Aperiodic multilayer structures in soft X-ray radiation optics

We review the works related to the development of aperiodic multilayer structures — optical elements for the soft X-ray range. The potentialities of aperiodic multilayer mirrors as regards reflection of soft X-ray radiation in a broad wavelength range, first and foremost at normal radiation incidence, as well as the capabilities of broadband polariser mirrors are investigated. The results of multiparametric optimisation and experimental results for Mo/Si aperiodic mirrors (λ ≥ 12.5 nm) as well as calculations for several promising material pairs (Pd/Y, Ag/Y, etc.) for λ ≤ 12.5 nm are outlined. The effect of transition layers on the reflectivity is considered, in particular by taking into account the smooth variation of the permittivity near interfacial boundaries. The use of broadband mirrors in laser-plasma spectroscopic experiments is discussed.