Optics: Anomalous Dispersion & Absorption Explained

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Discussion Overview

The discussion revolves around the concepts of anomalous dispersion and absorption in optics, specifically exploring the relationship between the derivative of the index of refraction with respect to frequency and absorption characteristics. Participants also delve into the derivation of the complex index of refraction from Maxwell's equations and the Helmholtz equation.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that anomalous dispersion occurs where dn/dω<0 and suggests a connection to strong absorption, but seeks clarification on this relationship.
  • Another participant references the Kramers-Kronig relations, indicating that anomalous dispersion and high dispersion generally coincide with absorption peaks.
  • A participant questions the formula n²=k for the complex index of refraction, expressing uncertainty about its derivation and suggesting a potential error in their teacher's explanation.
  • Another participant challenges the formula by pointing out the difference in units between the complex index of refraction (unitless) and the complex wavevector (units of inverse length), while affirming that the relationship k = n*ω/c remains valid.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between anomalous dispersion and absorption, as well as the validity of the formula for the complex index of refraction. The discussion remains unresolved regarding these points.

Contextual Notes

There are limitations regarding the assumptions made about the relationship between dispersion and absorption, as well as the derivation of the complex index of refraction, which are not fully explored in the discussion.

Niles
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Hi

I am reading about dispersion and index of refraction, and I have encountered the term "anomalous dispersion". This is where dn/dω<0, and where the absorption is strongest because of damping. Now, I can't seem to connect those two dots (and my book does not explain it nor Wiki): Why does dn/dω<0 imply strong absorption and vice versa?
 
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Briefly- it's the Kramers-Kronig relations. Anamolous dispersion (and high dispersion generally) occurs in the same spectral region as absorption peaks.
 
Thanks.

I have another question related to optics, so I will ask it here rather than creating a new thread. When we solve Maxwells equations inside a conductor, we eventually get the Helmholtz equation, where the wavevector k is complex.

In order to find the index of refraction, my teacher told me that n2=k, where n is the complex index of refraction and k is the complex wavevector. Did he make an error? I really have to idea where that formula comes from.
 
Niles said:
In order to find the index of refraction, my teacher told me that n2=k, where n is the complex index of refraction and k is the complex wavevector. Did he make an error? I really have to idea where that formula comes from.

Sounds like a mistake to me--the quickest way to see this is to note that n is unitless while k has units of inverse length. If you're treating n as the complex index of refraction, and k as the complex wavevector, then the formula k = n*omega/c still holds.
 

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