitude but the fit is good enough and in the Lorentz-Drude model form (and the fit does not converge with only positive amplitudes).

I still have a question regarding how to fit the permittivity. The convention used by the software is: a loss correspond to a negative imaginary part. For the example shown on the attached picture on the post from November 18, 2015 at 11:38 am, it means the imaginary part should be set as negative before fitting ?

Thank you,
Nathalie

[Damian Marek]

Yes you should use the negative of the imaginary component and then do the fitting.

[kasun gankanda]

Hi
i am a new user to this software and is planning on using it for my research work.
currently i am making a simple simulation of SPR on a gold layer with a small slit on a glass surface .

My problom is i cannot find how to make the glass material
and since i am planning to use Kretschmann cofiguration for SPR excitation .
what type of excitatio wave is good for the simulation.

thank you in advance .

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