Figure 1
The absorption
or gain (green), index of refraction (blue) and time delay (red) for optical
propagation, all as a function of optical frequency (x-axis). In (a) there are
two frequencies at which absorption occurs. Although the index of refraction is
close to 1 everywhere, it varies rapidly near these absorption regions. For this
reason, light travels increasingly slowly as its frequency approaches the
frequency of either of the absorption lines ("normal dispersion"). Directly on
top of either absorption line so-called "anomalous dispersion" may lead to group
velocities that are faster than light, or even negative. However, this only
happens at frequencies where there is strong absorption and pulse distortion. In
(b) the two absorption peaks are replaced by two gain (amplification)
peaks. This swaps the role of normal and anomalous dispersion, so that light at
the gain frequency travels very slowly, while light at other frequencies (where
the medium is essentially transparent) travels faster than the speed of light or
at negative speeds.