Ultraslow Light Propagation in an Inhomogeneously Broadened Rare-Earth Ion-Doped Crystal

We show that coherent population oscillations effect allows us to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits us to achieve a group velocity as low as $2.7\mathrm{m}/\mathrm{s}$ with a transmission of 40%. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40% to 90%, and the light group velocity from $2.7\mathrm{m}/\mathrm{s}$ to $100\mathrm{m}/\mathrm{s}$.