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Reflection from Thin Films Plot

  1. Aug 20, 2014 #1
    1. The problem statement, all variables and given/known data

    Here is a plot of Reflectance vs Wavelength from a multilayer thin film stack of alternating high and low refractive indices:

    abqooi.jpg

    I want plot this in Matlab, but what equation was used for this plot?

    The diagram is from my textbook, there are many like it online but there is no indication as to what equation was used for the plot.


    2. Relevant equations

    Reflectance from and N layers of film:

    ##\left( \frac{(n_0/n_s)(n_L/n_H)^{2N} -1}{(n_0/n_s)(n_L/n_H)^{2N} +1} \right)^2##


    Spectral width of the high reflectance zone:

    ##\Delta \lambda = \frac{4}{\pi} \lambda_0 sin^{-1} \left( \frac{n_H-n_L}{n_H+n_L} \right)##

    3. The attempt at a solution

    Well the first equation is not a function of wavelength. So I tried using the second one, with setting ##\lambda_0=514 \ nm## as the center wavelength:

    Code (Text):
    n0=1; ns=1.52; nh=2.35; nl=1.38;
    l=[300e-9 : 1 : 900e-9];
    dl=((4/pi)*l.*asin((nh-nl)/(nh+nl)));
    plot(l,dl);
    However Matlab does not return any plots (it's blank). I also think this is not the correct equation since it is independent of the number of layers (N). I need an equation that takes both width and height of the curve into account...

    What equation do I need to use here? :confused:
     
  2. jcsd
  3. Aug 21, 2014 #2

    ehild

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    Homework Helper
    Gold Member

    Your formulae are not functions of the wavelength. The plots show reflectance in terms of wavelength. To model R(λ) you can use the method based on the optical admittance. You find it in the book Optics of thin films by Vasicek, for example, or at http://www.edmundoptics.com/technical-resources-center/optics/an-introduction-to-optical-coatings [Broken].

    ehild
     
    Last edited by a moderator: May 6, 2017
  4. Aug 21, 2014 #3
    Thank you for the reply. The only parameters that I have are the refractive indices, N, and λ. So how do I plug these into the equation in your link?

    Here is the equation:

    ##R= \left( \frac{\eta_0-Y}{\eta_0+Y} \right) \left( \frac{\eta_0-Y}{\eta_0+Y} \right)##

    How does wavelength factor into this equation?

    The link doesn't really explain much. Unfortunately I don't have the textbook, and we haven't covered optical admittances.
     
  5. Aug 21, 2014 #4

    ehild

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    Through Y.
    By the way, the formula is not quite correct. The second factor has to be complex conjugated.

    Y depends on A and B.


    A and B are obtained by the product of matrices.
    The elements of the matrices depend on lambda and the refractive indices and thickness of the layers.
    Every necessary formula is given.
    Try search for a book about Optics of Thin Layers .

    ehild
     
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