Delay systems and phase retarders based on multilayers coated mirrors for FEL beam manipulation

The intensity of the radiation produced by a Free Electron Laser (FEL) is more intense, coherent, and with much higher photon density with respect to the radiation generated by storage rings undulators. FERMI@Elettra will use a seeding technique which provides near Gaussian temporal structure of the pulse with a bandwidth close to the transform limit. In order to preserve the properties of such pulse, the beam manipulation towards the ending station is performed by the use of multilayer coatings (MLs). The primary application is in the delay line systems, useful in pump and probe experiment: the beam is split and one of the arm is equipped with multilayer mirrors which are able to reject the fundamental harmonic, selecting the third; the two beams are then recombined and the relative delay can be controlled by changing the mirrors distance. Specific designs and working principle of such MLs are presented elsewhere. In this work the time delay of pulse travelling in the nanostructures is investigated and photoemission experiment applied to its evaluation conceived. MLs are also studied for verifying their possible application in a phase shifter set-up, useful to have control of the source polarization or to produce elliptical and circularly polarized light. In this way, the FELs circular polarized radiation, which is emitted out of the electron plane and therefore it is very difficult to be manipulated, can be generated from a plane pulse linearly polarized. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)