Multilayer optics to be used as FEL fundamental suppressors for harmonics selection

Pump and probe experiments are one of the most attractive and powerful tools offered by a free electron laser facility. In these experiments it is fundamental to pump the system with a particular wavelength (usually the fundamental one) and to probe it with a second wavelength (e.g. higher emitted harmonics). Radiation emitted at the fundamental wavelength is 100 or more times the number of photons emitted in higher harmonics; selection of higher harmonics can be therefore critical. Multilayer (ML) mirrors that are able to provide high reflectivity peak at the desired harmonic wavelength while rejecting the fundamental have been designed and their application is foreseen to the FERMI@Elettra FEL beam transport system. The photon beam will be split into two and one of these will pass through a set of ML mirrors optimized for third harmonics selection; this scheme is also useable to realize a delay line in which the few nanosecond time delay will be controlled by changing the mirrors distance. A set of target wavelengths has been considered and multilayer structure materials have been selected. The MLs have been designed using a method based on the control of standing wave distribution at the fundamental and third harmonics wavelength, using a capping layer as a key element to achieve the willing rejection. Fabrication and test of such structures are foreseen.