Polarization transformation of high-order harmonic radiation in an optically modulated plasma-based x-ray laser: Account of the nonlinearity and finite width of the harmonic spectral lines

The method proposed by Khairulin et al. [JETP Lett. 117, 652 (2023)] for converting the linearly polarized radiation of high-order harmonics of an optical field into the circularly polarized in an optically modulated active medium of a plasma-based x-ray laser has been developed. This method is extended to the case of finite width of spectral lines (finite pulse duration) of high-harmonic radiation and nonlinear interaction of radiation with matter. The transformation of the polarization of the harmonic field is achieved by introducing a phase shift of pi /2 between its polarization components, parallel and orthogonal to the polarization of the modulating field, through resonant dispersion on the spectral wings of the induced gain lines of the active medium and is accompanied by an increase in the power of high harmonics. It is shown that for harmonics with a finite bandwidth, the polarization of the output radiation is nonuniform in time. The reasons for such nonuniformity are analyzed and the conditions for its minimization are found. The influence of nonlinearity and amplified spontaneous emission of the active medium on the high-harmonic polarization is investigated. The possibilities for the experimental implementation of this effect in a neonlike active medium of a plasma x-ray laser based on Ti12+ ions with an unperturbed wavelength of the inverted transition at 32.6 nm are analyzed.