Polarization state of the output of soft-x-ray lasers through the paraxial Maxwell-Bloch approach

The polarization state of the output of collisionally pumped soft-x-ray lasers is studied through the Maxwell-Bloch (MB) approach considering level degeneracy. We have used the MB theory, so as to account fur the propagation of linearly polarized waves, in the laser-target conditions of two recent experiments on neonlike germanium. In the case where a linearly polarized x-rap beam is injected into an amplifying plasma, we conclude that the polarization state of the output will be basically unaltered by the amplifier. We have also investigated intrinsic polarization of the output of an amplified-spontaneous-emission system, i.e., in the absence of a seeded radiation. Applied to the J=0-J=1 lasing line, the calculations suggest that the emerging signal is not polarized. However, one of the experiments referred to above has shown a spontaneous polarization, but the mechanism invoked by the authors to interpret this result, namely a selective trapping of the resonance line, is shown to be negligible, especially when elastic electron-ion collisions are taken into account. The influence of these collisions on the polarization properties of the amplifier is investigated through the atomic orientation and alignment. A spontaneous contribution to the output polarization is obtained when a circularly polarized beam is injected in the amplifier in order to create the necessary population differences between the quantum states of each laser level. [S1050-2947(97)07512-4].