Freezing Light via hot gases
  We prove that it is possible to freeze a light pulse (i.e., to bring it to a full stop) or even to make its group velocity negative in a coherently driven Doppler broadened atomic medium via electromagnetically induced transparency (EIT). This remarkable phenomenon of the ultra-slow EIT polariton is based on the spatial dispersion of the refraction index $n(\w,k)$, i.e., its wavenumber dependence, which is due to atomic motion and provides a negative contribution to the group velocity. This is related to, but qualitatively different from, the recently observed light slowing caused by large temporal (frequency) dispersion. 
Olga Kocharovskaya, Yuri Rostovtsev, and Marlan O. Scully