FIGURE 3. Measured refractive index and gain coefficient.
From the following article:
Gain-assisted superluminal light propagation
L. J. Wang, A. Kuzmich and A. Dogariu
Nature 406, 277-279(20 July 2000)
doi:10.1038/35018520
The superimposed curve over the index data is obtained using equation (1)
with parameters v1, v2 and 
obtained
experimentally.
