  We report controllable manipulation of slow and fast light in a
whispering-gallery-mode (WGM) microtoroid resonator fabricated from Erbium
(Er3+) doped silica. We observe continuous transition of the coupling between
the fiber-taper waveguide and the microresonator from undercoupling to critical
coupling and then to overcoupling regimes by increasing the pump power even
though the spatial distance between the resonator and the waveguide was kept
fixed. This, in turn, enables switching from fast to slow light and vice versa
just by increasing the optical gain. An enhancement of delay of two-fold over
the passive silica resonator (no optical gain) was observed in the slow light
regime. Moreover, we show dynamic pulse splitting and its control in slow/fast
light systems using optical gain.
