Anomalous Dispersion: Negative Refractive Index & How to Achieve

[n0_3sc]

Anomalous Dispersion?

All I know about anomalous dispersion is that it has a negative second order propagation vector.
That would mean the refractive index is negative...how?

How do you also "achieve" or get "into" the anomalous dispersion regime?

 

[Meir Achuz]

The group velocity of a wave is given by
v_g=\frac{c}{n}\left(1+\frac{\lambda}{n}\frac{dn}{d\lambda}\right).
This means when dn/d\lambda is positive, the group velocity could be greater than c. The refractive spectrum would also be reversed. For these reasons, dn/d\lambda>0 is called "anomalous dispersion".
n is still usually postive, but the lambda derivative is positive.
Anomalous dispersion usually occurs near a resonant frequency of the material, where n varies rapidly with wavelength. Because of this rapid variation, v_g does not represent the propagation velocity of a pulse, so that relativity is not violated.