Abstract

Recent progress in X-ray laser research has created a technological basis for observation of nonlinear optical phenomena in new, X-ray domain. Apart from scientific interest (in particular, for X-ray spectroscopy), X-ray nonlinear optics appears to have great potential for applications in X-ray technology such as frequency upconversion, control of X-ray laser beam propagation, etc. At the present stage of X-ray laser technology, the main problem of X-ray nonlinear optics is the search for appropriate materials. For such short wavelengths and not very high output power available now, only resonantly enhanced nonlinear effects may be successfully observed. For this reason, most of our efforts have been devoted to identification and estimation of couples “X-ray laser line + transitions in a medium” which are resonant enough for the feasible X-ray nonlinear effects. Plasmas, in particular X-ray laser active plasmas, are the first obvious candidates for X-ray nonlinear media. After theoretical demonstration of the feasibility of X-ray saturation-related effects inside an X-ray laser, we identified, for a dozen of X-ray laser lines, resonant plasmas in which one may observe X-ray nonlinear effects based on nonlinear refractive index. For each of these couples “X-ray laser + plasma”, X-ray laser radiation is resonant to a transition from the ionic ground level. Required plasma parameters are readily attainable in standard discharge devices. The next step was the identification of resonant plasma media for the X-ray third-harmonic generation. Conversion efficiency up to 10^-6 is expected if phase matching is provided by adding “buffer ions”. In an attempt to move closer to the state-of-the-art experimental capabilities, we identified resonant couples “X-ray laser + plasma” for difference-frequency mixing of an X-ray and an optical laser. Conversion efficiency up to 0.05 is expected for this process which would allow one to achieve X-ray frequency doubling and to generate tunable coherent X-ray radiation. We also demonstrated potentials for X-ray nonlinear optics of nonionized media. In particular, we showed feasibility of stimulated electronic Raman scattering in Li vapor and He excited by Se X-ray laser radiation, using inner-shell transition in Li and He atoms. In our future research, we are planning to make use of resonances to inner-shell atomic excitation in liquids and solids.

© 1992 IQEC