Slow-light solitons: influence of relaxation
  We have applied the transformation of the slow light equations to Liouville theory that we developed in our previous work, to study the influence of relaxation on the soliton dynamics. We solved the problem of the soliton dynamics in the presence of relaxation and found that the spontaneous emission from the upper atomic level is strongly suppressed. Our solution proves that the spatial shape of the soliton is well preserved even if the relaxation time is much shorter than the soliton time length. This fact is of great importance for applications of the slow-light soliton concept in optical information processing. We also demonstrate that the relaxation plays a role of resistance to the soliton motion and slows the soliton down even if the controlling field is constant. 
A.V. Rybin, I.P. Vadeiko, A. R. Bishop