  A common strategy to compensate for losses in optical nanostructures is to
add gain material in the system. By exploiting slow-light effects it is
expected that the gain may be enhanced beyond its bulk value. Here we show that
this route cannot be followed uncritically: inclusion of gain inevitably
modifies the underlying dispersion law, and thereby may degrade the slow-light
properties underlying the device operation and the anticipated gain enhancement
itself. This degradation is generic; we demonstrate it for three different
systems of current interest (coupled resonator optical waveguides, Bragg
stacks, and photonic crystal waveguides). Nevertheless, a small amount of added
gain may be beneficial.
