Ultraslow light propagation in a solid

With the spectral hole-burning effect resulting from coherent population oscillation in the homogenously broadened material the ultraslow light propagation phenomenon with the group velocity as slow as 27.55±0.05m/s at room temperature in a ruby crystal is observed. And the group velocity in ruby crystal depends on the amplitude modulation frequency, the power of laser beam and the orientation of the crystal lattice. Lower frequency or higher power leads to slower group velocity.