Multilayer mirrors for FERMI@ELETTRA beam transport system

Experiments performed in a Free Electron Laser (FEL) facility can require a selection of higher harmonics; a typical example is the pump and probe experiments in which the system under test is pumped with a fundamental wavelength and probed with its third harmonic. The wavelengths selection performed by a monochromator can affect beam properties such as wavefront deformation or time elongation and its usage in the beam manipulation should be avoided. Nevertheless, for a limited number of wavelengths, the selection can be performed using periodic multilayer coatings (MLs) with the reflectivity peak tuned at the desired harmonic: this technique is already foreseen at the new FERMI@Elettra FEL facility for selecting 20nm, 16nm, 13.5nm and 6.66nm harmonics. In order to improve the fundamental rejection, the MLs have been overcoated by different capping-layers; in particular at shortest wavelength higher rejection ratio have been obtained by the use of a third absorbent material in the capping layer. However, this same approach has not showed considerable improvements at the longest wavelengths, where interferential aperiodic capping-layers designed using a method based on the control of standing wave distribution are to be preferred.