Knob for changing light propagation from subluminal to superluminal

What is it about?

We show how the application of a coupling field connecting the two lower metastable states of a Λ system can produce a variety of effects on the propagation of a weak electromagnetic pulse. In principle the light propagation can be changed from subluminal to superluminal. The negative group index results from regions of anomalous dispersion and gain in susceptibility.

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Why is it important?

we have demonstrated how the $\Lambda$ system can produce a variety of results if we apply an additional LL coupling field. In particular, we have demonstrated how the application of LL coupling can produce regions of anomalous dispersion with gain and how this results in superluminal propagation of a weak pulse of light.