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Why no transparent materials with large refractive index?

  1. Dec 18, 2014 #1

    jfizzix

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    I know that the refractive index is determined by a material's dielectric constant and magnetic permeability.

    It's also true that we can treat the refractive index as a complex function with the imaginary part giving you an absorption spectrum.

    You can then get the index of refraction from the absorption spectrum with the Kramers-Kronig relations.

    My question is, what makes it that there are so few if any transparent materials with an index of refraction greater than 4?
     
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  3. Dec 18, 2014 #2

    Vanadium 50

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    How many materials irrespective of transparancy have an index that high?
     
  4. Dec 18, 2014 #3

    jfizzix

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    Silicon at 500 nm has a refractive index of 4.3 or so. Coming up with more specific examples would be a challenge, but there's a whole field of slow light optics for narrowband light sources tuned to particular spectral lines, say of Rubidium vapor
     
  5. Dec 18, 2014 #4

    jfizzix

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    Long story short, there are lots of materials that have a high index at one wavelength or another, but not that also are transparent. I don't know why you can't have both, though
     
  6. Dec 19, 2014 #5

    DrDu

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    The index of refraction averaged over some frequency range is seriously restricted by sum rules. This does not preclude n to become very large near a resonance. However, near a resonance, you will also get absorption. The absorption follows a Lorentzian line shape, hence the imaginary part decays like ##1/(\Delta f)^2## with distance from the center of the line. On the other hand, the real part will decay like ##1/(\Delta f)## So for suffiently narrow absorption lines, you can get very close to the center of the line without getting appreciable absorption but a high index of refraction. In solid state this requires low temperatures to reduce spectral broadening.
     
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