Light propagation in a solid doped with erbium ions: From ultraslow light to the superluminal regime

In this article we report on ultraslow or fast light propagation, exciting behavior that become achievable thanks to the Coherent Population Oscillations effect (CPO). We show that the group velocity, the speed at which light propagates, can be drastically modified and engineered using the CPO effect in a crystal doped with triply ionized erbium ions, namely: Er3+:Y2SiO5. The steep dispersion of the index of refraction caused by the CPO effect is at the origin of group velocities as low as v g = 3 m / s . By inverting the slope of the index dispersion, we can make, in the same crystal, light go faster than if it was propagating in vacuum. This particular regime is characterized by larger than c or negative group velocities. In the 3-mm-long Er3+:Y2SiO5 crystal, we have achieved a delay of − 0.2 ms corresponding to a group velocity of v g = − 15 m / s . To cite this article: E. Baldit et al., C. R. Physique 10 (2009).