Controlling slow and fast light and dynamic pulse-splitting with tunable   optical gain in a whispering-gallery-mode microcavity
  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. 
Motoki Asano, \c{S}ahin Kaya \"Ozdemir, Weijian Chen, Rikizo Ikuta,   Lan Yang, Nobuyuki Imoto, and Takashi Yamamoto
physics.optics