How Do You Calculate Optical Properties of an Unknown Thin Film?

[Tzontonel]

Homework Statement

I need to calculate n, k and d of an "unknown" thin film.
I have the transmission spectra of the substrate (MgF2) and substrate + "unknown" thin film
See the attachments with: three files
- two screenshots of the transmission spectra (see file name for description)
- one .txt file with divided values (can be plotted to see the T spectrum of the "unknown" thin film).

The Attempt at a Solution

If think very clear, I need to divide values of %T of substrate+thin film at values of %T of substrate, and I obtain transmission values/spectrum of thin film. After that I apply the equations from Swanepoel, 1983. It's correct?
Also, I want to know if it's possible to have the %T ~100 of the thin film. I think it is an oxide (MgO/ZrO). And I'm confusing to assign the Ts to %100; I said that because when I import the spectrum in PARAV 2.0, the software put the Ts value at %95.42, why?!

Please help me, I have backgrounds in Geology not in Physics (but I do my best), and It's very hard to understand without any help/advice.

 

[Tzontonel]

Someone can help me with an advice? Please.

 

[ehild]

It is no use to divide the spectrum with that of the substrate. Have you got the original spectra of both substrate and layer?

ehild

 

[Tzontonel]

Here you go.

Note: The both spectra presents an collapse around ~350 and 800 nm, probably due to the change of the parameters of the instrument (lamp and so on - I think).

 

[ehild]

Is both sides of the substrate covered by identical layers? Or does the layer cover only one side of the substrate?

ehild

 

[Tzontonel]

The file MgF2 spectrum.txt is that obtained from substrate MgF2 (without thin film), and the second spectrum (Substrate (MgF2) + thinfilm spectrum.txt/MgF2 coated) is obtained from the same substrate with an "unknown" thin film.
I attach the "original" file, where you have the T values for the both spectra, also at the bottom of the columns you have some parameters used (if that help you to understand much more things for this files and spectra).

Thanks for your patience!

 

[ehild]

I do not understand why is the measured transmittance of the substrate is so low. MgF2 has the refractive index about 1.38, and transmittance of about 95%.

When a transparent thin film is on a transparent substrate, the transmittance at one of the extrema is identical to that of the substrate. In case the refractive index of the layer is higher than that of the substrate they are the maxima where the layered sample and the bare substrate have the same transmittances.
Multiply the divided transmittance data with the theoretical transmittance of the substrate, (0.9542) and apply the equations to the corrected transmittance spectrum.

ehild

 

[Tzontonel]

I attach the paper. In fact what I need, is to calculate d, k, n and absorbance of the thin film and finally this values are used to apply the formula no. 4 (see Swanepoel-paper.pdf).

And all of this things from that spectra attached (transmission spectra of MgF2 substrate and MgF2 coated). Also, I use PARAV software (http://www.chalcogenide.eu.org/index.php?option=com_content&task=view&id=1&Itemid=2" ), but I want to make an excel file to calculate step by step and to compare with the results obtained in PARAV.

If the layer covered both sides of the substrate, I guess n=1.66 refractive index and 1.4 micrometer thickness for the layer

The layer don't cover the both sides, just one (it is the substrate, and after him is a thin film). You are so fast with calculation, how?!

 

[ehild]

Thank you for the paper!
Well, with one side covered, the situation is quite different.

Find the refractive index of the substrate from Ts= 95.42%, Multiply all relative transmittance data with Ts, and use this value to get the refractive index of the layer from the transmittance at the two longest-wavelength minima.

(My new result is 1.83 for the refractive index and 1.06 micrometer for the thickness. I wonder if you get similar result)

ehild