Self induced focusing and defocusing of femtosecond pulses in Raman-active media

The results of analytical and numerical investigation of the propagation of space-limited ultrashort (pulse duration is shorter than all the relaxation times and the time scale of the medium response) light pulse in Raman-active media are represented. This process shown reveals a number of novel specific features. It is found that at definite pulse energy the self- induced focusing is replaced by the self-induced defocusing which is again followed by the selffocusing. As a result a light channel which consists of a train of moving focuses occurs on the light beam axis. The rate of self-induced focusing and defocusing processes for this propagation regime is inversely proportional to the pulse field intensity, which is contrary to the conventional selffocusing theory.